Compound Interest Calculator
Finance & MoneyCalculate compound interest on investments with configurable rate, frequency, duration, and optional monthly contributions.. Free, private — all processing in your browser.
Estimates assume a constant annual rate and do not account for taxes, fees, or inflation. Real returns vary year to year. Use as a planning tool, not a forecast.
A dollar invested at 7% doesn't stay a dollar for long. That's the whole reason this calculator exists: to turn a vague feeling about "compound interest is powerful" into a specific number for your starting balance, your monthly contribution, your expected rate, and your time horizon. The future balance at the top is what you'd end up with if the rate held steady, contributions happened on schedule, and you didn't touch the money.
Everything runs in your browser. Your savings goal, your retirement year, the anxiety about starting late — none of it gets logged, none of it ships to an analytics endpoint, none of it sits in a session record. There's no signup. There's no "upgrade to see the year-by-year breakdown" gate. The math is the whole product.
Six things set this one apart from the calculators on Bankrate, NerdWallet, or Investor.gov. First, compounding frequency is actually honored and the effective annual yield row shows what each one converts to, so you can see that 5% compounded daily is really 5.127% APY. Second, the year-by-year table is one click away, not buried behind an email capture. Third, presets cover the five scenarios most people actually run — emergency fund, house down payment, 529, index fund, retirement — so you can get a number in three seconds if you don't care about the inputs. Fourth, the breakdown bar shows how much of the final balance is principal, how much is your contributions, and how much is interest. That third slice is the one that matters for the "is this worth it" question. Fifth, the contribution-timing toggle exposes the difference between start-of-period and end-of-period contributions (annuity-due vs annuity-immediate) because the ~0.6% spread on a 30-year $500/month run is about $3,600 — small but real, and surfaced so your model can match reality. Sixth, doubling time and the rule of 72 sit side by side, so you can eyeball whether the rate you're using is realistic or wishful.
One thing it won't do: predict returns. The rate you type is the rate the math uses. Pick 10% because you heard the S&P returns 10% on average and you'll get a future balance that assumes 10% every year for 30 years in a row — which is not what stocks do. Sequence-of-returns risk, inflation, taxes, fees — the calculator doesn't model any of them. Treat it as a planning tool, not a forecast, and run the worst-case rate next to the best-case rate while you're here.
Compound Interest Calculator — key features
Real APY from your compounding choice
Pick daily, monthly, quarterly, or annual, and the effective annual yield row shows what your nominal rate actually earns. 5% compounded daily is 5.127% APY; 5% compounded monthly is 5.116%. Small differences, surfaced honestly.
Breakdown bar
Stacked visual showing what portion of the final balance is starting principal, what portion is your own contributions, and what portion is interest the money earned on your behalf. The interest slice is the one that answers 'is this worth it?'
Six one-click presets
Emergency fund, house down payment, 529 college fund, S&P 500 index fund, retirement IRA, and high-yield savings. Tap any preset to load a realistic scenario in three seconds, then tweak the numbers that don't match your situation.
Year-by-year table
Expandable schedule with starting balance, contributions, interest earned, and ending balance for every year. Aggregated from the underlying compounding schedule, so no precision is lost — just rows that would be unreadable at daily granularity.
Contribution timing toggle
Choose start-of-period (annuity-due) or end-of-period (annuity-immediate). The ~0.6% difference over 30 years on $500/month contributions adds up to roughly $3,600 — small but real, and surfaced so your model can match reality.
Doubling time and rule of 72 side by side
Exact doubling time via ln(2)/ln(1+r), plus the 72/r shortcut. Useful for sanity-checking any claimed rate of return — if someone promises doubling every 5 years, that's 14.4% compound, which is a red flag for anything without FDIC insurance.
Principal growth multiplier
Shows how many times over your starting amount grew. A 2.5x multiplier means every dollar you started with is now $2.50. A fast gut check for whether your assumed rate and time horizon produce a believable result.
