Temperature Converter

How it works

Celsius and Fahrenheit are offset scales: water freezes at 0 °C and 32 °F and boils at 100 °C and 212 °F, so a Fahrenheit degree is 5/9 of a Celsius degree and the scales cross at −40. Kelvin is Celsius shifted so that zero is absolute zero: K = °C + 273.15. Rankine does the same for Fahrenheit: °R = °F + 459.67.

The calculator converts the reading to Celsius, then from Celsius to the scale you asked for, and lists the same reading on all four scales. A reading below absolute zero on the from scale is rejected, because no such temperature exists.

A temperature difference converts differently from a reading. A rise of 10 °C is a rise of 18 °F, not 50 °F, because the 32 offset applies to points on the scale, not to gaps between them. Pick "Temperature difference" for that case; the offsets are dropped and only the 9/5 ratio between Fahrenheit and Celsius degrees remains.

Formula

°F = °C × 9/5 + 32          °C = (°F − 32) × 5/9
K  = °C + 273.15            °R = °F + 459.67 = K × 9/5
difference:  Δ°F = Δ°C × 9/5    ΔK = Δ°C    Δ°R = Δ°F
absolute zero: 0 K = 0 °R = −273.15 °C = −459.67 °F

Example

The default, 100 °C to Fahrenheit: 100 × 9/5 + 32 = 212 °F. The same reading is 373.15 K and 671.67 °R.

−40 °C is −40 °F, the one reading where the two scales agree. Normal body temperature, 98.6 °F, is (98.6 − 32) × 5/9 = 37 °C. A difference of 10 °C is a difference of 18 °F.

Assumptions and limitations

• Scale definitions follow the SI Brochure (9th edition, 2019), which fixes the kelvin and defines Celsius as t/°C = T/K − 273.15, and NIST Special Publication 811 Appendix B.8 for Fahrenheit (t/°C = (t/°F − 32)/1.8) and Rankine (T/K = T/°R ÷ 1.8).
• The 1990 International Temperature Scale and older thermometer scales differ from the thermodynamic scale by at most a few hundredths of a degree at ordinary temperatures; such differences are ignored.
• A difference may be negative (a fall in temperature). A reading may not be below absolute zero.
• Results are rounded to six decimal places for display; the arithmetic is done in full double precision.

Frequently asked questions