Temperature Converter

Convert between Celsius (°C), Fahrenheit (°F), Kelvin (K), Rankine (°R) and Réaumur (°Re) with live formula display.

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The Three Common Temperature Scales

Celsius is the everyday standard in most of the world, anchored to water's freezing point (0°C) and boiling point (100°C) at sea level. Fahrenheit is used in the United States and a few other countries, with water freezing at 32°F and boiling at 212°F. Kelvin is the scientific absolute scale — zero Kelvin is absolute zero, the coldest possible temperature (-273.15°C), making it ideal for thermodynamics and physics.

The -40 Curiosity

At exactly -40°, Celsius and Fahrenheit are equal. This is the only point where both scales agree, making it a useful mental anchor. Below -40 the Fahrenheit value is numerically higher; above -40 the Celsius value is lower.

Frequently Asked Questions

°F = °C × 9/5 + 32. To convert in the other direction: °C = (°F − 32) × 5/9. A quick mental approximation: double the Celsius value and add 30 (this gives a rough result — 20°C ≈ 70°F using the exact formula, 68°F).
Absolute zero is 0 Kelvin, which equals -273.15°C and -459.67°F. It is the theoretical minimum temperature — the point at which all thermal motion stops. It has never been fully achieved in practice, though physicists have gotten within nanokelvins of it.
The UK officially switched to Celsius in the 1970s for scientific and official use, but Fahrenheit persists colloquially — particularly for body temperature, weather above 80°F, and cooking (many British recipes still list oven temperatures in Fahrenheit). Older generations often think in both.
The commonly quoted figure is 37°C (98.6°F), but research suggests the average has shifted to around 36.6°C (97.9°F). Normal body temperature varies by individual, time of day and measurement location. A fever is typically defined as above 38°C (100.4°F).
Rankine is an absolute temperature scale like Kelvin but based on Fahrenheit degrees. Zero Rankine is absolute zero, and the degree size is the same as Fahrenheit. It is used almost exclusively in some US engineering and thermodynamics applications. Outside those contexts, Kelvin is the universal scientific standard.

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