Temperature Converter

Kelvin and Rankine

Kelvin is the SI unit of temperature and is written without a degree sign, as 300 K. It starts at absolute zero, the lowest possible temperature, which is −273.15 °C or −459.67 °F. Rankine is the Fahrenheit-sized equivalent used in some American engineering work: °R = °F + 459.67, so room temperature of 68 °F is about 527.67 °R. Gas laws and thermodynamics formulas must use an absolute scale like Kelvin, otherwise ratios of temperatures give meaningless answers.

Useful reference points

A few fixed points make quick checks easy. Water freezes at 0 °C (32 °F, 273.15 K) and boils at 100 °C (212 °F, 373.15 K) at sea level. Normal body temperature is about 37 °C, or 98.6 °F. The two scales meet at −40, where −40 °C equals −40 °F.

For a rough mental conversion from Celsius to Fahrenheit, double the number and add 30: 20 °C becomes about 70 °F (the exact answer is 68 °F). The shortcut works well for weather temperatures but drifts at extremes, so use the exact formula for cooking, science or medical readings.

The conversion formulas

Celsius and Fahrenheit have different zero points and different degree sizes. °F = °C × 9/5 + 32 and °C = (°F − 32) × 5/9. Kelvin uses the same degree size as Celsius but starts at absolute zero: K = °C + 273.15. Rankine is the Fahrenheit-sized equivalent of Kelvin: °R = °F + 459.67.

Water freezes at 0 °C (32 °F, 273.15 K) and boils at 100 °C (212 °F) at sea-level pressure.

Worked examples

Normal body temperature of 37 °C is 37 × 1.8 + 32 = 98.6 °F. An oven setting of 350 °F is (350 − 32) × 5/9 = 176.7 °C, usually rounded to 180 °C on recipes. The two scales meet at −40, where −40 °C equals −40 °F.

Temperature differences

When converting a change in temperature rather than a reading, ignore the 32 offset: a rise of 10 °C is a rise of 18 °F, and a rise of 10 °C is also 10 K. Mixing up readings and differences is a common source of errors in science homework.