Temperature Converter.
Convert between Celsius, Fahrenheit, Kelvin, Rankine, and historical scales (Delisle, Newton, Réaumur, Rømer) instantly. Click a reference temperature to load it directly.
Precision
How many significant digits to display in results.
Number Format
Always show exponential form (e.g. 3.73e+2 K). Enabled automatically for very large or very small values.
Scale Reference
Celsius — 0 °C = freezing, 100 °C = boiling (at 1 atm).
Fahrenheit — 32 °F = freezing, 212 °F = boiling.
Kelvin — SI base unit. 0 K = absolute zero. No degree symbol.
Rankine — Absolute scale in °F steps. 0 °R = absolute zero.
Historical — Delisle, Newton, Réaumur, Rømer are pre-SI scales no longer in scientific use.
Scale Reference
Celsius & Fahrenheit
The most widely used everyday scales. Daniel Fahrenheit set 32 °F and 212 °F at the ice and steam points, giving the formula °F = °C × 9/5 + 32. Anders Celsius inverted the original 0–100 scale to put freezing at 0 and boiling at 100.
Kelvin — The SI Scale
The Kelvin is the SI base unit for thermodynamic temperature. 0 K is absolute zero — the lowest possible temperature — and equals −273.15 °C. The 2019 SI redefinition fixed the Boltzmann constant k = 1.380 649 × 10⁻²³ J/K exactly.
Rankine
Used primarily in US engineering thermodynamics. Like Kelvin, 0 °R is absolute zero, but degree steps equal Fahrenheit steps. 1 K = 9/5 °R. Steam tables in US aerospace and HVAC textbooks are often published in Rankine.
Historical Scales
Delisle (1732) counts down from boiling — 0 °De = 100 °C. Newton (1701) used linseed oil expansion; 0 °N = 0 °C, 33 °N = 100 °C. Réaumur (1730) set boiling at 80 °Ré. Rømer (1701) inspired Fahrenheit and set 7.5 °Rø = 0 °C.
How temperature conversion works
Unlike length or mass, temperature scales are not all proportional — they have both different zero points and different step sizes. The standard approach is to convert any input to Kelvin first, then from Kelvin to the target scale. Kelvin uses the same step size as Celsius, so °C → K simply adds 273.15.
Celsius to Fahrenheit
°F = °C × 9/5 + 32. A quick mental shortcut: double the Celsius value, subtract 10%, then add 32. For example, 20 °C → 20 × 2 = 40, −4 = 36, +32 = 68 °F (exact: 68 °F).
Fahrenheit to Celsius
°C = (°F − 32) × 5/9. Memorise: 32 °F = 0 °C, 98.6 °F = 37 °C, 212 °F = 100 °C. The crossover point where the two scales coincide is −40 (−40 °C = −40 °F).
Absolute zero
Absolute zero — the lowest theoretically attainable temperature — is 0 K = −273.15 °C = −459.67 °F = 0 °R. At this point particles have minimum quantum mechanical motion. It has never been exactly reached in practice but laboratory temperatures have approached within a few billionths of a kelvin.
Body temperature & everyday references
Normal human body temperature is 37 °C / 98.6 °F / 310.15 K. Room temperature is typically 20–22 °C (68–72 °F). Oven temperatures range from 150 °C (300 °F) for slow cooking to 260 °C (500 °F) for high-heat baking.
Frequently Asked Questions
- How do I convert Celsius to Fahrenheit?
- °F = °C × 9/5 + 32. Quick mental shortcut: double the Celsius value, subtract 10%, then add 32. For example, 20 °C → 40 − 4 + 32 = 68 °F (the exact answer).
- How do I convert Fahrenheit to Celsius?
- °C = (°F − 32) × 5/9. Key reference points: 32 °F = 0 °C, 98.6 °F = 37 °C, 212 °F = 100 °C. The two scales coincide at exactly −40 (−40 °C = −40 °F).
- What is absolute zero in Celsius and Fahrenheit?
- Absolute zero is 0 K = −273.15 °C = −459.67 °F = 0 °R. It is the lowest theoretically achievable temperature, where particles have minimum quantum mechanical motion.
- Why are Kelvin values used in science?
- Kelvin is the SI base unit for thermodynamic temperature. Starting at absolute zero means equations for thermal energy, radiation, and gas laws give physically meaningful results — unlike Celsius or Fahrenheit, which have arbitrary zero points.
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