Temperature Converter
Convert any temperature — Celsius, Fahrenheit, Kelvin, Rankine, or Réaumur. Exact under the SI definitions; absolute zero is bounded.
100 Celsius (°C) =
212
Converted via Celsius as intermediate. Definitions: 0 °C = 273.15 K exactly; F = C × 9/5 + 32; °R = K × 9/5; 80 °Ré = 100 °C. Result rounded for display only; the underlying value is exact under SI definitions. Absolute zero in Fahrenheit (°F) is -459.67.
How to use this calculator
Enter a temperature, choose your starting scale in "From", then pick the target scale in "To". The result updates instantly. Pick Celsius for everyday metric, Fahrenheit for US weather and cooking, Kelvin for physics and chemistry, Rankine for US thermodynamics, or Réaumur for historical European texts. The calculator refuses inputs below absolute zero on the chosen scale.
How the calculation works
Temperature scales are affine — each has its own zero and step size, so you can't just multiply by a factor like you would with length or weight. We convert via Celsius as the intermediate. Definitions: 0 °C = 273.15 K exactly (SI); F = C × 9/5 + 32 exactly (NIST SP 811); °R = K × 9/5, so 0 °R = absolute zero with Fahrenheit-sized degrees; 80 °Ré = 100 °C exactly (Réaumur scale, 1730). No rounding is applied during computation — only at display.
Worked example
100 °C → Fahrenheit: 100 × 9/5 + 32 = 180 + 32 = 212 °F (boiling point of water at 1 atm). → Kelvin: 100 + 273.15 = 373.15 K. The well-known crossover point is −40: −40 °C × 9/5 + 32 = −72 + 32 = −40 °F. Absolute zero (0 K) = −273.15 °C = −459.67 °F = 0 °R.
Frequently asked questions
How do I convert Celsius to Fahrenheit by hand?
Use F = C × 9/5 + 32. So 20 °C × 1.8 = 36, plus 32 = 68 °F (room temperature). For a fast mental approximation, double the Celsius value and add 30 — gives 70 °F for 20 °C, close enough for weather. The exact formula matters for cooking, body temperature, and anything technical.
Why is the freezing point of water 32 °F instead of 0?
Daniel Fahrenheit set 0 °F at the freezing point of a brine of ammonium chloride and ice — the coldest stable mix he could make in 1724 — and 96 °F at human body temperature. Calibration drift later landed the freezing point of pure water at 32 °F and boiling at 212 °F. The 180-degree span between freezing and boiling explains the 9/5 ratio with Celsius (100 °C span).
What is absolute zero in each scale?
0 K = −273.15 °C = −459.67 °F = 0 °R = −218.52 °Ré. It is the temperature at which classical thermal motion stops — no system can be colder. Kelvin and Rankine are "absolute" scales that start at this floor; Celsius and Fahrenheit are offset from it. Real labs reach within a few nanokelvin of absolute zero with laser cooling and BEC traps, but never zero itself.
Why do Celsius and Fahrenheit cross at −40?
Because F = C × 9/5 + 32, the two scales agree where the linear functions intersect. Setting F = C gives x = x × 9/5 + 32 → −4x/5 = 32 → x = −40. So −40 °C and −40 °F are the same temperature. This is the only crossing point and a useful sanity check for any conversion tool.
When should I use Kelvin instead of Celsius?
Use Kelvin for any calculation that depends on absolute temperature: gas laws (PV = nRT), Stefan–Boltzmann radiation, ratios like T₂/T₁, thermal expansion coefficients, and most physics and chemistry. Use Celsius for everyday temperature differences and human-friendly readings. A 10 K change equals a 10 °C change in size, but only Kelvin gives meaningful ratios (300 K is twice as hot as 150 K; 30 °C is not twice as hot as 15 °C).
What is Rankine and where is it used?
Rankine is the Fahrenheit equivalent of Kelvin — an absolute scale with Fahrenheit-sized degrees. 0 °R is absolute zero; °R = K × 9/5; 491.67 °R = 0 °C. It is used in US engineering thermodynamics (combustion, HVAC, aerospace) where Fahrenheit is the working scale but ratios and gas-law calculations need an absolute reference. Outside the US it is rare.