Celsius to Fahrenheit Converter
Type in either box. °F = °C × 1.8 + 32 — both steps matter, and the working below shows why.
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Why 20 °C is 68 °F and not 36
Most unit conversions are a single multiplication. Kilograms to pounds is one number, 2.2046, applied and done. Temperature is not, and the reason people get it wrong is that two separate things differ between the scales at once.
The first is the size of a degree. Both scales use water's freezing and boiling points as their landmarks, but Celsius puts 100 steps between them and Fahrenheit puts 180. A Fahrenheit degree is therefore a smaller slice of the same range — 100/180, or five ninths of a Celsius degree.
The second is where zero sits. Celsius zero is water freezing. Fahrenheit zero is roughly the coldest temperature Fahrenheit could reproduce in his laboratory in 1724, well below freezing. The two scales count from different places, so after you have fixed the size of the degree you still have to slide the whole reading across by 32.
°F = °C × 1.8 + 32
Both operations are needed, and in that order. Multiply first and you have converted the gap above freezing into Fahrenheit-sized degrees. Add 32 and you have moved that gap to start where Fahrenheit starts. Do it the other way round and you get a number that is wrong by a predictable 58 degrees.
There is one temperature where the arithmetic collapses and both scales agree: −40. Below that, Fahrenheit readings are the lower of the two; above it, they are the higher. Type −40 into the box above and you will see the two scales meet on the graph.
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
Where this comes up
- Weather. The everyday band runs about −10 to 40 °C, which is 14 to 104 °F. A useful anchor: 28 °C is 82 °F, so once the Celsius figure passes the high twenties you are into Fahrenheit's eighties.
- Ovens. Recipes rarely need precision, and both traditions round to convenient marks. 180 °C is 356 °F but every American recipe says 350 °F; 200 °C is 392 °F and gets written as 400 °F. Use the round number, not the exact conversion.
- Fever. 37 °C is 98.6 °F. That decimal point is an artefact of conversion, not a measurement — the original figure was 37 to the nearest whole degree, and normal body temperature varies by about half a degree Celsius over a day anyway.
- Air conditioning and thermostats. 21 °C is about 70 °F, which is why those two numbers appear as the default setting on units sold either side of the Atlantic.
- Cooking meat. Here the exact figure matters. Poultry served at 74 °C is 165 °F; rounding to 160 leaves it three degrees short of the food-safety threshold.
A shortcut for weather
Double it and add 30. For 20 °C that gives 70 °F against a true 68. The error grows with the number — at 35 °C the shortcut says 100 °F when the answer is 95 — so it is fine for deciding what to wear and useless for an oven or a medical reading.
The reason it works at all is that doubling is close to multiplying by 1.8, and adding 30 is close to adding 32. Both errors happen to push in opposite directions in the middle of the everyday range, which is why it is at its best between about 5 and 25 °C.
Mistakes worth avoiding
- Do not convert a difference the same way as a reading. A rise of 10 °C is a rise of 18 °F, not 50 °F. For a change in temperature you use only the 1.8; the +32 applies to positions on the scale, not to gaps between them.
- Do not double a Celsius reading to double the heat. 40 °C is not twice as hot as 20 °C in any physical sense. Ratios of temperature only mean something on an absolute scale like kelvin.
- Watch the minus sign. −5 °C is 23 °F, a long way above zero Fahrenheit. Cold Celsius numbers map to Fahrenheit numbers that still look mild.
Readings people look up most
Each one opens the converter with the number already in it.
- -40 °C to °F
- -10 °C to °F
- 0 °C to °F
- 10 °C to °F
- 15 °C to °F
- 18 °C to °F
- 20 °C to °F
- 21 °C to °F
- 22 °C to °F
- 25 °C to °F
- 27 °C to °F
- 28 °C to °F
- 30 °C to °F
- 32 °C to °F
- 35 °C to °F
- 37 °C to °F
- 40 °C to °F
- 100 °C to °F
- 180 °C to °F
- 200 °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
What is the formula for Celsius to Fahrenheit?
Multiply by 1.8, then add 32. Written as a fraction it is °F = °C × 9/5 + 32, which is the same thing.
Why do you add 32?
Because the two scales do not start in the same place. Zero Celsius is water freezing; zero Fahrenheit is about 18 Celsius degrees colder. The 32 slides the reading across to account for that.
At what temperature are Celsius and Fahrenheit the same?
−40. It is the one reading where the stretch and the shift cancel out exactly, so −40 °C is −40 °F.
Is 180 °C really 350 °F?
Not exactly: 180 °C is 356 °F. Recipes round to 350 because oven dials are marked in fifties and the difference is well inside the accuracy of a domestic oven.
How do I convert a temperature difference rather than a reading?
Use only the 1.8. A change of 10 Celsius degrees is a change of 18 Fahrenheit degrees. The +32 applies to positions on the scale, not to the gaps between them.
Why is normal body temperature 98.6 °F?
It is 37 °C converted. The original nineteenth-century figure was 37 to the nearest whole degree, and converting it produced a spurious decimal place that has been quoted ever since.