Temperature Converter

Why temperature conversion needs an offset

Length and weight conversions are pure multiplications because both systems start from zero at the same physical point. Temperature scales do not: Celsius puts zero at the freezing point of water, while Fahrenheit puts zero roughly at the freezing point of a brine solution. Because the zero points differ, converting requires shifting as well as scaling.

The formulas are °F = (°C × 9/5) + 32 and °C = (°F − 32) × 5/9. Kelvin shares Celsius's degree size but starts at absolute zero, so K = °C + 273.15 with no scaling at all.

Reference points

These anchors make it easy to sanity-check a conversion.

• Absolute zero: −273.15 °C = −459.67 °F = 0 K
• Water freezes: 0 °C = 32 °F = 273.15 K
• Typical room temperature: 20–22 °C = 68–72 °F
• Normal body temperature: 37 °C = 98.6 °F
• Water boils at sea level: 100 °C = 212 °F = 373.15 K
• The scales cross at −40: −40 °C = −40 °F

Mental conversion shortcuts

Doubling the Celsius value and adding 30 gives a usable Fahrenheit estimate for everyday temperatures: 20 °C becomes 70, against the exact 68. Reversing it — subtract 30, then halve — converts Fahrenheit to Celsius approximately.

The approximation drifts at extremes because it uses 2 instead of 1.8 as the multiplier. For a more accurate mental method, subtract 30 from the Fahrenheit value, halve it, then add back a tenth of the result. But for deciding what to wear, doubling and adding 30 is entirely sufficient.

Absolute zero and why Kelvin has no degrees

Kelvin measures from absolute zero, the temperature at which particles reach their minimum possible motion. Because the scale has a true zero, ratios in kelvin are physically meaningful — 200 K really is twice as hot as 100 K in terms of thermal energy, while 20 °C is not twice as hot as 10 °C.

That is also why kelvin values are written without a degree symbol: 300 K, not 300 °K. Since 2019 the kelvin has been defined by fixing the Boltzmann constant, tying the unit to fundamental physics rather than to the properties of water. Absolute zero has never been reached experimentally, though laboratories have come within billionths of a kelvin.

Oven temperatures

Recipes cross borders more than most measurements, and oven dials disagree: American recipes use Fahrenheit, most others Celsius, and older British recipes give a gas mark. Fan (convection) ovens run hotter, so reduce the Celsius figure by about 20 degrees, or the Fahrenheit by about 25, when converting a conventional-oven recipe.