What it does
This converter changes a temperature between Celsius and Fahrenheit, in either direction. Type a number and the answer appears as you type; the swap button flips which scale you are converting from. Nothing is sent to a server — the arithmetic runs in your browser.
How it works
The two scales disagree about two things at once: where zero sits and how big one degree is. On the Celsius scale, water freezes at 0 and boils at 100, so there are 100 degrees between the two fixed points. Fahrenheit puts freezing at 32 and boiling at 212 — a gap of 180 degrees for the very same span. That makes one Celsius degree exactly 1.8 Fahrenheit degrees, and it means you cannot simply add a number: you have to rescale and shift.
Going from Celsius to Fahrenheit you first multiply by 9/5 (that is the same 1.8, written as a fraction) to stretch the degrees to the right size, then add 32 to line the zeros up. To reverse it you undo those steps in the opposite order: subtract 32 first, then divide by 9/5. Because the operations are exact inverses, converting a value and converting it back returns the number you started with.
Worked example
Take 100 °C, the boiling point of water at sea level. Multiplying by 9/5 gives 100 × 9/5 = 180, and adding the offset gives 180 + 32 = 212. So 100 °C is exactly 212 °F — the two numbers you will find printed on either side of a kitchen thermometer.
Common uses
- Weather: UK and European forecasts are given in Celsius, while US forecasts and many American websites use Fahrenheit — so a “90-degree day” abroad is a warm 32 °C, not a lethal one.
- Cooking: older British recipes and most American ones quote oven temperatures in Fahrenheit, whereas modern UK ovens are marked in Celsius.
- Health: a thermometer bought in one country may read in the other scale. Body temperature is traditionally quoted as 37 °C (98.6 °F), though the NHS notes normal varies from person to person and across the day.
- Travel and study: reading a room thermostat, a swimming-pool sign or a science worksheet that uses the unfamiliar scale.
Frequently asked
Is there a quick way to estimate in my head?
For a rough figure, double the Celsius value and add 30: 20 °C becomes about 70 °F (the exact answer is 68). It is close enough for weather, but for anything precise use the full × 9/5 + 32.
Why add 32 rather than start both scales at zero?
The scales were defined independently, long before they were reconciled, and their zero points were never meant to match. Fahrenheit’s zero sits well below water’s freezing point, so freezing lands at 32 rather than 0 — and that 32 has to be built into every conversion.
Is there a temperature that reads the same on both scales?
Yes: −40. At minus forty the two formulas cross, so −40 °C and −40 °F are the same temperature. It is the one value you can quote without saying which scale you mean.