Temperature Converter

This converter moves a temperature between the four scales it supports: Celsius, Fahrenheit, Kelvin and Rankine. Enter a value, choose the scale it is in and the scale you want, and read the result. It handles negative values correctly, which matters more here than with most units.

Temperature is the one quantity on this site where conversion is not simply multiplication. Three of these four scales have different zero points as well as different degree sizes, so the formulas involve an offset. That is why a quick mental estimate goes wrong so easily.

What are the formulas?

All four scales are worth writing out, because guessing at them is where errors creep in:

• °C to °F: (C × 9/5) + 32
• °F to °C: (F − 32) × 5/9
• °C to K: C + 273.15
• °F to °R: F + 459.67

Kelvin and Rankine both start at absolute zero, the point where there is no thermal energy left to remove. That is why they have no negative values and why Kelvin carries no degree symbol — it is simply 293.15 K, not 293.15 °K.

A worked example: an oven temperature

A recipe from an American site says to bake at 350 °F and your oven is marked in Celsius.

1. Subtract 32: 350 − 32 = 318
2. Multiply by 5/9: 318 × 5 ÷ 9 = 176.7 °C

In practice you would set it to 180 °C, which is the standard gas-mark-4 equivalent. Going the other way, a European recipe at 180 °C is (180 × 9/5) + 32 = 356 °F.

And a curiosity worth knowing: −40 °C is exactly −40 °F. It is the one point where the two scales cross, which makes it a useful sanity check on any conversion routine.

Why does the offset matter so much?

Because it breaks the intuition that conversion is scaling. If you double a length in metres, the figure in feet doubles too. Temperature does not behave that way: 20 °C is 68 °F, but 40 °C is 104 °F, not 136.

The same trap catches temperature differences. A rise of 10 °C is a rise of 18 °F, not 50 °F — for a difference you apply only the 9/5 ratio and drop the offset entirely. If you are converting a tolerance, a thermal gradient or a "plus or minus" figure rather than an absolute reading, that distinction is the whole answer.

How to use the converter

1. Enter the temperature you have, including a minus sign if it is below zero.
2. Pick the scale it is measured in.
3. Pick the scale you want it in.
4. Read the converted value. It recalculates as you type.

Where each scale is used

Celsius, introduced by the Swedish astronomer Anders Celsius, is the everyday scale almost everywhere outside the United States and the scientific standard for ordinary work. Its reference points — 0 for the freezing point of water, 100 for boiling at standard pressure — make it easy to reason about. You will still occasionally see it called centigrade.

Fahrenheit, devised by Daniel Gabriel Fahrenheit, anchored zero to a salt-and-ice brine mixture and the upper end near human body temperature. It remains the domestic standard in the United States, so it turns up in recipes, weather reports and thermostat settings from there.

Kelvin, named for William Thomson, Lord Kelvin, is the SI unit and the one used in physics and chemistry, because gas laws and thermodynamic equations only work on an absolute scale. It also describes the colour temperature of light bulbs — a "2700K warm white" is describing a colour, not a hazard.

Rankine, after the Scottish engineer William Rankine, is absolute temperature with Fahrenheit-sized degrees. It is used in some American engineering and thermodynamics work for the same reason Kelvin is used elsewhere: the formulas need an absolute scale, and the surrounding data is already in Fahrenheit.

Frequently asked questions

What is 100 °F in Celsius?

37.8 °C. Close to human body temperature, which is where the number comes from historically.

Why is it 273.15 and not 273?

Because absolute zero sits at −273.15 °C. Rounding to 273 introduces an error of 0.15 degrees, which is negligible for weather and significant in a laboratory.

Is it correct to write degrees Kelvin?

No. Kelvin is written without the degree symbol and without the word "degrees" — 300 K, read as "three hundred kelvin". Celsius and Fahrenheit keep theirs.

How do I convert a temperature range?

Convert each end separately. Converting the midpoint and then scaling the width will give you the wrong bounds because of the offset.

What is the highest and lowest this tool will take?

There is no practical upper limit. At the bottom, values below absolute zero are physically meaningless — if you enter one, check the scale you selected, because it usually means Kelvin and Celsius have been swapped.

Related tools

• Celsius to Fahrenheit Converter — a dedicated tool with a full reference table.
• Fahrenheit to Celsius Converter — the same, in reverse.
• Pressure Converter — gas calculations need both.
• Power Converter — for heating and cooling loads.

For the formula explained step by step with a printable chart, see our Celsius to Fahrenheit conversion guide.