Mortgage Calculator

A mortgage calculator should answer one question: what am I actually going to write a check for each month? Most of them answer a different question — what's my principal and interest — and leave you to figure out why the real payment your lender quotes is $500 higher. This one does the full calculation up front. Property tax, homeowners insurance, PMI when it applies, HOA dues when you have them. The number at the top of the page is the number that leaves your account.

Everything runs in your browser. Your home price, your income considerations, the neighborhoods you're pricing out — none of it leaves your device. There's no signup, no rate-shop popup, no lender affiliate pitch waiting for your email. The math is the whole product.

Five things set this calculator apart from the ones you'll find on Bankrate, Zillow, or Rocket Mortgage. First, PMI is handled honestly: when your down payment is less than 20% the calculator computes it into your payment, shows you the exact month it drops off, and reflects that in the amortization table as a column that goes to zero. Second, the amortization schedule is one click away, not gated behind a signup. Third, bidirectional down payment input means you can type $75,000 or 15% and the other auto-fills. Fourth, CSV export for your tax planning, refinance comparison, or just for pasting into a spreadsheet. Fifth, a biweekly payment toggle that shows the actual savings instead of making you take the claim on faith.

One thing it does not do: quote you a rate. Rates change daily and depend on your credit, loan-to-value ratio, and local market conditions. We leave that to lenders and stay in our lane of doing the math once you know the rate. Use this to decide what house you can afford, what rate feels worth it, or whether putting an extra $200 toward principal each month is worth the coffee you'd give up. It's a planning tool, not a loan quote.

A fixed-rate mortgage is amortized using a geometric series. The monthly principal and interest payment for a loan of amount P, monthly interest rate r = annualRate / 12, and total period count n = years * 12 is:

``
payment = P × (r × (1 + r)^n) / ((1 + r)^n − 1)
``

Every monthly payment is split between interest (the balance times the monthly rate) and principal (whatever's left). Early in the loan the balance is high, so interest dominates. Late in the loan the balance is low, so almost all of each payment goes to principal. This is why year-one looks like you're not making any progress — about 72% of an early payment on a 30-year loan at 6.5% is interest. By year 15 it's closer to 50/50. By year 25 it flips to 72% principal. The formula is correct; it's just non-intuitive. If you want to see this on your specific loan, expand the amortization schedule and look at the Principal and Interest columns over time.

PMI — what it is and when it legally comes off. Private Mortgage Insurance protects the *lender* if you default. You pay it, they benefit from it. PMI is required when your down payment is less than 20% on a conventional loan. Under the Homeowners Protection Act of 1998, PMI must be automatically cancelled when your loan-to-value ratio reaches 78% based on the original amortization schedule. You can request removal once you hit 80% LTV. Both of those cutoffs are calculated from the *original* home value and the original payment schedule — not from the current market value of your home. If your home appreciated and you want PMI off sooner, you can pay for a new appraisal and ask your servicer to cancel based on current LTV. This calculator uses 80% as the practical removal point since most homeowners request it rather than waiting for the 78% automatic trigger.

Biweekly payments: the math behind the claim. Biweekly payment programs claim to save tens of thousands of dollars in interest. They're telling the truth, but the mechanism is worse-than-advertised simple. If you pay half your monthly payment every two weeks, you make 26 half-payments per year. That equals 13 monthly payments — one more than the 12 you'd make on a monthly schedule. That extra payment goes 100% to principal, which lowers the interest charged on the next statement, which means the following month pays down more principal, and the savings compound. On a $300,000 loan at 4% for 30 years, a biweekly schedule pays off the loan about 4 years early and saves about $33,000 in interest. On a $400,000 loan at 7%, the savings are closer to $100,000. The catch is you need your lender to *actually apply* the extra payments to principal, not hold them in escrow. Some lenders charge a fee for formal biweekly programs. An identical-and-free alternative is to pay an extra 1/12th of your monthly payment each month yourself; the principal reduction math works out the same.

