Temperature Converter (Celsius, Fahrenheit, Kelvin)
Convert between Celsius, Fahrenheit and Kelvin, with the exact formula shown for every conversion.
This tool runs entirely in your browser. Nothing you enter is uploaded to a server.
Convert between Celsius, Fahrenheit and Kelvin instantly, in either direction. The exact formula used is shown beneath the result, and values below absolute zero are rejected rather than silently converted. Everything is calculated in your browser.
How to use it
- Enter a temperatureType into either box. Conversion works both ways, so start from whichever figure you already have.
- Choose the scalesPick from the two dropdowns, or press Swap scales to reverse the direction.
- Check the formulaThe line beneath the result shows the formula applied, for example °F = °C × 9/5 + 32.
The formulas
- Celsius to Fahrenheit: °F = °C × 9/5 + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5/9
- Celsius to Kelvin: K = °C + 273.15
- Kelvin to Celsius: °C = K − 273.15
Each conversion routes through Celsius internally, which is why there are four formulas rather than six. The tool shows whichever one applies to the pair you have selected, so you can check the arithmetic yourself rather than trusting the output.
Why temperature is different from other conversions
Most unit conversions are a single multiplication. A metre is always 3.28 feet, whether you are measuring a pencil or a runway, because both scales start from the same zero. Temperature does not work that way. Zero degrees Celsius and zero degrees Fahrenheit describe completely different conditions, so converting between them needs an offset as well as a scale factor.
This has a practical consequence people get wrong regularly: a temperature differenceconverts differently from a temperature. A rise of 10 °C is a rise of 18 °F, not 50 °F — for a difference you apply the 9/5 factor but not the +32 offset. If you are converting a change rather than a reading, use the factor alone.
Which scale is used where
Celsius is standard in almost every country and throughout science. Fahrenheit remains in everyday use in the United States and a handful of territories. Kelvin is the SI base unit and is used wherever absolute temperature matters — thermodynamics, cryogenics, astronomy, and the colour temperature of light sources.
Kelvin has the same degree size as Celsius, just shifted so its zero sits at absolute zero. That is why converting between them is a simple addition, and why a 1 K change and a 1 °C change are exactly the same amount of warming.
Limitations
Results are rounded to six decimal places for display. Because 5/9 is a repeating decimal, some conversions cannot be written exactly in either direction — 100 °F is 37.777778 °C, not a tidy figure. The underlying calculation keeps full precision.
Historical scales such as Rankine, Réaumur and Delisle are not included. If you need one, it is worth telling us, since it is a short addition to the same formula table.
Frequently asked questions
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 and add 32. So 20 °C × 1.8 = 36, plus 32 = 68 °F. Going the other way, subtract 32 first and then multiply by 5/9.
Why can't I just multiply, like with metres and feet?
Because Celsius and Fahrenheit have different zero points as well as different degree sizes. Length scales share a zero — nothing is zero metres and also five feet — so one multiplication is enough. Temperature scales do not, so every conversion needs both a scale factor and an offset. That is also why this tool has its own logic rather than sharing the one used by our other converters.
What is absolute zero?
The lowest temperature physically possible: −273.15 °C, −459.67 °F, or 0 K. Nothing can be colder, so if you enter a value below it this tool tells you rather than returning a number that looks valid but describes nothing real.
Why is there no degree symbol on Kelvin?
Kelvin is an absolute scale, not a scale of degrees relative to an arbitrary zero, so by SI convention it is written 300 K and not 300 °K. The symbol was officially dropped in 1967.
At what temperature do Celsius and Fahrenheit agree?
At −40. Minus forty degrees Celsius is exactly minus forty degrees Fahrenheit — the one point where the two scales cross.
What are the everyday reference points?
Water freezes at 0 °C / 32 °F / 273.15 K and boils at sea level at 100 °C / 212 °F / 373.15 K. Normal human body temperature is about 37 °C / 98.6 °F, and comfortable room temperature is around 20–22 °C / 68–72 °F.
Is anything sent to a server?
No. The formulas ship inside the page and run on your device, so the tool keeps working with no connection.