Stays entirely in your browser
No signup, no email capture, no server round-trip. Your retirement plan, your savings goal, and your assumptions never leave your device. Refresh the page and it's gone.
How to use the Compound Interest Calculator
- 1
Tap a preset or start from scratch
The six preset buttons cover the common scenarios (emergency fund, house savings, college fund, index fund, retirement, high-yield savings). Tap one to load realistic inputs, then tweak from there. If your situation doesn't match any preset, type your own starting amount and keep going.
- 2
Type your starting amount and monthly contribution
Starting amount is what you already have saved. Monthly contribution is what you'll add each month from paychecks or transfers. Putting $0 as the starting amount is fine if you're starting fresh. Putting $0 as the monthly contribution is fine if you just want to see how a lump sum grows on its own.
- 3
Set the annual rate you're willing to assume
For high-yield savings, use the current APY (roughly 4-5% in 2026). For CDs, use the contract rate. For diversified stock-market investing, most planners use 6-8% for post-inflation nominal returns, or 9-10% for nominal returns without an inflation haircut. Pick one and be consistent across scenarios.
- 4
Pick your time horizon
How many years before you need the money. For retirement, this is years until you retire. For a down payment, years until you buy. For a 529, years until your kid starts college. Rate and horizon matter more than starting amount when the horizon is long — compound interest needs runway.
- 5
Open the compounding and timing options if you care about them
Most accounts compound monthly by default. Some high-yield savings accounts compound daily. CDs often compound quarterly. The difference between them is tenths of a percent, but the selector is there if you want accuracy. Contribution timing defaults to end-of-period, which matches how auto-transfers actually work.
- 6
Read the breakdown and open the year-by-year table
The breakdown bar at the top shows the three pieces of your final balance: starting principal, your contributions, and interest. That last slice tells you how much the market did for you versus how much you did yourself. The year-by-year table is one click away if you want the full growth path.
Common use cases for the Compound Interest Calculator
Planning long-horizon goals (retirement, college, house)
- →Model a retirement savings trajectory: Plug in your current 401(k) or IRA balance, your monthly contribution rate, an assumed 7% return, and years until retirement. The future balance is the rough number your accounts should hit if markets behave historically. Run it again at 5% as a stress test — if the result still works, your plan has room.
- →Estimate how much a 529 plan grows before college: Start with your current 529 balance, add your monthly contribution, use 6-7% for a balanced age-based portfolio, and set years equal to 18 minus the child's current age. If the number falls short of projected tuition, the honest options are contribute more, take more equity risk while there's still runway, or plan for loans.
- →Run a down-payment savings timeline: Type how much down payment you need, then solve backward by trying different monthly contributions and years until the future balance hits your target. Use 4-5% for a high-yield savings account or 5-6% for a short-term bond fund. The right number isn't the one that feels easy; it's the one you'll actually save.
Comparing account types and strategies
- →Compare a high-yield savings account against a CD: Punch in the HYSA rate with monthly compounding, then switch to the CD contract rate with quarterly compounding. The difference in future balance at 3 years tells you whether locking up the money is worth the extra basis points. Often the HYSA wins on flexibility even if the CD wins on rate.
- →Check if extra contributions beat a lump-sum deposit: Run scenario A with a $50,000 starting balance and $0/month. Then scenario B with $10,000 starting and $500/month. At 7% over 20 years, they roughly land in the same place — the lump sum wins on early compounding, the contributions win on dollar-cost averaging. Useful for deciding whether to invest a bonus all at once.
- →See what 1% in expense ratio actually costs you: Run the same scenario at 7% and at 6%. The 1% delta compounds into about a 22% lower final balance over 30 years. This is the number that explains why Vanguard exists — and why picking the cheap index fund matters more than picking the 'best' one.
Education and sanity-checking claims
- →Teach compound interest to a kid or a beginner: The breakdown bar makes the abstract concept concrete. 'See that green slice? You didn't put that money in. The market earned it for you.' That visual beats most textbook explanations. Works for a nephew, a niece, or a new employee who doesn't know what a 401(k) is yet.
