Temperature Unit Conversion Reference

Convert between all four temperature scales

Temperature is the one common conversion where you cannot just multiply by a factor, because the scales start at different zero points. This reference converts between Celsius, Fahrenheit, Kelvin and Rankine using the exact formulas, and shows where your value sits against fixed physical reference points.

Why temperature conversion requires an offset

Unlike mass or length, where converting between units is a pure multiplication, temperature scales have different zero points. Celsius sets 0 at the freezing point of water. Fahrenheit sets 0 at an older reference point (a brine/ice mixture), so water freezes at 32°F. This offset means every Celsius-to-Fahrenheit conversion needs both a scale factor (9/5 for the degree size) and an offset (+32 for the zero shift). Kelvin and Rankine are absolute scales — their zero is physical absolute zero, the coldest possible temperature — so they relate to each other by a pure factor of 9/5 with no offset needed.

How it works

Each input is converted to Celsius as a pivot, then out to every scale:

C -> F :  F = C × 9/5 + 32
C -> K :  K = C + 273.15
C -> R :  R = (C + 273.15) × 9/5

To convert into Celsius first, the formulas are reversed: subtract the offset before applying the scale factor. Celsius and Fahrenheit need both a factor (9/5) and an offset (+32); Kelvin and Rankine are absolute scales that share zero with absolute zero (-273.15 C).

Key reference points

Knowing a few anchor values makes mental estimation easier:

The −40 coincidence and other tricks

Celsius and Fahrenheit cross at exactly −40 degrees — it is the one temperature where the two scales agree, which makes it a useful sanity-check anchor. For rough Celsius-to-Fahrenheit estimation without a calculator: double the Celsius value and add 30 (for example, 20°C → 40 + 30 = 70°F; the exact answer is 68°F). The approximation is off by a few degrees but quick to compute in your head.

Kelvin is used in physics and chemistry wherever you need a proportional scale — the ideal gas law, Stefan–Boltzmann radiation, Wien’s displacement law, and most thermodynamic equations require Kelvin because they involve ratios of temperature. Rankine is used the same way in US engineering thermodynamics that works in Fahrenheit-sized degrees.

Tips and notes

• Celsius and Fahrenheit read the same value at exactly −40 degrees.
• Absolute zero is 0 K, −273.15 C, −459.67 F, 0 R — nothing can be colder.
• Water freezes at 0 C / 32 F and boils at 100 C / 212 F at standard atmospheric pressure.
• A change of 1 K equals a change of 1 C (same degree size); a change of 1 R equals a change of 1 F.
• All conversions run locally in your browser; nothing is uploaded.