Convert temperature between Celsius, Fahrenheit, and Kelvin in your browser. Exact formulas, an absolute-zero guard, and all three scales at once. Private.
Runs entirely in your browser — your data never leaves your device.
How to use Temperature Converter
What it does & when you need it
Following a recipe written for a different country's oven, reading a science
problem, checking a weather forecast, or debugging a sensor reading — sooner or
later you need to move a temperature between Celsius, Fahrenheit, and Kelvin.
This converter does it with the exact formulas, shows all three scales at once,
and refuses any value that would fall below absolute zero. Everything is computed
in your browser.
How to use
Type a number in the Value field.
Pick the From and To units, or press Swap to flip the direction.
Read the converted value in the result panel; the footer lists the same
temperature in Celsius, Fahrenheit, and Kelvin together.
Press Copy result or Ctrl/Cmd + Enter to copy the number. Press
Sample to load 100 °C for a quick check.
Things worth knowing
The formulas. Celsius to Fahrenheit multiplies by 9/5 and adds 32; the
reverse subtracts 32 and multiplies by 5/9. Kelvin shares the Celsius degree
size but starts at absolute zero, so K = °C + 273.15 — which is why water
freezes at 273.15 K and boils at 373.15 K.
Minus forty is the magic number. The Celsius and Fahrenheit scales cross at
-40, so -40 °C and -40 °F are the exact same temperature. It is the quickest
sanity check that a conversion is correct.
Absolute zero is a hard floor. Nothing can be colder than 0 K (-273.15 °C,
-459.67 °F), so the tool converts your input to Kelvin internally and rejects
anything below that as physically impossible. Results are rounded to six decimal
places to strip floating-point noise like 98.60000001, so a normal body
temperature reads as a clean 37 °C rather than a long decimal tail.
Examples
Boiling water: C to F
100 (C → F)
Water boils at 100 °C, which is 212 °F at standard pressure.
Body temperature: F to C
98.6 (F → C)
The classic 98.6 °F normal body temperature is 37 °C.
Where the scales meet
-40 (C → F)
The one point where Celsius and Fahrenheit agree: -40 °C equals -40 °F.
Frequently asked questions
What are the formulas between Celsius and Fahrenheit?+
To go from Celsius to Fahrenheit, multiply by 9/5 and add 32; to reverse it, subtract 32 and multiply by 5/9. The two scales cross at -40, which is why -40 °C and -40 °F are the same temperature — a handy check that a conversion is working correctly.
Why does it reject values below a certain point?+
Absolute zero, 0 K (-273.15 °C, -459.67 °F), is the coldest temperature physically possible, so any input that would fall below it is rejected as invalid. The tool converts your input to Kelvin internally and refuses to continue if that value is negative.
How does Kelvin relate to Celsius?+
Kelvin uses the same degree size as Celsius but starts at absolute zero instead of the freezing point of water, so K = °C + 273.15. That offset means water freezes at 273.15 K and boils at 373.15 K, and there are no negative Kelvin values.
Why does it show all three scales at once?+
Alongside the single from-to result you pick, the tool also lists the value in Celsius, Fahrenheit, and Kelvin together, so you can sanity-check a reading — a fever, an oven setting, a science-class figure — against every scale without running three separate conversions.
How precise are the results?+
Conversions are computed with exact floating-point arithmetic and then rounded to six decimal places to remove noise like 98.60000001. Everything runs locally in your browser, so nothing you enter is sent to a server.