Temperature is the only common measurement on the converter that does not behave like a single multiplication. Two of the scales in everyday use have offsets, two have different sized degrees, and two start counting from absolute zero rather than from a property of water. That is why "1 degree Fahrenheit" and "1 degree Celsius" are not the same magnitude, and why you cannot convert between them with a single factor.

The Five Scales

Celsius (°C)

Defined so that water freezes at 0 °C and boils at 100 °C at one standard atmosphere. The degree size is one-hundredth of the gap between those two reference points. Used almost everywhere outside the United States for weather, cooking and general daily life.

Fahrenheit (°F)

Water freezes at 32 °F and boils at 212 °F. That makes the gap between freezing and boiling 180 Fahrenheit degrees, so a Fahrenheit degree is 5⁄9 the size of a Celsius degree. The 32-degree offset is the reason the conversion formulas look messy. Used for weather, cooking and body temperature in the United States.

Kelvin (K)

Same degree size as Celsius, but the zero point is absolute zero — the temperature at which classical thermal motion stops. 0 K = −273.15 °C. Kelvin is the SI base unit for temperature and is used in physics, chemistry and engineering wherever absolute temperatures matter. Note: kelvin is written without a degree symbol, and lowercase when used as a unit name.

Rankine (°R or °Ra)

Absolute scale that uses the Fahrenheit degree size. 0 °R is absolute zero, like Kelvin, but each Rankine degree is 5⁄9 of a Kelvin. Encountered mainly in older US engineering texts, particularly in thermodynamics and aerospace.

Réaumur (°Ré)

Sets water at 0 °Ré (freezing) and 80 °Ré (boiling), so the degree is 5⁄4 the size of a Celsius degree. Largely historical — you will meet it in older European recipes, especially for cheese and some traditional confectionery, and in older Russian literature.

The Conversion Formulas

Every formula below assumes you are converting between scalar temperature values, not temperature differences. Differences and offsets behave slightly differently — a "10-degree change" in Celsius is also a 10 K change but an 18 °F change.

• °F to °C: C = (F − 32) × 5⁄9
• °C to °F: F = C × 9⁄5 + 32
• °C to K: K = C + 273.15
• K to °C: C = K − 273.15
• °F to K: K = (F − 32) × 5⁄9 + 273.15
• K to °F: F = (K − 273.15) × 9⁄5 + 32
• °C to °R (Rankine): R = (C + 273.15) × 9⁄5
• °F to °R (Rankine): R = F + 459.67
• °C to °Ré: Ré = C × 4⁄5
• °Ré to °C: C = Ré × 5⁄4

Quick Mental Approximations

For everyday conversions where a degree or two does not matter, these shortcuts are accurate enough:

• Fahrenheit to Celsius, fast: subtract 30, halve. 70 °F → 40 → 20 °C (actual 21.1 °C).
• Celsius to Fahrenheit, fast: double, add 30. 20 °C → 40 → 70 °F (actual 68 °F).
• The two scales meet at −40. −40 °C = −40 °F. A useful sanity check.
• Body temperature reference: 37 °C ≈ 98.6 °F.
• Cool oven reference: 180 °C ≈ 350 °F. A useful baseline when adapting recipes.

Why the Offsets Exist

Celsius was designed for water — a substance every laboratory and every kitchen has — and chose two reproducible reference points. Fahrenheit predates Celsius and was originally calibrated to a brine mixture and human body temperature, neither of which is as easy to reproduce, but it survived in some countries by inertia. Kelvin and Rankine were introduced once thermodynamics required a scale that started at absolute zero so that ratios of temperatures (used in heat-engine equations, gas laws and so on) would behave correctly.

This is also why some equations only work in Kelvin or Rankine. The ideal-gas law PV = nRT, for example, requires T to be on an absolute scale; plugging in Celsius gives nonsense for cold temperatures.

Common Mistakes

• Forgetting the offset. Multiplying 32 °F by 5⁄9 gives 17.78, not 0. The minus-32 step is mandatory.
• Treating temperatures and temperature differences the same way. A "10 °C rise" is the same as a "10 K rise", but it is not a "10 °F rise".
• Using degree symbols on Kelvin. The unit is "K", not "°K".
• Mixing absolute and relative scales in equations. Anything involving ratios of temperatures (efficiency, gas behaviour) needs Kelvin or Rankine.
• Cooking-temperature mismatches. If a recipe says "180" without a unit and the writer is American, they probably mean Fahrenheit, which is barely warm. If they are European, they mean Celsius, which is a moderate oven.

Worked Example: Adapting an Oven Recipe

A recipe says to bake at 425 °F. Convert to Celsius:

• 425 − 32 = 393.
• 393 × 5⁄9 ≈ 218.3 °C.
• Round to the nearest oven dial: 220 °C, or 200 °C in a fan-assisted oven calibrated for 20 °C lower.

The same logic applies in reverse: a recipe written in 200 °C maps to (200 × 9⁄5) + 32 = 392 °F, usually rounded to 400 °F in a US oven.

Where to Go From Here

To run a specific conversion now, use the temperature category on the ConvertMe.org home page. For broader context on the metric system, see the metric vs. imperial guide; for SI base units more generally, the SI base units and prefixes page sets the framework that Kelvin sits inside. If your interest is cooking specifically, the cooking conversions page covers oven temperatures alongside volume-to-weight swaps.