How to Convert Celsius to Fahrenheit (Exactly and in Your Head)

To convert Celsius to Fahrenheit, multiply by 9/5 (or 1.8) and add 32: °F = °C × 1.8 + 32. So 25°C × 1.8 = 45, and 45 + 32 = 77°F. For a fast mental estimate, double the Celsius value and add 30: 25°C becomes 50 + 30 = 80°F, close enough to know what to wear.

The exact formulas

The two scales differ in two ways. Their zero points are different: water freezes at 0°C but 32°F. And their degrees are different sizes: water boils 100 Celsius degrees above freezing but 180 Fahrenheit degrees above, so one Celsius degree equals 180/100 = 1.8 Fahrenheit degrees. The formula scales by 1.8 and then shifts by 32.

Order matters in the reverse direction. Subtract 32 before multiplying by 5/9. Doing it the other way around gives the wrong answer.

Converting in your head

Quick estimate: double it and add 30

This gets you in the right neighborhood for weather.

The shortcut is exact at 10°C and drifts by 2°F for every 10°C away from it. It is fine for weather between about −10°C and 30°C and useless for cooking.

Exact mental method: double, minus 10%, plus 32

Multiplying by 1.8 is the same as doubling and then taking away a tenth of the result:

Fahrenheit to Celsius in your head

For an estimate, subtract 30 and halve: 80°F → 50 → 25°C (exact: 26.7°C). For a closer answer, subtract 32, halve, then add about 10% of that: 80 − 32 = 48 → 24 → 24 + 2.4 ≈ 26.4°C.

Landmarks worth memorizing

A handful of anchor points cover most situations:

There is also a well-known rhyme for weather: “30 is hot, 20 is nice, 10 is cold, 0 is ice.”

Oven temperatures

Recipes from outside the US list oven temperatures in Celsius, and converted values are usually rounded to the nearest 25°F setting.

For fan (convection) ovens, recipes often specify about 20°C lower than a conventional oven. The Celsius to Fahrenheit converter gives exact values for any temperature, and the Fahrenheit to Celsius converter goes the other way.

Converting a temperature change

A temperature difference converts without the 32. If the forecast says it will be 10°C warmer tomorrow, that is 10 × 1.8 = 18°F warmer, not 50°F.

The +32 belongs only to points on the scale, because it shifts the zero. Changes, tolerances and rates (such as “the oven heats 5°C per minute”) use the 1.8 factor alone. This is the most common mistake in science and engineering conversions.

Converting in a spreadsheet

Excel and Google Sheets have a built-in conversion function. =CONVERT(25,"C","F") returns 77, and =CONVERT(77,"F","C") returns 25. The same function accepts “K” for kelvin and “Rank” for Rankine. If you prefer a plain formula, =A2*9/5+32 and =(A2-32)*5/9 do the same job and make the arithmetic visible.

Why converted values sometimes look off by one

Thermometers and forecasts round to whole degrees, and a Fahrenheit degree is smaller than a Celsius degree. A forecast of 21°C could be anything from 20.5°C to 21.5°C, which is 68.9°F to 70.7°F. So a converted reading can legitimately show 69°F, 70°F or 71°F. When comparing two thermometers on different scales, allow for a degree or so of rounding before deciding one is wrong.

Kelvin and Rankine

Scientists use the kelvin, the SI base unit of temperature, which starts at absolute zero. A kelvin is the same size as a Celsius degree:

Room temperature of 25°C is 298.15 K. Note that kelvins are written without a degree sign. Engineers working in US units sometimes use the Rankine scale, which starts at absolute zero but uses Fahrenheit-sized degrees: °R = °F + 459.67, so 77°F is 536.67°R.

The boiling points are for standard sea-level pressure; water boils at a lower temperature at altitude. The modern definitions of these scales come from the SI, as maintained in the US by NIST. The Celsius to Kelvin converter and Fahrenheit to Kelvin converter handle the scientific scales, and the all-in-one temperature converter converts between any pair.

A short history

Daniel Gabriel Fahrenheit proposed his scale in 1724, and Anders Celsius proposed a 100-step scale in 1742, originally running in the reverse direction, with 0 for boiling and 100 for freezing. Today nearly every country uses Celsius for weather; the United States and a few others still use Fahrenheit for everyday purposes, which is why these conversions come up so often. Other everyday conversions follow similar shortcuts, such as converting kilograms to pounds.