Temperature Converter
Convert between Celsius, Fahrenheit, Kelvin and Rankine — the one conversion that needs an offset as well as a scale factor.
How it works
Temperature is the one family here that is not a simple multiplication. The scales have different zero points as well as different degree sizes, so every conversion needs an offset too — which is why you cannot convert a temperature by scaling alone.
Celsius and Fahrenheit cross at −40°, the only temperature where both scales read the same number. Kelvin and Rankine are absolute scales starting at absolute zero, so they never go negative; a negative Kelvin result means an input below absolute zero, which is not physically meaningful.
A temperature difference converts differently from a temperature. A change of 10°C is a change of 18°F, because only the degree size matters for a difference — the offset cancels out.
The formula
Celsius → Fahrenheit
°F = (°C × 9/5) + 32
Fahrenheit → Celsius
°C = (°F − 32) × 5/9
Celsius → Kelvin
K = °C + 273.15
Fahrenheit → Rankine
°R = °F + 459.67
Worked examples
| Scenario | Working | Result |
|---|---|---|
| 0°C to Fahrenheit | (0 × 9/5) + 32 | 32°F — water freezes |
| 37°C to Fahrenheit | (37 × 9/5) + 32 | 98.6°F — body temperature |
| −40°C to Fahrenheit | (−40 × 9/5) + 32 | −40°F — the scales meet |
When you'd use it
- Following a recipe with the oven in the other scale
- Reading a weather forecast from another country
- Converting a lab or datasheet figure to Kelvin
- Checking a fever reading against a familiar scale
Common questions
Why can't I just multiply to convert temperature?
Because the scales start in different places. Celsius puts zero at water's freezing point, Fahrenheit puts it 32 degrees lower, and Kelvin starts at absolute zero. You have to shift as well as scale, which is why the formula has a +32 or a −273.15 in it.
At what temperature do Celsius and Fahrenheit read the same?
−40. It is the single crossing point of the two scales, which makes it a useful sanity check on any conversion — if your formula does not give −40°F for −40°C, it is wrong.
How do I convert a temperature difference rather than a temperature?
Use only the ratio, not the offset. A 10°C difference is an 18°F difference (10 × 9/5), and a 1°C difference is exactly a 1 K difference. Applying the +32 to a difference is a common mistake.
Is anything I enter sent anywhere?
No. Every conversion runs in your browser, so nothing you type is uploaded, logged or stored. There is no account and no record of what you converted.