How to Convert Fahrenheit to Celsius

The formula

To convert Fahrenheit to Celsius, subtract 32 from the Fahrenheit temperature and multiply the result by 5/9. Written as a formula: °C = (°F − 32) × 5/9. Dividing by 1.8 gives the same answer, because 9/5 equals 1.8, so °C = (°F − 32) ÷ 1.8 is an equivalent form.

There are two steps because the scales differ in two ways. Their zero points are not the same: water freezes at 32 °F but at 0 °C. And their degrees are different sizes: between the freezing and boiling points of water at standard atmospheric pressure there are 180 Fahrenheit degrees (32 to 212) but only 100 Celsius degrees (0 to 100). Subtracting 32 aligns the zero points, and multiplying by 100/180, which simplifies to 5/9, rescales the degree.

The reverse conversion is °F = °C × 9/5 + 32. In both directions the order of operations matters: in the Fahrenheit-to-Celsius formula, the subtraction comes first, and the parentheses are not optional.

A worked example, step by step

Take a common oven setting, 350 °F, and follow the formula:

1. Subtract 32: 350 − 32 = 318.
2. Multiply by 5: 318 × 5 = 1,590.
3. Divide by 9: 1,590 ÷ 9 = 176.666…, which rounds to 176.7 °C.

A room temperature works the same way. For 72 °F, 72 − 32 = 40, and 40 × 5 ÷ 9 = 22.2 °C (rounded to one decimal place). For 100 °F, 100 − 32 = 68, and 68 × 5 ÷ 9 = 37.8 °C.

Doing the multiplication before the division, as above, keeps the arithmetic in whole numbers for longer, which makes mistakes less likely when you work by hand.

Reference points worth remembering

A handful of fixed points make it easy to sanity-check any answer you calculate:

• 32 °F = 0 °C: water freezes.
• 212 °F = 100 °C: water boils at standard atmospheric pressure. At higher altitude the boiling point is lower.
• 98.6 °F = 37 °C: the figure commonly quoted for normal human body temperature. Individual readings vary, and health questions belong with a medical professional, not a formula.
• −40 °F = −40 °C: the one temperature where the two scales agree. Check it: (−40 − 32) × 5/9 = −72 × 5/9 = −40.

If your result for a temperature above freezing comes out negative, or a result near boiling is far from 100, you have probably dropped the parentheses or swapped the formula for the reverse one.

Doing it in your head

A well-known shortcut is to subtract 30 and halve the result. For 72 °F that gives 42, then 21 °C, against a true value of 22.2 °C, so it is off by little more than a degree. For 50 °F it gives 10 °C, which is exactly right. The shortcut works because 5/9 is about 0.56, and halving uses 0.5.

The error grows as temperatures rise, because the missing 0.06 is applied to a bigger number. At 350 °F the shortcut gives 160 °C instead of 176.7 °C — too far off for an oven.

A better mental method: subtract 32, halve it, then add one tenth of that half. For 350 °F: 318, half is 159, a tenth of 159 is about 16, so 175 °C. That lands within two degrees of the true 176.7 °C. For 72 °F: 40, half is 20, plus 2 gives 22 °C, very close to 22.2 °C.

Negative temperatures and absolute zero

The formula works unchanged below freezing; just keep the signs. For 0 °F: 0 − 32 = −32, and −32 × 5 ÷ 9 = −17.8 °C. For 20 °F: 20 − 32 = −12, and −12 × 5 ÷ 9 = −6.7 °C. Any Fahrenheit value below 32 gives a negative Celsius value.

There is a lower limit. Absolute zero is −273.15 °C, which is −459.67 °F: (−459.67 − 32) × 5/9 = −273.15. Nothing can be colder, so a value below that is not a real temperature. A unit converter such as this one rejects temperatures below absolute zero for that reason.

Scientists usually work in kelvin, where K = °C + 273.15. The 350 °F example is 176.67 °C, which is about 449.82 K. Note that kelvin is written without a degree sign.

Rounding, and when to check the source

How many decimals you keep depends on the job. Home ovens are set in steps of 5 or 10 degrees, so 176.7 °C becomes 175 °C or 180 °C in practice. Weather is usually quoted in whole degrees. A laboratory protocol or a manufacturing specification may require a stated precision, and rounding too early can shift the result.

For anything where the number matters for safety or compliance — medical equipment, food safety limits, industrial processes — use the temperature given in the original source in its original unit, and convert only if you must, keeping the original value alongside it. A converted figure is a convenience, not a replacement for the instructions it came from.

The temperature tab of a converter also lets you choose the number of decimal places and shows the same value in all three scales at once, which is handy when you want Celsius and kelvin together.