Temperature Converter
Free temperature converter between Celsius (°C), Fahrenheit (°F) and Kelvin (K). Works in your browser, no sign-up.
Choose source and target units, type a value, and the calculator converts between Celsius, Fahrenheit and Kelvin.
The formulas
Unlike length or mass, temperature scales are affine — they have a
non-zero offset. So you can’t normalise with a single multiplier; you have
to do value/from → Celsius → value/to.
°C → °F: °F = °C × 9/5 + 32 °F → °C: °C = (°F − 32) × 5/9 °C → K: K = °C + 273.15 K → °C: °C = K − 273.15
Reference points
| Phenomenon | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 20–22 | 68–72 | 293–295 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils (1 atm) | 100 | 212 | 373.15 |
Why “K” and not “°K”
By international convention (SI, 1967), the kelvin is written K without the degree sign. The reasoning: ° marks a relative scale; the kelvin is an absolute thermodynamic scale (no negative values), so the degree symbol is dropped. Celsius and Fahrenheit both keep the °.
Mental conversion tricks
For Celsius → Fahrenheit, double it and add 30 is a quick (within ~5°) approximation. So 20 °C ≈ 70 °F (actual 68 °F); 30 °C ≈ 90 °F (actual 86 °F); 0 °C ≈ 30 °F (actual 32 °F). For precision use the calculator.
Worked examples
-
100 °C in Fahrenheit (boiling water)
100 °C = 212 °F
-
0 °C in Kelvin (freezing water)
0 °C = 273.15 K
Frequently asked questions
What are the conversion formulas?
Celsius ↔ Fahrenheit: °F = °C × 9/5 + 32; °C = (°F − 32) × 5/9. Celsius ↔ Kelvin: K = °C + 273.15; °C = K − 273.15. Internally the calculator normalises through Celsius — convert from-unit to Celsius, then Celsius to to-unit.
Why doesn't Kelvin use the degree symbol?
By the SI convention adopted in 1967, Kelvin is just "K", not "°K". The degree symbol marks a relative scale; the kelvin is an absolute thermodynamic scale with no negative values, so the convention is to drop the °. Celsius and Fahrenheit both keep the °.
Common reference points?
Water freezes at 0 °C = 32 °F = 273.15 K. Water boils (at standard pressure) at 100 °C = 212 °F = 373.15 K. Human body temperature is about 37 °C ≈ 98.6 °F. Room temperature is roughly 20–22 °C ≈ 68–72 °F. Absolute zero is −273.15 °C = −459.67 °F = 0 K.
Why the .15 in 273.15?
Historical accident: the Celsius scale was defined first (0 = freezing water, 100 = boiling water at 1 atm). Kelvin was later set so that 0 K = absolute zero, but the size of a kelvin = size of a degree Celsius. The empirical fact that absolute zero is at −273.15 °C is what produces the offset.
How does the calculator handle very small or very large values?
Same precision as the rest of the site (IEEE-754 doubles, ~15 significant digits). For everyday temperatures the limiting factor is the precision of the input (you typed it to one decimal place, the answer is good to one decimal place).
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