How this calculator works
Enter a value and choose the scale to convert from and to. Unlike length or weight, temperature scales are not simple ratios of each other: Fahrenheit and Kelvin each need both a multiplication and an offset relative to Celsius, so this converter always works through Celsius as the pivot rather than a single factor.
Celsius and Kelvin share the same degree size, just with a different zero point, which makes Kelvin useful in science because it never needs negative numbers for any physically real temperature. Fahrenheit degrees are smaller than Celsius or Kelvin degrees, by a ratio of 9 to 5.
The formulas
°F = °C × 9⁄5 + 32°C = (°F − 32) × 5⁄9K = °C + 273.15Absolute zero, the physical floor for temperature, is 0 K, equal to -273.15°C and -459.67°F. Values below this are not physically attainable, though the arithmetic here still computes them like any other conversion.
Worked example: 100°C to Fahrenheit and Kelvin
- Start with 100°C, the boiling point of water at sea level.
- Fahrenheit: 100 × 9⁄5 + 32 = 180 + 32 = 212°F.
- Kelvin: 100 + 273.15 = 373.15 K.
Frequently asked questions
Why do Celsius and Fahrenheit cross at -40?
Because -40 is the point where the linear formula F = C × 9/5 + 32 gives F = C. Solve C × 9/5 + 32 = C and the only solution is -40.
What is absolute zero?
Absolute zero, 0 K (-273.15°C, -459.67°F), is the theoretical lower limit of temperature, where a system has minimal thermal energy. It has never been reached experimentally, only approached extremely closely.
Can I convert Fahrenheit straight to Kelvin without going through Celsius?
You can combine the formulas into one step, but converting through Celsius first (as this calculator does) is easier to verify and avoids arithmetic mistakes in the combined constant.
Is Kelvin ever written with a degree symbol?
No. By SI convention it is written as '300 K', not '300°K', because the kelvin is an absolute unit rather than a degree offset from an arbitrary zero.
How do I convert Celsius to Fahrenheit without a calculator?
Multiply the Celsius value by 9/5, then add 32. Using this page's own worked example, 100°C (water's boiling point) becomes 100 × 9/5 + 32 = 180 + 32 = 212°F.