Temperature converter
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- Reviewed
Temperature conversion between Celsius and Fahrenheit uses °F = °C × 9/5 + 32, and the inverse °C = (°F − 32) × 5/9. Water freezes at 0 °C and 32 °F and boils at 100 °C and 212 °F at standard pressure. The two scales read the same value at −40 degrees, and normal human body temperature of 37 °C equals 98.6 °F.
Result
100 °C converted
212 °F
°F = °C × 9/5 + 32; °C = (°F − 32) × 5/9
- Celsius
- 100 °C
- Fahrenheit
- 212 °F
- Kelvin
- 373.15 K
- Rankine
- 671.67 °R
100 °C equals 212 °F. Temperature scales are affine rather than proportional, so conversion applies both a multiplier and an offset — unlike length or mass, doubling a Celsius reading does not double the Fahrenheit reading.
How to use the temperature converter
- 01
Enter the temperature
Type the reading you want to convert. Negative values are accepted for sub-zero temperatures.
- 02
Choose the source scale
Select Celsius, Fahrenheit, Kelvin or Rankine to match the reading you entered.
- 03
Choose the target scale
Select the scale you want the answer in. The result updates instantly as you type.
- 04
Compare all four
The rows beneath the result show the same temperature on all four scales simultaneously, which is useful for scientific work.
The formula
°F = °C × 9/5 + 32 °C = (°F − 32) × 5/9 K = °C + 273.15
- °C
- Degrees Celsius. Zero is the freezing point of water at standard pressure.
- °F
- Degrees Fahrenheit. The scale places water's freezing point at 32 and its boiling point at 212.
- K
- Kelvin — the SI base unit of thermodynamic temperature. Zero kelvin is absolute zero, and no degree symbol is used.
- 9/5
- The ratio of a Fahrenheit degree interval to a Celsius one: 180 Fahrenheit degrees span the same range as 100 Celsius degrees.
Temperature scales are affine rather than proportional: conversion applies both a multiplier and an offset. This is why doubling a Celsius reading does not double the Fahrenheit reading, and why a temperature difference converts differently from a temperature value — a rise of 10 °C is a rise of 18 °F, not 50 °F.
Worked example
- Temperature
- 100
- From
- Celsius
- To
- Fahrenheit
- Result
- 212 °F
Multiply the Celsius value by the interval ratio: 100 × 9/5 = 180. Add the offset that aligns the two zero points: 180 + 32 = 212 °F, the boiling point of water at standard atmospheric pressure. The same reading is 373.15 K, found by adding 273.15 to the Celsius value, and 671.67 °R on the Rankine scale.
Frequently asked questions
How do you convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5, which is 1.8, then add 32. For 20 °C: 20 × 1.8 = 36, and 36 + 32 = 68 °F. For a quick mental estimate, double the Celsius figure and add 30 — that gives 70 for 20 °C, within two degrees across the range of ordinary weather temperatures.
How do you convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. For 98.6 °F: 98.6 − 32 = 66.6, and 66.6 × 5/9 = 37 °C. The subtraction must come first, because it aligns the two zero points before the interval scaling is applied. Reversing the order produces an error of roughly 18 degrees.
At what temperature do Celsius and Fahrenheit read the same?
At −40 degrees. Setting the two scales equal gives x = 1.8x + 32, which solves to x = −40. It is the only point where the two agree, because the scales have different zero points and different interval sizes and therefore cross exactly once.
What is absolute zero?
Absolute zero is the lowest possible thermodynamic temperature, at which particles retain only quantum-mechanical zero-point motion. It sits at 0 K, −273.15 °C and −459.67 °F. The third law of thermodynamics establishes that it cannot be reached in a finite number of steps; laboratories have come within a few hundred picokelvin.
Why does the Fahrenheit scale use 32 and 212?
Daniel Gabriel Fahrenheit set zero at the temperature of a brine mixture of ice, water and ammonium chloride, and 96 at approximately human body temperature. Water's freezing and boiling points fell at 32 and 212 as a consequence. The scale was later redefined against those two water points, which shifted body temperature to 98.6.
When should Kelvin be used instead of Celsius?
Kelvin is required whenever temperature appears in a physical law, because gas laws, blackbody radiation and thermodynamic efficiency all depend on absolute temperature. Doubling absolute temperature doubles the mean kinetic energy of particles; doubling a Celsius reading has no physical meaning at all.
How do you convert a temperature difference rather than a reading?
Apply only the multiplier, never the offset. A difference of 10 °C is a difference of 18 °F, because 10 × 1.8 = 18 — the 32-degree offset would be wrong here, as it aligns zero points rather than scaling intervals. A difference of 1 °C is exactly a difference of 1 K, since the two scales share an interval size.
Sources
- The kelvin — SI base unit definition — Bureau International des Poids et Mesures
- ITS-90 international temperature scale — US National Institute of Standards and Technology
- Redefining the kelvin — US National Institute of Standards and Technology
Last reviewed: · Formula and sources verified by Syed Aqeel Ahmad Gillani. See the methodology for how every calculation is derived.