- →Sanity-check a high-return claim: If a newsletter promises 'double your money in 5 years,' the rule of 72 row shows that requires about a 14.4% annual return. Historical S&P 500 returns are closer to 10% nominal, so the claim is either extraordinary or dishonest. Same logic catches any pitch that sounds too clean.
- →Decide whether a loan makes sense against saving first: If the thing you want costs $X and the loan rate is 8%, compare that against what $X could grow into at 7% over the same loan term. Sometimes saving first beats borrowing; sometimes borrowing and keeping your investments intact wins. Depends on the specific rates, not on which feels more responsible.
Compound Interest Calculator — examples
Retirement starting at 25 — $500/month for 40 years at 7%
Starting amount: $0 Monthly contribution: $500 Annual rate: 7% Years: 40 Compounding: monthly Contributions: end of period
Future balance: $1,312,500 Total contributed: $240,000 Interest earned: $1,072,500 Interest share of final: 82% Balance doubles in: ~10.2 years at this rate
Retirement starting at 40 with $50K already saved — $500/month for 25 years at 7%
Starting amount: $50,000 Monthly contribution: $500 Annual rate: 7% Years: 25 Compounding: monthly Contributions: end of period
Future balance: $691,540 Total contributed: $200,000 ($50K start + $150K over time) Interest earned: $491,540 Interest share of final: 71% Late start tax: starting 15 years later almost halves the result
Five-year house down payment in a high-yield savings account
Starting amount: $10,000 Monthly contribution: $1,200 Annual rate: 4.5% Years: 5 Compounding: monthly Contributions: end of period
Future balance: $93,100 Total contributed: $82,000 Interest earned: $11,100 Effective annual yield: 4.594% Most of the growth is contribution, not interest — short horizons punish the rate
529 college fund from age 0 for 18 years
Starting amount: $5,000 Monthly contribution: $300 Annual rate: 7% Years: 18 Compounding: monthly Contributions: end of period
Future balance: $146,800 Total contributed: $69,800 ($5K start + $64,800 over time) Interest earned: $77,000 Interest share of final: 52% More than half the balance is growth — time does the work
Lump sum $25,000 in a 3-year CD at 5% quarterly compounding
Starting amount: $25,000 Monthly contribution: $0 Annual rate: 5.0% Years: 3 Compounding: quarterly Contributions: end of period
Future balance: $29,020 Total contributed: $25,000 Interest earned: $4,020 Effective annual yield: 5.094% Short-horizon, no-contribution case — the starting amount does almost all the work
Technical details
The formula for future value with regular contributions is two pieces added together: the growth of the initial principal, and the growth of a stream of contributions treated as an annuity.
``
FV = P × (1 + r/n)^(n×t) + PMT × [((1 + r/n)^(n×t) − 1) / (r/n)] × adjustment
P is starting principal, r is the annual rate as a decimal, n is compounds per year, t is years, PMT is the contribution per compounding period, and adjustment is 1 for end-of-period (annuity-immediate) or (1 + r/n) for start-of-period (annuity-due). The calculator converts your monthly contribution into a per-period contribution under the hood so that "$500/month" produces the same annual contribution regardless of whether you're compounding monthly or quarterly. That's a deliberate UX call — you think in "per month," not "per period."
What compounding frequency actually changes. More frequent compounding at the same nominal rate earns slightly more. The difference is smaller than marketing copy suggests. A 5% nominal rate compounded annually gives 5.000% APY. Compounded monthly it's 5.116%. Daily it's 5.127%. Continuously (the mathematical limit) it's also 5.127% to three decimals. On a $10,000 balance held for 10 years with no contributions, annual compounding ends at $16,289 and daily ends at $16,488 — about $200 difference across a decade. Don't switch banks over the compounding schedule. Switch over the APY they advertise, which already folds it in.
