Convert between Celsius, Fahrenheit and Kelvin.
Temperature is the one conversion on this site that needs both a scale factor and an offset, because the scales do not share a zero point. That is why you cannot convert a temperature by multiplying alone, and why converting a temperature difference uses a different rule from converting a temperature.
Converting 37 C, normal human body temperature, to Fahrenheit:
And 100 F back to Celsius: (100 - 32) x 5/9 = 68 x 5/9 = 37.8 C.
| Celsius | Fahrenheit | Kelvin | Reference |
|---|---|---|---|
| -273.15 | -459.67 | 0 | Absolute zero |
| -40 | -40 | 233.15 | Where the two scales cross |
| -18 | 0 | 255.15 | Typical freezer |
| 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 at sea level |
| 180 | 356 | 453.15 | Moderate oven |
The -40 crossing point is a genuine curiosity and a useful sanity check: it is the only temperature where the two scales read the same number.
For Celsius to Fahrenheit, double it and add 30. At 20 C that gives 70 against a true 68; at 30 C it gives 90 against 86. The approximation is good to within a few degrees across normal weather and drifts at extremes. Reversing it — subtract 30 and halve — works equally well for Fahrenheit to Celsius.
Some anchors worth memorising outright: 0 C is 32 F, 10 C is 50 F, 16 C is 61 F (the digits reverse), 28 C is 82 F (also reversed), and 100 C is 212 F.
This trips people up regularly. A temperature rise of 10 C is a rise of 18 F, not 50 F. When converting an interval or a change rather than a point on the scale, use only the ratio and drop the offset:
So "the temperature rose by 5 degrees" converts to 9 F, whereas "the temperature is 5 degrees" converts to 41 F. This matters in climate reporting, in materials specifications quoting thermal expansion per degree, and in any tolerance expressed as a range.
The kelvin shares the Celsius degree size but starts at absolute zero, the point where thermal motion is minimal. Because it has a true zero, it is the scale used in physics and chemistry, where ratios have to be meaningful — doubling the kelvin temperature of a gas doubles its average kinetic energy, whereas doubling a Celsius figure means nothing physical.
Note the convention: kelvin takes no degree symbol and no capital letter when written out. It is "300 kelvin" or "300 K", never "300 degrees Kelvin". Absolute zero is 0 K, -273.15 C, or -459.67 F, and it cannot be reached, only approached.
British recipes often specify gas marks, which follow their own progression of 25 F per mark.
| Gas mark | Celsius | Fahrenheit | Description |
|---|---|---|---|
| 1 | 140 | 275 | Very slow |
| 3 | 160 | 325 | Moderate |
| 4 | 180 | 350 | Moderate |
| 6 | 200 | 400 | Moderately hot |
| 8 | 230 | 450 | Hot |
One further adjustment: a fan or convection oven runs hotter than its setting implies because circulating air transfers heat more efficiently. Reduce the stated conventional temperature by about 20 C, or 25-30 F, when using one.
Double the Celsius figure and add 30. It is accurate within a few degrees across normal weather ranges — 25 C gives 80 against a true 77. For an exact answer, multiply by 1.8 and add 32.
At -40. It is the single point where the two scales intersect, which makes it a handy check that a conversion formula has been applied correctly.
Because a difference has no offset. Converting a point on the scale needs both the 1.8 factor and the 32 offset; converting an interval needs only the factor. A 10 C rise is an 18 F rise.
No. Kelvin is written without the degree symbol and lowercase when spelled out — "300 K" or "300 kelvin". The scale starts at absolute zero, so no offset is implied by the word "degrees".
Reduce the conventional temperature by about 20 C or 25-30 F. Circulating air transfers heat more efficiently, so a fan oven at 180 C browns considerably faster than a conventional one at the same setting.