Celsius to Fahrenheit, without the brain freeze
A weather app shows 28C and you have no instinct for whether that’s jacket weather or shorts weather. (It’s shorts weather, about 82F.) Temperature is the one conversion people never quite internalize, because unlike weight or distance, you can’t just multiply by a single factor. There’s an offset too.
Here’s the rule. F = C x 9/5 + 32. Multiply Celsius by 1.8, then add 32. Type a Celsius value and the Fahrenheit reading updates instantly. The dropdown flips the direction or swaps in Kelvin.
Why temperature isn’t a simple multiply
Most conversions are pure scaling: a meter is always 3.28 feet, no matter how big the number. Temperature is different because the two scales don’t start from the same zero. Celsius pegs zero to water freezing; Fahrenheit puts water freezing at 32. So you scale and shift. That “+32” is the part people forget, and it’s why “just double it” lands you wildly off at normal temperatures.
The 9/5 (which is 1.8) handles the fact that a Celsius degree is bigger than a Fahrenheit degree. One degree C is 1.8 degrees F worth of change.
Anchor points worth knowing by heart
| Celsius | Fahrenheit | What it is |
|---|---|---|
| -40 C | -40 F | the one place both scales meet |
| 0 C | 32 F | water freezes |
| 20 C | 68 F | comfortable room temperature |
| 25 C | 77 F | warm spring day |
| 37 C | 98.6 F | normal human body temperature |
| 40 C | 104 F | a fever to take seriously |
| 100 C | 212 F | water boils at sea level |
Lock in three of these and you can estimate almost anything. Body temp at 37C equals 98.6F. Room temp at 20C is 68F. Freezing at 0C is 32F. From those, you can feel your way to the rest.
Where you’ll reach for it
Weather and travel are the obvious ones. A forecast in Celsius means nothing to someone raised on Fahrenheit until it’s converted: 30C reads as a sweaty 86F, while 10C is a cool 50F.
Cooking pulls it in constantly. European recipes give oven temperatures in Celsius, and a US oven dial is in Fahrenheit. A “180C oven” is 356F, which most US recipes round to 350F. Get this wrong and your bake runs too hot or too cold.
Then there’s anything health-related. A thermometer bought abroad might read Celsius. Knowing that 38C is 100.4F, the usual fever threshold, tells you whether to worry.
A trick for mental math
Need a fast estimate without the exact formula? Double the Celsius and add 30. So 20C roughly doubles to 40, plus 30, is about 70F (the precise answer is 68). It drifts a little at the extremes, but for everyday weather it’s close enough to decide what to wear. For cooking or fever readings, use the real formula the tool runs.
All calculation happens in your browser. Nothing you enter is sent anywhere, and it works offline once the page loads.
FAQ
What’s the Celsius to Fahrenheit formula?
F = C x 9/5 + 32. Multiply by 1.8, then add 32. So 25C is 77F.
What is normal body temperature in Fahrenheit?
98.6F, which is exactly 37C. A reading of 38C equals 100.4F, the common fever line.
Is there a temperature where both scales read the same?
Yes, -40. Minus 40 Celsius and minus 40 Fahrenheit are the identical temperature.
Why can’t I just multiply by 1.8?
Because the scales have different zero points. You also have to add 32 to account for the offset, or your answer is off by 32 degrees.
Does anything get uploaded?
No. It’s plain in-page math, so your values never get sent anywhere.