Temperature Converter
Convert between Celsius, Fahrenheit and Kelvin at once, with reference points from absolute zero to boiling, and the formulas for each direction.
| °C | °F | K | |
|---|---|---|---|
| -273.15 | -459.67 | 0 | Absolute zero |
| -40 | -40 | 233.15 | Where the scales meet |
| 0 | 32 | 273.15 | Water freezes |
| 20 | 68 | 293.15 | Room temperature |
| 37 | 98.6 | 310.15 | Body temperature |
| 100 | 212 | 373.15 | Water boils |
A change in temperature does not convert like a temperature
This is the most common temperature mistake in print, and the tool above will happily help you make it. "It warmed up by 10°C" is not "it warmed up by 50°F", even though 10°C is 50°F. A rise of 10°C is a rise of 18°F.
The reason is that a conversion has two parts — a scale factor and an offset — and a difference
only takes the scale factor. A temperature is F = C × 9/5 + 32; a difference is
just ΔF = ΔC × 9/5. The 32 exists to line up the two zero points, and a difference
has no zero point to line up.
| A rise of | Really is a rise of | The common mistake | Wrong by |
|---|---|---|---|
| 1°C | 1.8°F | 33.8°F | 32 |
| 5°C | 9°F | 41°F | 32 |
| 10°C | 18°F | 50°F | 32 |
| 25°C | 45°F | 77°F | 32 |
| 100°C | 180°F | 212°F | 32 |
The error is always exactly 32 degrees, whatever the size of the change — it is the offset that should not have been applied. That makes it easy to spot once you know: if a converted temperature change looks about 32 too big, it was run through the wrong formula. The reliable way to avoid it is to convert both endpoints and subtract, which gives the right answer without needing to remember which formula you are in.
And "twice as hot" means nothing in either scale
Ratios need a true zero to be meaningful, and neither everyday scale has one. Celsius zero is the freezing point of water; Fahrenheit zero is roughly the freezing point of a particular brine mixture. Both are arbitrary choices, so any ratio you compute in them is arbitrary too — and the same physical change gives a different answer in each.
| Change | Times hotter in °C | In °F | In kelvin |
|---|---|---|---|
| 10°C → 20°C | 2.00× | 1.36× | 1.035× |
| 20°C → 40°C | 2.00× | 1.53× | 1.068× |
| 15°C → 30°C | 2.00× | 1.46× | 1.052× |
| -10°C → 10°C | -1.0 (negative!) | 3.57× | 1.076× |
Going from 10°C to 20°C looks like a doubling in Celsius, is 1.36× in Fahrenheit, and is 1.035× in kelvin — three answers for one physical change, and only the last one means anything. In energy terms that day got about 3.5% warmer, not 100% warmer.
The giveaway that the question is malformed rather than merely awkward: 0°C to 10°C is an infinite increase in Celsius, since you divided by zero, and a 3.7% increase in kelvin. Below freezing the Celsius ratio goes negative, which is worse than useless. If you need to say how much hotter something got, either give the difference in degrees or use kelvin, where zero really is the absence of heat at -273.15°C, or -459.67°F.
How to use
- Type a temperature into any of the three boxes.
- The other two update instantly.
- Negative values work in Celsius and Fahrenheit.
- Check the reference table for common benchmarks.
Frequently asked questions
What are the three formulas?
Celsius to Fahrenheit: multiply by 9/5 and add 32. Fahrenheit to Celsius: subtract 32, then multiply by 5/9. Celsius to Kelvin: add 273.15. The Kelvin conversion is the easy one, because the scales share a degree size and differ only in where zero sits.
What is absolute zero?
0 K, which is minus 273.15 C or minus 459.67 F — the point at which a system holds no thermal energy to give up. It cannot be reached, only approached, and laboratories have got within a few billionths of a degree of it.
Why does Kelvin not use degrees?
Because it is an absolute scale, not a relative one. Temperatures are written as 300 K, not 300 degrees K. Celsius and Fahrenheit measure from arbitrary zero points, so a difference of degrees is meaningful but a ratio is not — 20 C is not twice as hot as 10 C, whereas 300 K genuinely is twice 150 K.
What is the Rankine scale?
An absolute scale using Fahrenheit-sized degrees, where zero is absolute zero and water freezes at 491.67 R. It survives in a few corners of American engineering, particularly thermodynamics, and is otherwise a historical curiosity.
At what temperature do the scales cross?
Celsius and Fahrenheit read the same at minus 40. Celsius and Kelvin never coincide, since they differ by a constant 273.15. Fahrenheit and Kelvin meet at about 574.6.
Why is body temperature 98.6 F such a precise-looking number?
Because it is a conversion artefact. The original nineteenth-century measurement was 37 C, a rounded figure, and converting it produced the spurious precision of 98.6. Normal body temperature actually varies by around half a degree Celsius across individuals and through the day.
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