How to convert temperatures

Temperature conversion is one of those skills that feels trivial until you need to do it correctly in your head at a grocery store in Europe, or your oven is calibrated in one scale and the recipe in another, or a lab protocol specifies Kelvin and your instrument reads Celsius. Four scales show up in practice—Celsius, Fahrenheit, Kelvin, Rankine—and each has a reason to exist. Kelvin and Celsius share a step size but different zeros. Fahrenheit has a smaller step and a weirder zero point. Rankine is just Kelvin expressed with Fahrenheit steps. Mixing them up in engineering or medical contexts can kill people, so it pays to know the formulas cold and to carry a mental sanity check for the answer. This guide covers the four scales, the conversion formulas, absolute zero, human body temperature as a calibration point, and when each scale is the professional standard.

The four scales

Celsius (°C) sets water’s freezing point at 0 and boiling at 100 at standard atmospheric pressure. Fahrenheit (°F) sets them at 32 and 212. Kelvin (K, no degree sign) shares Celsius’s step size but starts at absolute zero: 0 K = −273.15 °C. Rankine(°R) shares Fahrenheit’s step size and starts at absolute zero: 0 °R = −459.67 °F.

                Freeze  Boil   Absolute zero
Celsius      C  0       100    −273.15
Fahrenheit   F  32      212    −459.67
Kelvin       K  273.15  373.15 0
Rankine      R  491.67  671.67 0

Celsius to Fahrenheit and back

F = C × 9/5 + 32. Equivalently, F = C × 1.8 + 32. Going the other way: C = (F − 32) × 5/9. The subtraction happens first—that’s the common arithmetic trap.

20 °C → 20 × 1.8 + 32 = 68 °F
0 °C → 32 °F
100 °C → 212 °F
37 °C (body temp) → 98.6 °F

68 °F → (68 − 32) × 5/9 = 20 °C
32 °F → 0 °C
−40 °F → −40 °C  (the scales cross here)

Mental shortcuts

To estimate C from F quickly: subtract 30, divide by 2. “86 °F” → 56 / 2 = 28 °C (actual: 30). Accurate enough for weather conversation. For F from C, double and add 30: 20 °C → 40 + 30 = 70 °F (actual: 68). These shortcuts lose accuracy at extremes but work for the range humans normally encounter.

Celsius and Kelvin

K = C + 273.15. The 0.15 matters for precision work; a rounded 273 is fine for weather but wrong for thermodynamics homework. Kelvin differences and Celsius differences are identical—a change of 10 K equals a change of 10 °C. This is why temperature coefficients in physics are always in Kelvin even when the starting temperature was in Celsius.

Fahrenheit and Rankine

R = F + 459.67. Rankine is used in some US thermodynamics and aerospace contexts because it preserves Fahrenheit steps while giving an absolute scale. Outside those fields, you’ll rarely encounter it.

Absolute zero

0 K = 0 °R = −273.15 °C = −459.67 °F. This is the temperature at which classical molecular motion stops. You cannot reach it (third law of thermodynamics); you can only asymptotically approach it. The coldest temperatures ever achieved in labs are a few billionths of a Kelvin above zero. Negative Kelvin or Rankine values are nonsensical in the normal sense.

Human body as a sanity check

Normal body temperature is ~37 °C = 98.6 °F = 310.15 K. A fever is 38 °C (100.4 °F) and above. Hypothermia begins below 35 °C (95 °F). If your conversion produces a body temperature of 200 or 30, you slipped a digit. Also useful: room temperature is ~20–22 °C = 68–72 °F. An ice bath is 0 °C. Boiling water is 100 °C. Keep these in mind as guardrails.

Cooking temperatures

Oven recipes collide with scale differences. Key conversions:

350 °F = 177 °C   (classic cookies, casseroles)
400 °F = 204 °C   (roasting vegetables)
425 °F = 218 °C   (pizza, browning)
450 °F = 232 °C   (high-heat roasting)
500 °F = 260 °C   (flatbreads, some pizza stones)
180 °C = 356 °F   (gentle baking)
200 °C = 392 °F
220 °C = 428 °F

Most ovens round to 5 °C or 25 °F increments, so a recipe’s exact value matters less than the range. A recipe calling for 177 °C will do fine at 175 °C or 180 °C.

Scientific conventions

Physics and chemistry default to Kelvin because thermodynamic equations require an absolute scale. Ideal gas law PV = nRT only works with Kelvin—plug in Celsius and you get wrong answers. Climate science uses Celsius because the deltas are small and Kelvin steps are identical. Biology and medicine use Celsius almost everywhere except the US consumer market.

When each scale is the standard

Celsius: most of the world, weather, cooking outside the US, biology, meteorology. Fahrenheit: US consumer weather, US home cooking, US medical charts. Kelvin: physics, chemistry, engineering thermodynamics, astronomy. Rankine: some US aerospace and HVAC calculations.

Precision and rounding

For weather reports, one decimal is usually enough. For body temperature, tenths of a degree matter clinically. For scientific work, use the full formula and carry as many significant figures as your input. When reporting, round to match the precision of your measurement—saying 98.6001 °F when your thermometer reads to 0.1 is fake precision.

Common mistakes

Doing the arithmetic in the wrong order. F = C × 9/5 + 32means multiply first, then add. If you add 32 first, you get a wrong answer by a multiplicative factor.

Using the offset for deltas. A temperature difference of 10 °C is 18 °F (just multiply by 9/5), not 50 °F. Offsets only apply to absolute temperatures, not differences.

Rounding 273.15 to 273 in precision work. The 0.15 offset matters for physics and thermodynamics. Only drop it in casual weather conversion.

Forgetting Kelvin omits the degree sign. It’s “300 K,” not “300 °K.” Celsius and Fahrenheit keep the degree sign, Kelvin does not. Small formatting, big credibility cost.

Running ideal-gas calculations in Celsius. Gas laws require absolute temperature. Using Celsius gives wildly wrong answers, especially near 0 °C where the error is largest relative to the value.

Misreading thermometer units. A US thermometer showing 37 is probably Celsius (it’s a medical thermometer); a weather app showing 37 in the UK is unusual; in the US it’s probably Fahrenheit. Always check the unit before trusting a number.

Assuming “negative” means dangerously cold in Fahrenheit.Zero °F (−18 °C) is cold, not catastrophic. −40 in either scale is seriously cold. Context matters.

Run the numbers

Stop doing the 9/5 multiplication under pressure; paste your value into our temperature converter and get all four scales at once. Pair it with the unit converter for the full set of measurement conversions, and the cooking converter when the temperature conversion is paired with volume or weight in a recipe.