Temperature Converter

Temperature is the one unit where the math isn't just a multiplication. Each scale has a different zero point, so there's both an offset and a scale factor — which is why nobody remembers the conversion formula and why '70 degrees' means room temperature to an American and a mild fever to a European. This calculator handles the math; the context below handles the confusion.

Fahrenheit (1724) fixed zero at the temperature of a brine solution (cold enough for winter Danzig) and 96°F at human body temperature. Celsius (1742) originally had zero at water's boiling point and 100 at freezing — the inverse of the modern scale — because Celsius was thinking about barometric pressure calibration, not weather. The scale was flipped after his death. Kelvin (1848) starts at absolute zero, the physical floor below which no more heat can be extracted. Rankine is Kelvin with Fahrenheit-sized degrees and appears only in US thermodynamics textbooks.

Human body temperature is not 98.6°F. That number comes from an 1868 German study by Carl Reinhold August Wunderlich, who measured 25,000 patients with an axillary (armpit) thermometer. Modern studies of oral and tympanic measurements find normal body temperature is closer to 97.9°F / 36.6°C, and averages have been drifting downward since the 1800s (possibly because of reduced chronic inflammation from modern medicine). If your thermometer reads 98.0°F, you are not cold-blooded — the reference is wrong.

°C to °F: multiply by 9/5 and add 32, or shortcut 'double and add 30' (works up to about 150°C, drifts above). °F to °C: subtract 32, divide by 9/5. Kelvin: °C + 273.15. Rankine: °F + 459.67. The point where Celsius and Fahrenheit read the same is -40°C = -40°F — a useful test case to remember.

For cooking: oven temps, 175°C = 350°F (moderate), 200°C = 400°F (hot), 230°C = 450°F (very hot). Internal meat temps: 63°C = 145°F (medium beef, pork, fish), 71°C = 160°F (ground meat), 74°C = 165°F (poultry instant-kill). For weather: 20°C = 68°F is comfortable indoor; 30°C = 86°F is hot summer; 40°C = 104°F is dangerous heat.

• Celsius
• Fahrenheit
• Kelvin
• Rankine

• Treating Kelvin as 'degrees Kelvin.' It's just 'kelvin' — no degree sign, lowercase. '300 K,' not '300°K.'
• Converting a difference as if it were a temperature. 'Warmed by 10°C' converts to 'warmed by 18°F' (no offset — differences use only the scale factor). Somebody who says '10°C warmer = 50°F warmer' is doing the conversion wrong.
• Wind chill and heat index as 'real' temperatures. They're indices, not thermodynamic temperatures. Wind chill of -30 is not the same physical condition as an air temperature of -30; it's a heat-loss-equivalent.
• Approximating °C × 2 for °F. The shortcut 'double and add 30' is not precise. At 40°C it gives 110°F, real answer 104°F. Fine for weather; not fine for the oven.
• Ignoring the Celsius/centigrade distinction. 'Centigrade' was renamed to 'Celsius' in 1948 to avoid confusion with French gradians. They're the same scale; centigrade is just the older name.

• Water freezes: 0°C / 32°F / 273.15 K.
• Water boils at sea level: 100°C / 212°F / 373.15 K.
• Room temperature (comfortable): 20°C / 68°F.
• Body temperature (modern normal): 36.6°C / 97.9°F.
• Fever territory: above 38°C / 100.4°F.
• Moderate oven: 175°C / 350°F.
• Hot summer day: 30°C / 86°F.
• Arctic winter: -40°C / -40°F (the Celsius-Fahrenheit crossing point).
• Absolute zero: -273.15°C / -459.67°F / 0 K.
• Sun's surface: 5,500°C / 9,932°F.

• Memorize two pairs: 0°C = 32°F and 100°C = 212°F. Every other common temperature interpolates between them.
• For weather forecasts abroad, '10s' in °C is coat weather, '20s' is spring, '30s' is summer, '40s' is dangerous. '10s' in °F is freezing, '30s' is cool, '70s' is perfect, '90s' is hot.
• Cooking thermometers should read within 1-2°C of a reference. Check yours in boiling water (should read 100°C at sea level) and ice water (should read 0°C). If they're off, calibrate or replace.

Related: Specific Heat Converter · Thermal Conductivity Converter · Energy Converter.