Celsius and Fahrenheit
The Celsius scale is tied to the kelvin, the SI unit of thermodynamic temperature: a temperature in °C is the kelvin value minus 273.15. The Fahrenheit scale, still used for weather and cooking in the United States, has degrees that are 5/9 of a kelvin and an offset that puts water's freezing point at 32 °F and its boiling point at about 212 °F at sea level.
Because the scales differ in both size and zero, converting is not a simple multiplication: you scale by 9/5 and then add 32.
Worked example
Body temperature of 37 °C: 37 × 9/5 + 32 = 66.6 + 32 = 98.6 °F. The two scales meet at −40, where −40 °C is also −40 °F.
Quick mental conversion
Double the Celsius value and add 30: 20 °C → 70 °F (exact: 68 °F). The shortcut is within 2 °F between 0 °C and 25 °C.
How this works
The page applies the exact affine formula from NIST SP 811:
°F = °C × 9/5 + 32
°C = (°F − 32) × 5/9
Internally both values pass through kelvin (K = °C + 273.15), which is how every temperature unit on UtilNest is defined, so °C, °F, K and °R pages always agree. Results are cleaned of floating-point noise and shown to the precision you pick.
Frequently asked
What is the formula from Celsius to Fahrenheit?
Multiply by 9/5 (or 1.8) and add 32. For example 25 °C × 1.8 + 32 = 77 °F.
At what temperature are Celsius and Fahrenheit equal?
At −40. Solving C = C × 9/5 + 32 gives C = −40.
What is absolute zero in Fahrenheit?
−459.67 °F, which is 0 K or −273.15 °C.
Why is a normal body temperature 98.6 °F?
It is 37 °C converted exactly. Measured healthy body temperatures actually vary by about a degree around it.
Sources
- NIST — SI units: temperature
- NIST Special Publication 811 — Guide for the Use of the International System of Units
- Wikipedia — Conversion of scales of temperature
Reviewed Oct 17, 2026 · test vectors fixtures/units/temperature/celsius-to-fahrenheit/