Extra payments toward principal. Any payment above the scheduled amount, applied to principal, shortens the loan and saves interest. The savings come from two places: fewer total payments at the end of the loan, and less interest accruing on the smaller balance during the remaining months. A $100/month extra payment on a 30-year $350,000 loan at 6.5% saves around $70,000 in interest and pays the loan off roughly 5 years early. The amortization schedule in this calculator recomputes period by period, so the numbers it shows reflect this compounding effect — not a linear approximation.

Property tax is not a fixed value. The annual property tax you see on your county assessor's website is this year's bill. It will go up. Most jurisdictions reassess periodically and raise the rate or the assessed value (or both). Over a 30-year loan, your property tax at year 30 will often be double what it is today, sometimes more. This calculator assumes a fixed tax for simplicity, which means your real total PITI will drift upward over the loan's life. Same caveat applies to homeowners insurance, which has been trending up faster than inflation in most US markets since 2022.

APR vs interest rate. The rate in this calculator is the nominal annual interest rate — the number your lender quotes and the number that shows up in the amortization math. APR (Annual Percentage Rate) is a separate number that includes certain closing costs amortized over the loan term. APR is always higher than the nominal rate and is what lenders are legally required to disclose in Reg Z statements, so you can compare loan offers apples-to-apples. For "what's my monthly payment," use the interest rate. For "is this lender ripping me off on fees," compare APRs.

What this calculator doesn't model. Property value change (zero appreciation assumed), adjustable rates (this is a fixed-rate calculator), refinancing mid-loan, lump-sum payments at specific points (recurring extras only), points / rate buydowns, escrow shortfalls, or changes in your PMI premium. Those are all real things that affect your mortgage. We'll build separate tools for them. This one optimizes for "what is the real monthly payment and total cost" — the two numbers that matter most when you're deciding what house to buy.

The mortgage calculator field is dominated by lender-adjacent and real-estate-marketplace sites — Bankrate, Zillow, Redfin, Rocket Mortgage, NerdWallet. Each has a calculator with a distinct tradeoff.

Bankrate's calculator is feature-complete and gets the math right. The page is fast. The downside is that the result is surrounded by rate-shopping CTAs that follow you around the page, and the amortization schedule pushes you toward their rate-comparison funnel. If you just want the number and don't mind ignoring the ads, it's good.

Zillow and Redfin tie their calculators to listing pages. They're excellent when you're calculating the payment on a specific home they have listings for — property tax pre-fills from the assessor record, insurance estimates come from regional averages. They're worse as standalone tools because you always feel like they're trying to hand you to an agent.

Rocket Mortgage's calculator is minimalist. The amortization schedule is gated behind a login — you have to create a Rocket account to see it. The user experience is clean until you try to do anything useful beyond the monthly payment number, and then you're signing up.

Chase, Wells Fargo, and other bank calculators are mostly lead-generation fronts. They compute correctly but are missing features like extra payments or biweekly comparisons, and they push you toward loan officers the moment you interact.

Calculator.net and similar utility-calculator sites have thorough math — every variant of mortgage calculator, PMI calculator, amortization calculator, extra-payment calculator as separate pages. If you're a power user who wants to see every variable, they're excellent. The downside is fragmentation: you bounce between three pages to model one realistic scenario.

Our calculator tries to consolidate: PITI, PMI with the 80% drop-off handled correctly, extra payments, biweekly, full amortization with equity tracking, and CSV export — all on one page, with progressive disclosure so the simple case stays simple. There's no signup, no rate-shop redirect, no affiliate bias. The data never leaves your browser. That's the tradeoff: we don't know your local property tax rate, so you enter it yourself; we don't know today's market rate, so you enter that yourself. In exchange, you get an honest calculator that doesn't care whether you ever click anything else on the site.

If you're price-shopping a specific home, Zillow's calculator is fine because the property-specific inputs pre-fill. If you want to understand the math or compare scenarios across multiple prices, this one is better. If you need a formal quote, no calculator helps — you need a lender.