Temperature Converter

How to convert temperatures

Type a value into any of the fields. The rest update on their own, following the conversion formulas that count as the international standard.

Kelvin is the temperature unit in the SI, and it carries no ° symbol. Rankine, in turn, is the absolute scale within the imperial system.

Four scales, four conversion formulas

The four common temperature scales — Celsius, Fahrenheit, Kelvin and Rankine — measure the same physical quantity with different anchors and step sizes. Anders Celsius, a Swedish astronomer, proposed his scale in 1742; oddly, his original version had 0°C as the boiling point of water and 100°C as the freezing point, and it was inverted to today's convention shortly after his death. Daniel Gabriel Fahrenheit, a Danzig-born physicist, defined his scale in 1724 using a brine ice bath (0°F), the freezing point of water (32°F) and human body temperature (originally 96°F, later refined to 98.6°F).

William Thomson, later Lord Kelvin, introduced the absolute thermodynamic scale in 1848, anchored at absolute zero (the theoretical point where molecular motion stops) and using the same degree size as Celsius. Since the 2019 SI redefinition, the kelvin is defined by fixing the Boltzmann constant at 1.380649 × 10⁻²³ J/K — no longer by water's triple point. Rankine, proposed by William Rankine in 1859, is the absolute analogue of Fahrenheit: 0°R = absolute zero, with Fahrenheit-sized degrees. Engineers in legacy US thermodynamics still use it.

The four conversion formulas

Celsius to Fahrenheit: F = C · 9/5 + 32 — the multiplier 9/5 = 1.8 accounts for Fahrenheit's smaller degree, and +32 anchors at water's freezing point. Celsius to Kelvin: K = C + 273.15 — same step size, different zero. Fahrenheit to Kelvin: K = (F − 32) · 5/9 + 273.15. Rankine: R = F + 459.67, and R = K · 9/5. The two scales meet at exactly −40° — that is the only temperature where Celsius and Fahrenheit numerically agree.

Useful reference points: water freezes at 0°C / 32°F / 273.15 K / 491.67°R, boils (at 1 atm) at 100°C / 212°F / 373.15 K / 671.67°R, and absolute zero is 0 K / −273.15°C / −459.67°F / 0°R. Human body temperature is approximately 37°C / 98.6°F / 310.15 K. The Sun's surface sits at roughly 5,778 K, and the cosmic microwave background measures 2.725 K.

FAQ

Why does Fahrenheit use such an odd freezing point of 32? Fahrenheit initially calibrated using three reference points, and water's freezing landed at 32 by construction. He revised the scale several times; the final definition fixed water's boiling at 212°F, making 180 degrees between freezing and boiling — divisible neatly by many factors (2, 3, 4, 5, 6, 9, 10, 12...).

Why does Kelvin not use a degree symbol? Since 1967 the SI convention dropped the degree sign for kelvin — you write 300 K, not 300°K. Celsius, Fahrenheit and Rankine still take the degree sign because they are relative scales.

Which countries officially use which? Fahrenheit remains official only in the United States, the Bahamas, Belize, the Cayman Islands and Liberia. The rest of the world uses Celsius for everyday measurements and Kelvin in scientific contexts.

Is there a quick mental approximation between °C and °F? Yes — double the Celsius value and add 30 gets you close. For 20°C: 2 · 20 + 30 = 70, the exact answer is 68°F. The error stays within 3 degrees between −10°C and 40°C, the range that covers most daily weather.