Temperature Converter
Type a reading and every scale updates together.
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Four scales, one temperature
Temperature is unusual in having four scales still in live use, and they disagree in two different ways: how big a degree is, and where zero sits. Type a reading above and every scale updates at once.
Celsius and kelvin share a degree size and differ only in their zero. Fahrenheit and Rankine do the same. Across the two families the degree changes size by a factor of 1.8 — because Celsius puts 100 steps between water freezing and boiling where Fahrenheit puts 180.
°F = °C × 1.8 + 32 K = °C + 273.15 R = °F + 459.67
Kelvin and Rankine are the absolute scales: they start at absolute zero, so their readings are never negative, and only on those scales does it mean anything to say one temperature is twice another.
The working, step by step
Every line updates with whatever you type above. Temperature is the one conversion where the method genuinely matters, because two things change at once: the size of a degree and the position of zero.
Why it is a line that misses the origin
Plotting one scale against the other shows the whole relationship at a glance. The slope is the size of a degree. The height at which the line crosses the vertical axis is the offset — the reason multiplying alone never works. Converting kilograms to pounds gives a line through (0, 0); this one does not.
The same temperature, two different numbers
One physical temperature, read off two scales. The tick marks are spaced differently and shifted along, which is exactly what the two numbers in the formula are doing.
Where does that sit?
A reading is easier to judge against temperatures you already know.
Your turn
Reading a method and being able to use it are different things. Answer within half a per cent and it counts as correct.
Score: 0 of 0
How to use this converter
- Type into either box. The other side updates as you go. There is no button to press.
- Negative readings are fine. Type a minus sign and the graph and scale below follow the value below zero.
- Swap the direction with the button between the boxes if you want to work the other way round.
- Set the decimals if the default is more or less precision than you need.
- Copy the answer with its unit, ready to paste.
- Share the result. The address bar tracks what you type, so the link you copy opens on the same reading.
If you enter something colder than absolute zero, the tool will say so rather than quietly converting a temperature that cannot exist.
Common uses and tips
Which scale to use when
- Celsius for everyday life nearly everywhere, and for science outside the United States.
- Fahrenheit for weather, cooking and body temperature in the United States.
- Kelvin whenever a formula divides or multiplies by temperature — gas laws, radiation, efficiency — and for colour temperature.
- Rankine for absolute temperature in American engineering, where everything else is already in Fahrenheit.
Tips worth knowing
- Differences behave differently from readings. A change of 1 °C equals 1 K but 1.8 °F. Offsets never apply to a difference.
- Ratios need an absolute scale. 600 K is twice the absolute temperature of 300 K; 60 °C is not twice 30 °C.
- −40 is the crossover where Celsius and Fahrenheit read the same.
- Notation. Kelvin takes no degree sign. Rankine is written R or °R. Celsius and Fahrenheit always take one.
Readings people look up most
Each one opens the converter with the number already in it.
- -273.15 °C to °F
- -40 °C to °F
- 0 °C to °F
- 10 °C to °F
- 20 °C to °F
- 25 °C to °F
- 30 °C to °F
- 37 °C to °F
- 100 °C to °F
- 180 °C to °F
Quick reference
| Celsius | Fahrenheit | Usually this is… |
|---|---|---|
| −40 °C | −40 °F | the point both scales agree |
| −18 °C | 0 °F | home freezer |
| 0 °C | 32 °F | water freezes |
| 10 °C | 50 °F | cool day |
| 20 °C | 68 °F | room temperature |
| 25 °C | 77 °F | warm day |
| 30 °C | 86 °F | hot day |
| 37 °C | 98.6 °F | body temperature |
| 40 °C | 104 °F | heatwave, or a high fever |
| 100 °C | 212 °F | water boils |
| 180 °C | 356 °F | moderate oven (written 350) |
| 220 °C | 428 °F | hot oven (written 425) |
Questions people ask
Which temperature scale should I use?
Celsius for everyday use in most of the world, Fahrenheit in the United States, and kelvin whenever a calculation needs absolute temperature.
What is absolute zero on each scale?
0 K, −273.15 °C, −459.67 °F and 0 R.
At what temperature do Celsius and Fahrenheit agree?
−40. It is the only reading where the two scales give the same number.
Why does kelvin have no degree sign?
Because it is an absolute unit rather than a scale of degrees. It is written 300 K, and spoken as three hundred kelvin.