Temperature Converter

How to use the temperature converter

1. Type the temperature. Negative numbers are fine, for example -40 or -17.8.
2. Choose the scale you have in From and the scale you want in To. Use ⇄ to flip them.
3. The answer appears at once, with the same temperature in all four scales listed underneath. Change Decimal places if you want a whole number for a recipe or more precision for a lab report.

If you type a value below absolute zero, such as -300 °C, the tool tells you it is not possible instead of printing a meaningless number.

Formulas

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

Celsius and Fahrenheit are not simple multiples of each other, because their zero points differ. A change of 1 °C equals a change of 1.8 °F, but 0 °C equals 32 °F, not 0 °F. That is why the tool treats each scale as a straight line from absolute zero: it converts the input to kelvin, then from kelvin to the target scale. Kelvin and Rankine start at absolute zero, so they have no negative values.

Worked examples

• Body temperature, 37 °C to °F: 37 × 9/5 = 66.6, and 66.6 + 32 = 98.6 °F.
• Oven at 350 °F to °C: 350 − 32 = 318, and 318 × 5/9 = 176.67 °C. Most European ovens would be set to 175 or 180 °C.
• Where the scales meet: −40 °C is exactly −40 °F. Set both formulas equal to each other and that is the only solution.
• Absolute zero: 0 K = −273.15 °C = −459.67 °F = 0 °R. Nothing can be colder.

Everyday reference points

Oven temperatures and gas marks

Gas marks are rounded to the nearest 10 °C when sold in recipes, and fan ovens usually need about 20 °C lower than the figure for a conventional oven. The converter gives the exact figure; a recipe's printed Celsius value is rounded.

Temperature points versus temperature differences

The formulas above convert a reading, such as 20 °C on a thermometer. A difference converts differently, because it ignores the zero points. If a room warms up by 10 °C, it warms up by 10 × 1.8 = 18 °F, not by 50 °F. A temperature difference of 1 kelvin is exactly the same size as 1 °C, and a difference of 1 °R is exactly 1 °F.

This matters in engineering and cooking. A recipe that says "reduce by 25 °F" means 14 °C less, not 25 × 5/9 added to anything. If you use this converter on the two readings separately you will always get the right answer for each. Never use it on a difference, where you only need the multiplication.

Where the scales came from

Daniel Fahrenheit proposed his scale in the early 1700s, setting 0 °F at the freezing point of a brine mixture and 96 °F near body temperature, which was later adjusted so water freezes at 32 °F and boils at 212 °F, 180 degrees apart. Anders Celsius published a scale in 1742 with 100 degrees between the freezing and boiling points of water, originally upside down, and it was reversed to the form used today. Lord Kelvin proposed an absolute scale in 1848 that keeps Celsius-sized degrees but starts at the lowest possible temperature. Since 2019, the kelvin is defined by fixing the value of the Boltzmann constant, not by the triple point of water, though the practical result is the same to many decimal places.

Accuracy and limits

The conversions use the exact definitions of the scales, with 0 °C fixed at 273.15 K. The display is rounded to the number of decimal places you choose, and tiny floating-point differences such as 98.60000001 are removed by rounding. Water boils at 100 °C only at standard sea-level pressure; at 2,000 m it boils near 93 °C, which changes cooking times but not the conversion.

Frequently asked questions