Calculate the Fahrenheit equivalent of any Celsius temperature, and convert freely between Celsius, Fahrenheit, and Kelvin.
Celsius to Fahrenheit Formula
To convert a Celsius temperature to Fahrenheit, multiply it by 9/5 and add 32.
F = C * 9/5 + 32
To go the other direction, from Fahrenheit to Celsius, subtract 32 first and then multiply by 5/9.
C = (F - 32) * 5/9
The converter also supports Kelvin, which uses the same size degree as Celsius but starts at absolute zero.
K = C + 273.15
- C = temperature in degrees Celsius
- F = temperature in degrees Fahrenheit
- K = temperature in Kelvin
- 9/5 = number of Fahrenheit degrees in a temperature interval of one Celsius degree
- 32 = the Fahrenheit reading at the freezing point of water, where Celsius reads 0
- 273.15 = the number of Celsius degrees between absolute zero and the freezing point of water
The “Convert from” and “Convert to” selectors set the temperature scales. Enter an absolute temperature reading at or above absolute zero. Expand “Display precision” to choose 0–4 decimal places; the result shows all three scales, and “Calculation details” gives the conversion equations. Temperature differences require scale factors without the reading offsets.
Common Temperature Reference Points
These illustrative values show the same temperature on each scale. The water values are conventional approximate reference points for pure water near standard atmospheric pressure; room and body values are examples, not universal constants.
| Reference | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | -273.15 | -459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 21 | 69.8 | 294.15 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils | 100 | 212 | 373.15 |
The two scales read the same value at -40, where -40 Celsius equals -40 Fahrenheit. This is a quick way to confirm the formula is working.
Example Problems
Example 1. Convert 25 degrees Celsius to Fahrenheit.
Multiply 25 by 9/5 to get 45, then add 32. The result is 77 degrees Fahrenheit.
F = 25 * 9/5 + 32 = 77
Example 2. Convert 98.6 degrees Fahrenheit to Celsius.
Subtract 32 from 98.6 to get 66.6, then multiply by 5/9. The result is 37 degrees Celsius.
C = (98.6 - 32) * 5/9 = 37
Frequently Asked Questions
Why do you multiply by 9/5 instead of just adding a fixed number?
A Fahrenheit degree is smaller than a Celsius degree. There are 180 Fahrenheit degrees between freezing and boiling water but only 100 Celsius degrees over the same range, and 180/100 reduces to 9/5. The multiplication rescales the gap between degrees, and the +32 shifts the zero point so that freezing lines up at 32 instead of 0.
Is there a temperature where Celsius and Fahrenheit are equal?
Yes. At -40 the two scales meet, so -40 Celsius is exactly -40 Fahrenheit. Below and above that point the readings diverge.
What is the quick mental estimate for Celsius to Fahrenheit?
Double the Celsius value and add 30. For 25 Celsius that gives roughly 80, close to the exact answer of 77. Use the full formula when you need an accurate figure, since the shortcut drifts at higher temperatures.
