How to Convert Celsius to Fahrenheit

This converter handles temperature readings, including negative values down to absolute zero. It does not handle temperature differences with the same formula; the difference is explained below. Calculations run in your browser.

Multiply the Celsius value by 9, divide by 5, then add 32. Multiplying by 1.8 is equivalent to the first two steps. Add 32 after the multiplication, not before it.

For a negative input, keep the sign through the multiplication: −10 × 1.8 = −18. Adding 32 then gives 14°F. Do not remove the minus sign because the output happens to be positive.

These exact conversion formulas are published in NIST’s SI Units – Temperature reference. The diagrams and worked examples here apply those same formulas.

Subtract 32 first, multiply by 5, then divide by 9. The parentheses in (°F − 32) × 5/9 matter: the subtraction applies to the Fahrenheit reading before scaling it.

Keep the full intermediate value until the final step. An exact formula can produce a repeating decimal, so a rounded display is not an exact fraction. Converting the displayed 21.11°C back gives 69.998°F before rounding; it is not exactly 70°F.

Celsius and Fahrenheit have different degree sizes and different zero points. A change of 1°C corresponds to a change of 1.8°F, which explains the multiplication. A reading of 0°C corresponds to 32°F, which explains the added offset.

Use both parts for a reading. Multiplying 20 by 1.8 gives 36, but 20°C is 68°F after adding 32. Adding 32 alone would also be wrong because it ignores the different degree sizes.

To find where the two scales have the same number, set C = 1.8C + 32. Subtracting 1.8C gives −0.8C = 32, so C = −40. That is why −40°C and −40°F represent the same temperature.

The mental shortcut 2 × °C + 30 is an estimate. It replaces the exact expression 1.8 × °C + 32, so its error is (2C + 30) − (1.8C + 32) = 0.2C − 2°F. The error changes with the input; it is not always two degrees.

For exact mental arithmetic, double the Celsius value, subtract one fifth of the original value, then add 32. At 25°C: 50 − 5 + 32 = 77°F. This is the exact formula rearranged, not another approximation.

The reverse shortcut—subtract 30 from Fahrenheit and divide by 2—is also approximate. For 70°F it gives 20°C, about 1.11°C below the exact result. Use the full formula or converter whenever the difference matters.

For a change or difference, multiply Celsius degrees by 9/5 without adding 32. For a Fahrenheit difference, multiply by 5/9 without subtracting 32. The offset cancels when you subtract two readings.

The calculator above converts readings. To compare a start and end temperature with it, convert each reading separately and subtract the results. Do not type a difference into the reading field and interpret the output as a difference.

Try the calculation before reading the last column. Write the input unit first so you choose the correct formula.

For repeat conversions, keep the original value and unit alongside any rounded result. If you use a desktop calculator or widget, check that it applies the right direction; a convenient display does not remove the need to distinguish readings from differences.