The rule of 72. For any rate r in percent, the number of years for your money to double is approximately 72 / r. The exact formula is ln(2) / ln(1 + r/100), which is what the "balance doubles in" row uses. At 5% the rule of 72 gives 14.4 years; the exact formula gives 14.2 years. It's a fast back-of-napkin check. If someone tells you their investment doubles every 5 years, they're implying roughly 14.4% compound returns — either a great year in the stock market or a red flag, depending on the source.
Contribution timing and why the toggle exists. Textbook finance calls end-of-period contributions "annuity-immediate" and start-of-period contributions "annuity-due." The practical difference: contributions at the start of a period earn one extra period of interest. Over 30 years at 7% compounded monthly, $500/month produces about $610,000 at end-of-period and about $613,600 at start-of-period — roughly $3,600 more, or 0.6%. Not life-changing. If your contribution comes from a paycheck that lands on the 1st, start-of-period is the more accurate model. If it's an auto-transfer on the 28th, end-of-period is closer. The default is end-of-period because that's what most calculators assume and what most auto-invest schedules actually do.
Taxes, fees, and inflation — what the math skips. The future balance at 30 years assumes:
- No capital gains tax on realized gains. A taxable brokerage hits this every time you rebalance.
- No dividend tax. Qualified dividends get taxed at 15-20% for most earners.
- No account fees or expense ratios. A 1% expense ratio on an index fund quietly shaves roughly 22% off the final balance over 30 years — this is why Vanguard and Fidelity's cheap index funds exist.
- No inflation adjustment. A $1,000,000 balance in 2056 dollars is worth roughly $480,000 in 2026 dollars at 2.5% annual inflation.
For 401(k) and Roth IRA balances the tax caveat is mostly absorbed — taxes are deferred or avoided entirely. For taxable brokerage money, shave 15-25% off the interest column to approximate post-tax reality. For real purchasing power decades out, divide the final number by roughly 2 to convert to today's dollars.
Why the table aggregates to years instead of periods. Internally the calculator compounds at whatever frequency you pick — daily, weekly, monthly, quarterly. The display rolls up to years because a 30-year table at daily frequency is 10,950 rows, which nobody wants to scroll. The yearly aggregate is the exact sum of within-year compounding; you're not losing precision, just rows.
Common problems and solutions
⚠Assuming the average rate will happen every year
If you use 10% because that's the long-run S&P 500 average, the calculator assumes 10% every single year for 30 years. Real returns swing between roughly -40% and +40% in any given year. The average matters for the endpoint; the sequence matters for whether you panic-sell along the way. For retirement planning, stress-test at a rate 2-3% lower than the historical average and see if the number still works.
⚠Forgetting that inflation eats the future dollar
A $1,000,000 future balance in 2056 dollars has roughly the purchasing power of $480,000 today if inflation averages 2.5%. The calculator gives you nominal dollars. For planning that reflects your future lifestyle, either subtract inflation from your rate (7% nominal becomes roughly 4.5% real) or divide the final balance by 2 for a rough 30-year real-dollar estimate.
⚠Ignoring taxes on a taxable brokerage account
The calculator assumes tax-free growth. That's accurate for Roth IRAs and Roth 401(k)s, and accurate for traditional 401(k) growth during the accumulation phase (the tax hits at withdrawal). For a taxable brokerage account, every dividend and every realized gain gets taxed, which shaves roughly 15-25% off the interest column over long horizons. Fill tax-advantaged accounts first; use taxable accounts only after.
⚠Not paying attention to fees and expense ratios
A 1% expense ratio on an index fund feels trivial. Over 30 years at a 7% gross return, a 1% drag takes about 22% off the final balance — hundreds of thousands of dollars on a serious retirement account. The fix is easy: check the fund's expense ratio, and if it's above 0.2% for a broad index, there's almost certainly a cheaper equivalent. Vanguard, Fidelity, and Schwab all offer sub-0.05% index funds.
