Celsius, Fahrenheit, and Kelvin: How Temperature Conversion Works
September 18, 2026
°F = (°C × 9/5) + 32. °C = (°F − 32) × 5/9. K = °C + 273.15. Celsius and Fahrenheit differ in both their zero point and their degree size, which is why converting between them needs both multiplication and addition — Kelvin, by contrast, uses the same degree size as Celsius and only shifts the zero point.
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Open the Temperature Converter →Why Celsius to Fahrenheit Needs Two Steps
Celsius sets 0° at water's freezing point and 100° at boiling; Fahrenheit sets 32° at freezing and 212° at boiling — a 100-degree span in Celsius covers a 180-degree span in Fahrenheit, which is where the 9/5 (or 180/100) scaling factor comes from. The +32 then shifts for the different zero point.
Why Kelvin Is Simpler
Kelvin uses the exact same degree size as Celsius — a 1-degree change in Kelvin equals a 1-degree change in Celsius — it just moves the zero point to absolute zero (−273.15°C), the coldest theoretically possible temperature. That's why the Kelvin conversion is just addition, with no multiplication needed.
The One Temperature Where Celsius and Fahrenheit Agree
Celsius and Fahrenheit intersect at exactly −40 degrees — −40°C equals −40°F. This can be proven algebraically by setting °F = °C in the conversion formula and solving, and it's a handy piece of trivia for remembering both scales.
Frequently Asked Questions
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 (or 1.8), then add 32: °F = (°C × 1.8) + 32. For example, 20°C = (20 × 1.8) + 32 = 68°F.
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9: °C = (°F − 32) × 5/9. For example, 98.6°F = (98.6 − 32) × 5/9 = 37°C.
How do I convert Celsius to Kelvin?
Add 273.15: K = °C + 273.15. For example, 25°C = 298.15 K.
At what temperature are Celsius and Fahrenheit equal?
−40 degrees — both scales read exactly −40 at this single point, which can be proven by setting °F equal to °C in the conversion formula and solving.
Why does Kelvin have no negative numbers?
Because Kelvin's zero point is absolute zero — the coldest temperature theoretically possible, where all thermal motion stops — so there's nothing colder to represent with a negative number.