Everyday Life · Unit Conversions
Unit Converter (Temperature)
Convert temperatures between Celsius, Fahrenheit, Kelvin, and Rankine using exact NIST conversion formulas.
Calculator
Formula
All conversions route through Celsius as the intermediate scale. T_C is Celsius, T_F is Fahrenheit, T_K is Kelvin, T_R is Rankine.
Source: NIST Special Publication 811 (2008 Edition), Table B.9 — Conversion Factors for Temperature.
How it works
Unlike unit conversions for length or mass, temperature cannot be converted by multiplication alone. This is because the zero point of each scale is defined differently. The Celsius scale sets 0 °C at the freezing point of water, while Fahrenheit sets 32 °F at the same physical point.
This calculator routes every input through Celsius as an intermediate. First the input is converted to Celsius, then all three output formulas are applied simultaneously.
Worked example
Converting 100 °C (boiling point of water):
Fahrenheit = 100 × 9/5 + 32 = 212.0000 °F
Kelvin = 100 + 273.15 = 373.1500 K
Rankine = 212 + 459.67 = 671.6700 °R
Limitations & notes
This calculator uses NIST standard constants. Inputs below −273.15 °C (absolute zero) produce physically meaningless results.
Frequently asked questions
What is the formula to convert Celsius to Fahrenheit?
T_F = (9/5) × T_C + 32. The inverse is T_C = (T_F − 32) × 5/9.
Why can't you convert temperatures by multiplying?
Because most scales don't share the same zero point. 0 °C equals 32 °F, not 0 °F, so a ratio alone gives the wrong answer.
What is absolute zero in each scale?
Absolute zero is −273.15 °C, −459.67 °F, 0 K, and 0 °R.
When is Rankine used?
Rankine is used in some US engineering fields, particularly thermodynamics, where absolute temperatures in Fahrenheit-sized degrees are needed.
Is 'degrees Kelvin' correct?
No — since 1967 the unit is simply 'kelvin' (K), not 'degrees kelvin'. Celsius and Fahrenheit are still 'degrees'.
Last updated: 2025-01-15 · Formula verified against primary sources.