⚠Starting late and trying to catch up with a higher rate
The calculator rewards time more than rate. $500/month at 7% for 40 years hits about $1.31M; $500/month at 10% for 30 years hits about $1.14M. If you're starting late, the honest moves are increase the contribution, delay retirement, or both. Dialing up the assumed rate just sets up disappointment — and often pushes people into riskier products than they should hold.
⚠Confusing nominal rate and APY
A 5% nominal rate compounded monthly is 5.116% APY. Banks advertise both, sometimes interchangeably. When comparing high-yield savings accounts, compare APY to APY — it already bakes in the compounding schedule. In this calculator, the rate you type is nominal; the effective annual yield row shows the APY equivalent.
⚠Trusting a 'guaranteed 12%' pitch because the calculator makes it look real
Calculators produce outputs for any input, including fraudulent rates. Historical post-inflation returns on a diversified stock portfolio are roughly 6-7% real or 9-10% nominal. Guaranteed rates above 5-6% for anything without FDIC insurance are a red flag. If someone is offering 12% guaranteed, the guarantee is worth less than the promise. Use the rule of 72 row as a check: 72/12 = 6, so 'doubles every 6 years' is what they're really claiming — then ask yourself how.
Compound Interest Calculator — comparisons and alternatives
Compound interest calculators cluster into three camps — broker-and-bank sites, financial-media sites, and independent utility sites. Each camp has a different bias worth knowing about.
Broker and bank calculators — Fidelity, Schwab, Vanguard, Ally, Marcus — are accurate and minimal. Most assume monthly compounding, don't let you change it, and don't expose the effective annual yield row. If you already have an account with one of them, they're fine. If you're comparison-shopping, they feel like product pages that happen to do math.
Financial-media calculators — Bankrate, NerdWallet, Forbes Advisor, Investor.gov — are feature-complete and get the math right. Investor.gov is worth a bookmark; it's the SEC's utility with no monetization angle, which is rare. The downside of the commercial media sites is the same as their mortgage calculators: you land in a funnel of "best high-yield savings accounts" affiliate content whether you want it or not. Bankrate's calculator is solid but surrounds the result with four rate-comparison CTAs that chase you around the viewport.
Independent utility sites — Calculator.net, The Calculator Site, Omni Calculator — are thorough and don't push affiliate products. Calculator.net exposes every knob, including non-monthly contribution schedules, variable rates, and tax-adjusted sub-calculators. The downside is fragmentation: modeling one realistic scenario takes three pages. Omni Calculator gets closer to a single-page experience but is heavier on UI chrome than on math.
Our version consolidates the useful parts onto one page. Presets cover the five questions most people are actually asking. Compounding frequency is adjustable and its effect is visible via the effective annual yield row. Contribution timing is exposed for people who care about the 0.6% edge. The breakdown bar makes "how much of the end number is free money from the market" visible at a glance — the actual question behind most compound interest searches. The year-by-year table is there but hidden by default because most visitors don't need it.
Honest tradeoffs we made:
- No variable rates. If you want to model rising rates or historical S&P returns year by year, Calculator.net's tool does that. We'll build a separate variable-rate tool when demand shows up.
- No inflation adjustment. You enter nominal rates, you get nominal dollars. Divide the final balance by about 2 to approximate real purchasing power on a 30-year run at 2.5% inflation.
- No tax modeling. Taxable brokerage accounts will underperform the calculator by 15-25% on the interest column over long horizons. 401(k) and Roth IRA results are close to accurate.
- No Monte Carlo or sequence-of-returns simulation. If your planning hinges on "what if returns are negative the first five years," Portfolio Visualizer is the right tool.
For the default case — what does $X/month for Y years at Z% become — this is the fastest path to the number. For everything past that, we'll happily name the sites that specialize.
Frequently asked questions about the Compound Interest Calculator
▶Why is my final balance different from another calculator with the same inputs?
