Celsius to Fahrenheit Formula
The two formulas, why the 32 exists, and the mental-math shortcuts for everyday temperatures.
To convert Celsius to Fahrenheit: multiply by 9/5 and add 32 (F = C x 9/5 + 32). To go back: subtract 32 and multiply by 5/9 (C = (F - 32) x 5/9). Room temperature, 20 C, is 68 F; body temperature, 37 C, is 98.6 F; water freezes at 0 C (32 F) and boils at 100 C (212 F).
The formulas
Celsius to Fahrenheit: F = C x 9/5 + 32. Multiply the Celsius temperature by 9/5 (1.8) and add 32. Fahrenheit to Celsius: C = (F - 32) x 5/9. Subtract 32 first, then multiply by 5/9 (about 0.556). Order matters: adding before multiplying gives a wrong answer.
Why these numbers? The two scales agree that water's freezing and boiling points are 180 Fahrenheit degrees apart (32 to 212) but 100 Celsius degrees apart (0 to 100). The ratio 180/100 simplifies to 9/5, and the +32 aligns the different zero points.
Landmark temperatures
Memorize these anchors and interpolate between them: -40 is where both scales meet (-40 C = -40 F). 0 C = 32 F (freezing). 10 C = 50 F (cool day). 20 C = 68 F (room temperature). 30 C = 86 F (hot day). 37 C = 98.6 F (body temperature). 100 C = 212 F (boiling).
Oven temperatures: 180 C is 356 F (usually rounded to 350), 200 C is 392 F (rounded to 400), 220 C is 428 F (rounded to 425). Recipe conversions round to the nearest 25 degrees because ovens are not that precise anyway.
Mental-math shortcuts
The double-and-add-30 trick: double the Celsius, subtract 10 percent of that, add 32. For 20 C: double to 40, subtract 4, add 32 = 68 F. Faster rough version: double Celsius and add 30 (20 C -> 70 F, close enough for weather).
Going the other way: subtract 30 from Fahrenheit and halve it. 86 F -> 56 / 2 = 28 C (actual 30 C). Good enough for deciding on a jacket; use the exact formula for cooking, science, or fevers.
Why the US still uses Fahrenheit
Fahrenheit's finer degree size (180 divisions versus 100 between freezing and boiling) gives more granular weather reporting without decimals, which suited 18th-century thermometers. The US kept it through industrial-era inertia while science and the rest of the world standardized on Celsius.
For weather, each scale's defenders have a point: 0-100 F covers most human weather experience in whole numbers, while Celsius ties neatly to water's phase changes. Know both and you will never be confused by a forecast again.
Skip the arithmetic
Convert any temperature exactly with the free unit converter.
Temperature conversion questions
What is the formula to convert Celsius to Fahrenheit?
Multiply the Celsius temperature by 9/5 (1.8) and add 32: F = C x 9/5 + 32. For example, 20 degrees Celsius: 20 x 1.8 = 36, plus 32 = 68 Fahrenheit.
What is 98.6 F in Celsius?
37 degrees Celsius. Subtract 32 to get 66.6, multiply by 5/9 to get 37. That is the classic normal human body temperature.
At what temperature are Celsius and Fahrenheit equal?
At -40 degrees, the one temperature where both scales agree: -40 C equals -40 F. Solve C = C x 9/5 + 32 and you get C = -40.