Temperature Converter
Convert between Celsius, Fahrenheit and Kelvin.
Temperature conversion: the offset that breaks the pattern
Temperature is the one conversion on this site that is not a simple multiplication. Celsius and Fahrenheit have different zero points as well as different degree sizes, so the formula needs an offset - and that offset is why temperature differences convert differently from temperatures.
Formula
F = (C x 9/5) + 32 | C = (F - 32) x 5/9 | K = C + 273.15
Subtract 32 before multiplying when going from Fahrenheit, and multiply before adding when going to it. Reversing the order is the most common error.
A degree of difference is not a temperature
A temperature of 10 degrees Celsius is 50 degrees Fahrenheit. But a temperature rise of 10 degrees Celsius is a rise of 18 degrees Fahrenheit, not 50.
The reason is that the offset of 32 applies to the position on the scale, not to the size of the interval. When converting a difference, use only the ratio 9/5 and drop the offset entirely.
This matters in engineering specifications, weather reporting, and cooking. "Reduce the oven by 20 degrees" means 20 Celsius degrees or 36 Fahrenheit degrees depending on the original units, and getting it wrong is a substantial change.
Where the scales came from
Daniel Fahrenheit set his 1724 scale using a brine freezing point as zero and human body temperature as 96, chosen because it made the divisions easy to mark by repeated halving. Water then froze at 32 and boiled at 212.
Anders Celsius proposed his scale in 1742 with 0 at the boiling point and 100 at freezing - inverted from modern use. It was reversed shortly after his death, and the unit was renamed from "centigrade" to "Celsius" in 1948.
Kelvin, introduced by William Thomson in 1848, starts at absolute zero, the point where thermal motion is minimal. It uses the same degree size as Celsius, which is why the conversion is a simple addition of 273.15.
Reference points worth knowing
- Absolute zero: -273.15 C, -459.67 F, 0 K. Unreachable in practice; laboratories have come within a billionth of a degree.
- -40 is the crossing point where Celsius and Fahrenheit read the same. There is exactly one such point, since the two scales are lines with different slopes.
- Water freezes at 0 C, 32 F, 273.15 K and boils at 100 C, 212 F, 373.15 K at standard atmospheric pressure. Both are pressure dependent - water boils around 93 C at 2,000 m altitude.
- Normal human body temperature averages about 36.8 C, not 37. The 37 figure comes from Wunderlich's 1868 study, and modern cohorts measure consistently lower.
- A fever is generally defined as 38 C or 100.4 F and above.
Worked example
A recipe specifies 180 C in a fan oven and you have a Fahrenheit dial. F = (180 x 9/5) + 32 = 324 + 32 = 356 F, so 350 F is the practical setting.
The recipe then says to reduce by 20 degrees for a conventional oven. Because this is a difference, not a temperature, the conversion is 20 x 9/5 = 36 Fahrenheit degrees. The new setting is 320 F, not 68 F.
Checking with the full conversion: 160 C is (160 x 9/5) + 32 = 320 F. Both routes agree, which confirms the offset was correctly excluded from the difference.
Reference temperatures across scales
| Point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | -273.15 | -459.67 | 0 |
| Scales cross | -40 | -40 | 233.15 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 20 | 68 | 293.15 |
| Body temperature | 36.8 | 98.2 | 309.95 |
| Fever threshold | 38 | 100.4 | 311.15 |
| Water boils | 100 | 212 | 373.15 |
Frequently Asked Questions
Related calculators
References
- BIPM - The International System of Units (SI Brochure, 9th edition)
- NIST - International Temperature Scale of 1990 (ITS-90)
Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.