Most differences come from two places — compounding frequency and contribution timing. Some calculators hardcode annual compounding; others hardcode monthly. Some assume contributions at the start of each period, others at the end. A 30-year run with $500/month at 7% can come out anywhere from about $566,000 (annual compounding, end-of-period) to about $614,000 (monthly compounding, start-of-period) depending on these two choices. This calculator exposes both settings so you can match whatever your account actually does.
▶What rate should I use for stock-market investing?
For long-horizon planning, 7% is a reasonable default for nominal returns after typical expense ratios, based on historical S&P 500 data. Some planners use 10% (historical nominal return without inflation adjustment); others use 4-5% (post-inflation real return). Conservative: 5%. Middle: 7%. Aggressive: 10%. Run the same scenario at all three and look at the range instead of picking one number.
▶How does compounding frequency actually change the result?
At the same nominal rate, more frequent compounding earns slightly more. A 5% rate compounded annually is 5.000% APY, monthly is 5.116%, daily is 5.127%. The difference between monthly and daily is tiny — about $200 on a $10,000 balance over 10 years with no contributions. Don't switch accounts over the compounding schedule; switch over the advertised APY, which folds it in.
▶Should I pick start-of-period or end-of-period contributions?
End-of-period is the default and matches how most auto-transfers work. Start-of-period is more accurate if contributions come from a paycheck that lands on the 1st of the month. On a 30-year run with $500/month at 7%, start-of-period produces about $3,600 more than end-of-period. Not a deal-breaker, but useful if you want the model to match reality.
▶Does this include taxes?
No. The calculator assumes tax-free growth, which matches Roth IRA and Roth 401(k) behavior. For traditional 401(k) and IRA accounts the tax happens at withdrawal, not during accumulation, so the calculator's number is also roughly accurate for the growth phase. For taxable brokerage accounts, shave 15-25% off the interest column to approximate real post-tax results.
▶What's the rule of 72 and is it accurate?
Rule of 72 estimates doubling time as 72 divided by your rate in percent. At 6%, your money doubles in about 12 years (72/6). The exact formula is ln(2)/ln(1+r/100), which gives 11.9 years. The rule is within 3% for rates between 4% and 10%. Above 10%, it starts to drift. The calculator shows both the exact doubling time and the rule-of-72 estimate so you can compare.
▶Why doesn't the year-by-year table show monthly rows if I picked monthly compounding?
The underlying math compounds at whatever frequency you pick. The table aggregates to yearly rows because a 40-year daily schedule would be 14,600 rows. The yearly total is the exact sum of within-year periods — no rounding, no approximation. If you need month-level detail, export the year-ending balances and compute the within-year breakdown in a spreadsheet.
▶Can I model a contribution that increases over time?
Not directly — this calculator assumes a fixed monthly contribution for the full period. A rough workaround is to run the calculation in segments: five years at your current contribution, then reset the starting balance to that result and run the next stretch at a higher contribution. For accurate career-arc modeling with raises and bonuses, Portfolio Visualizer and similar tools handle variable contributions natively.
Additional resources
- Investor.gov Compound Interest Calculator — The SEC's official compound interest utility. No monetization angle, no affiliate ads. Matches our math for most scenarios and is worth bookmarking as a sanity-check.
- Investopedia — Average Annual Return on the S&P 500 — Summary of historical S&P 500 returns with clear caveats about inflation, time periods, and why the '10% average' is both true and misleading.
- Portfolio Visualizer — Monte Carlo Simulation — Free tool for modeling sequence-of-returns risk. Use this when your planning needs to answer 'what if markets crash in the first five years of retirement?'
- Vanguard — Principles for Investing Success — Vanguard's research paper on low-cost investing, diversification, and why expense ratios compound painfully over long horizons.
- CFPB — Saving and Investing Basics — Federal consumer education on building savings, aimed at teenagers through pre-retirees. Unbiased, no product pitches.
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