How to Convert Between Celsius, Fahrenheit and Kelvin

Published 18 October 2026Guide for the Temperature Converter

Temperature is the everyday quantity where the world still uses three scales, and where a wrong conversion has real consequences: a 400-degree oven setpoint is 200 C, not 400 C. The formulas are short; the understanding is in where the constants come from and what each scale is actually for.

This guide explains the three scales, derives the conversion formulas, and gives practical rules of thumb for cooking, weather and lab work.

Three scales, three jobs

Celsius (C) is the everyday metric scale: water freezes at 0 C and boils at 100 C (at standard pressure). It is the scientific and daily scale for most of the world.

Fahrenheit (F) is the everyday scale of the United States. Its constants are historical: 32 F is the freezing point of water and 212 F its boiling point, so the 180-degree span between them is divided into 180 degrees - each F degree is 5/9 the size of a C degree, which is where the factor 1.8 (9/5) in the formula comes from.

Kelvin (K) is the SI thermodynamic scale, starting at absolute zero (0 K = -273.15 C), the temperature at which thermal motion reaches its minimum. Size of a kelvin equals size of a degree Celsius; only the zero points differ. Kelvin is used wherever temperature ratios matter (gases, statistics, radiation) - and it has no negative values.

The formulas, and why they look the way they do

C to F: F = C x 9/5 + 32. The multiplication rescales the degree size; the +32 shifts the zero point to Fahrenheit's. F to C inverts both: C = (F - 32) x 5/9 - subtract the offset first, then rescale.

C to K: K = C + 273.15. K to C: C = K - 273.15. No multiplication - same degree size, different zero.

The order of operations is the whole skill. Converting 212 F to C: subtract 32 first (180), then multiply by 5/9 (100 C). Doing it the other way (multiply, then subtract) gives 84 C - confidently wrong.

Worked example: oven temperature

Recipe: 375 F (US recipe, gas oven off)
C = (375 - 32) x 5/9 = 343 x 0.5556 = 190.6 C  ->  set 190 C
K = 190.6 + 273.15 = 463.7 K

Practical rules by domain

The same formula serves every domain, but the tolerance that matters differs:

Cooking: ovens are approximate equipment - a setpoint is good to maybe 10 F (about 5-6 C). Convert, round to the nearest oven increment, and do not chase the last degree. A recipe that says 180 C fan / 200 C conventional is telling you the same physics twice.
Weather: convert with one decimal at most; people say "about 70" and "about 21". Note the scale you are quoting - 50 without a unit is a communication hazard.
Science and engineering: Kelvin for ratios and gas laws, Celsius for everyday lab and industrial work. Triple-point and absolute-zero references are Kelvin-only.

The mental shortcuts that are actually reliable

Two approximations cover most of life. For C to F: double it and add 30 - 20 C is "70 F" (true: 68), 30 C is "90 F" (true: 86), 40 C is "110 F" (true: 104). Good to within a few degrees for weather and cooking.

The anchor points: 0 C = 32 F (freezing), 100 C = 212 F (boiling), 20 C is room temperature, and -40 is the one temperature where both scales agree. If your converted number lands on the wrong side of an anchor you know (water "boiling" at 200 F?), the order of operations went wrong - go back and re-check, because the error is in the method, not the arithmetic.

Frequently asked questions

Where do the 32 and the 1.8 come from?

The 32 is Fahrenheit's freezing point for water; the 1.8 is 9/5, the ratio of the Celsius span (100) to the Fahrenheit span (180) between freezing and boiling.

Can temperature be negative in Kelvin?

No - 0 K is absolute zero, the floor. You can reach temperatures arbitrarily close but never below it. Negative Celsius and Fahrenheit values are perfectly ordinary; negative kelvin is not.

What is 20 C in Fahrenheit?

68 F - the standard "room temperature" anchor. The shortcut (double and add 30) gives 70, close enough for weather talk.

Why do US recipes use Fahrenheit while everything else uses Celsius?

Historical inertia - Fahrenheit preceded metric adoption in American daily life. The conversion is mechanical; the real skill is knowing that ovens are approximate and rounding to the nearest sensible setpoint is the right call.