Temperature Converter

The Temperature Converter translates any temperature value between the five common scales — Celsius, Fahrenheit, Kelvin, Rankine, and Réaumur — simultaneously and with the conversion formula shown for every transformation. Most people meet this problem when reading weather reports while traveling, cooking from a foreign recipe, or following scientific content that mixes metric and imperial units. Celsius and Fahrenheit are the two you will encounter in daily life; Kelvin shows up in science, astronomy, and cold-chain logistics; Rankine appears in US engineering thermodynamics; Réaumur survives in a few European cheese and syrup recipes.

This tool handles all five at once. You type 100 in the Celsius field and the Fahrenheit (212), Kelvin (373.15), Rankine (671.67), and Réaumur (80) fields update in real time. Every result shows the exact formula used (for example, F = C times 9/5 + 32), which is useful for students learning the conversion and anyone who wants to verify the math manually. The converter also handles negative temperatures all the way down to absolute zero (-273.15 °C, 0 K) with an explicit warning if you try to enter a value below the physically possible minimum.

Temperature scales differ in two properties: the zero point and the size of one degree.

Celsius (formerly centigrade) is defined by the freezing and boiling points of water at standard atmospheric pressure — 0 °C and 100 °C. Since 2019 the scale is formally tied to the Boltzmann constant rather than water, but the practical numbers are identical.

Fahrenheit places the freezing point of water at 32 °F and boiling at 212 °F, which means 180 Fahrenheit degrees span the same range as 100 Celsius degrees. One Fahrenheit degree is therefore 5/9 of a Celsius degree. The conversion formulas are F = C × 9/5 + 32 and C = (F - 32) × 5/9.

Kelvin is the SI unit of thermodynamic temperature. It shares the Celsius degree size (1 K = 1 °C interval) but starts at absolute zero: 0 K = -273.15 °C. The conversion is simple addition: K = C + 273.15 and C = K - 273.15. Kelvin has no negative values because absolute zero is a physical minimum — no molecular motion is possible below it.

Rankine is to Fahrenheit what Kelvin is to Celsius: same degree size, absolute zero at zero. 0 °R = 0 K = -459.67 °F. R = F + 459.67. Rankine appears mostly in US engineering contexts — HVAC, aerospace, thermodynamic cycle calculations.

Réaumur fixes freezing at 0 °Ré and boiling at 80 °Ré, so one Réaumur degree is 1.25 Celsius degrees. Ré = C × 4/5 and C = Ré × 5/4. Mostly obsolete, still encountered in historical European cooking references.

This converter uses double-precision floating point internally, which is more than enough precision for any practical temperature (error below 10^-10 for all realistic values). Results are rounded to a user-configurable decimal precision for display, defaulting to two decimals for weather-style data and four for scientific work.

Compared to a quick Google search (type 95 F in C), this tool shows every scale at once plus the formula. You get the context that saves follow-up searches when you realize you need Kelvin too. For single quick conversions Google is fine; for teaching, scientific work, or comparing across multiple scales, the side-by-side view wins.

Compared to a physical calculator or phone app, this tool works in any browser without install and handles all five common scales with formulas visible. Simple unit converter apps often hide the formula, so they are less useful for learning — you copy the number without understanding it.

Compared to Wolfram Alpha, this tool is faster for the narrow task of temperature conversion. Wolfram handles arbitrary unit expressions and is more powerful for complex calculations, but you get a heavier UI and more typing for the basic five-scale lookup most people actually need.