The formula
How to convert Celsius to Fahrenheit
To convert Celsius to Fahrenheit, multiply the Celsius value by 1.8 (the same as nine-fifths) and add 32. The multiplication accounts for Fahrenheit's smaller degree — 180 Fahrenheit degrees span the same range as 100 Celsius degrees — and the 32 accounts for the two scales starting at different points.
A quick mental shortcut
A quick mental shortcut for rough conversions: double the Celsius figure and add 30. It overstates slightly at higher temperatures and understates slightly at lower ones, but it lands within a couple of degrees across most everyday weather, which is close enough to decide what to wear without doing exact arithmetic.
Worked example
At the default of 20 °C: 20 × 1.8 + 32 = 68 °F.
Reading the result
A handful of fixed points are worth memorising: 0 °C is 32 °F (freezing), 10 °C is 50 °F (cool), 20 °C is 68 °F (mild room temperature), 30 °C is 86 °F (hot), and 37 °C is 98.6 °F (body temperature). A common slip is forgetting the +32 and just multiplying by 1.8 — that gives the size of the temperature *change* in Fahrenheit degrees, not the converted reading itself. A 10 °C rise is an 18 °F rise, but 10 °C itself is 50 °F, not 18 °F.
0 °C is exactly 32 °F — the freezing point of water on both scales, and the anchor the rest of the Fahrenheit scale was built from.
Normal human body temperature, usually quoted as 37 °C, converts to 98.6 °F. Individual normal readings vary by roughly half a degree Celsius either way, so a reading close to but not exactly 98.6 °F is not automatically a fever.