What this tool does
Temperature scales do not share a zero, which is why they cannot be converted by multiplication alone. A reading of 20°C is not double a reading of 10°C once it is in Fahrenheit, and the Kelvin scale stops at absolute zero. This converter handles all three directions — Celsius, Fahrenheit and Kelvin — with the exact formula shown and the reference points that let you sanity-check any answer.
How it works
Celsius and Fahrenheit differ in two ways at once: the size of one degree and the position of zero. A Fahrenheit degree is five-ninths of a Celsius degree, and the Fahrenheit scale sits 32 degrees below Celsius at the freezing point of water. Multiply by nine-fifths and add 32 to go up, then subtract 32 and multiply by five-ninths to come back. Zero Celsius is 32°F, not 0°F.
Kelvin is different again: the degree is the same size as a Celsius degree, but the scale starts at absolute zero, the coldest state physics allows, so there are no negative values. Kelvin is simply Celsius plus 273.15. Absolute zero is −273.15°C, which is −459.67°F, and 0 K is 273.15 degrees below the freezing point of water on the Celsius scale.
Because the conversion is offset as well as scaled, a few fixed points catch most mistakes. −40 is the one temperature where Celsius and Fahrenheit read identically, body temperature is 37°C or 98.6°F, and water boils at 100°C or 212°F. If a conversion gives the same number for both scales anywhere other than −40, the 32-degree offset was dropped.
Worked example
Converting 25°C to Fahrenheit and Kelvin.
- °F = 25 × 9/5 + 32 = 45 + 32 = 77
- K = 25 + 273.15 = 298.15
- Reverse check: (77 − 32) × 5/9 = 25
25°C = 77°F = 298.15 K. The same formulas give 100°C = 212°F and 0°C = 32°F = 273.15 K.
Accuracy and limitations
- The formulas follow the ITS-90 practical temperature scale used by everyday and laboratory thermometers; specialised cryogenic work uses a different scale.
- Fahrenheit carries visible rounding at extremes, such as absolute zero at −459.67°F, because it is defined from Celsius by a fraction.
- Fever thresholds vary by age, measurement method and guideline, so 37°C or 98.6°F is a reference point rather than a clinical cut-off.
Frequently asked questions
- What is the Celsius to Fahrenheit formula?
- °F = °C × 9/5 + 32. For 100°C that is 100 × 1.8 = 180, plus 32, giving 212°F. The reverse direction, °C = (°F − 32) × 5/9, unwinds the same two operations.
- What is 0 Celsius in Fahrenheit?
- 32°F — the freezing point of water on both scales. Getting 0°F instead means the value was multiplied without adding the 32-degree offset.
- What is normal body temperature in Fahrenheit?
- 37°C is 98.6°F, the long-standing reference value. Real readings move through the day and vary by measurement site, so treat 98.6 as a midpoint rather than a pass or fail line.
- How do I convert Fahrenheit to Celsius?
- Subtract 32, then multiply by 5/9. For 98.6°F that is 98.6 − 32 = 66.6, and 66.6 × 5/9 = 37°C exactly.
- How do I convert Kelvin to Celsius?
- Subtract 273.15. 298.15 K is 25°C, and 0 K is absolute zero at −273.15°C. Kelvin never goes negative, so any reading below 0 K is invalid.
- Why is minus 40 the same in Celsius and Fahrenheit?
- Because −40 × 1.8 = −72 and −72 + 32 = −40, so the scale factor and the offset cancel at exactly that point. It is the only temperature where the two scales read the same number.