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Temperature Converter - Celsius, Fahrenheit, Kelvin and Rankine with Formulas

Type a temperature, pick the scale you have and the scale you want, and get the exact answer with the formula in words.

Maintained by Hirantha Sandaruwan Lakshan. Updated 13 October 2026.

About this tool

Celsius, Fahrenheit, Kelvin and Rankine are covered, anything below absolute zero is refused with a clear message, and a table lists the usual reference points: water freezing and boiling, absolute zero, the one number that reads the same in Celsius and Fahrenheit (-40) and the 37 °C body temperature convention.

How it works

Temperature scales are not simple ratios, because their zero points differ: 0 °C is 32 °F. So each scale is written as a slope and an offset against the kelvin, and every conversion uses exact fractions, with whole-number arithmetic and no floating point. That is why 37 °C comes out as exactly 98.6 °F and 98.6 °F returns exactly 37 °C. The answer is rounded only when it is shown.

The formulas are the exact ones in NIST SP 811 and NIST Handbook 44. To go from Celsius to Fahrenheit, multiply by 9/5 and add 32. From Fahrenheit to Celsius, subtract 32 and multiply by 5/9. From Celsius to kelvin, add 273.15. From Fahrenheit to Rankine, add 459.67. NIST states that 0 °C is 273.15 K, so a Celsius degree and a kelvin are the same size. A Fahrenheit degree and a Rankine degree are the same size too, and each is 5/9 of a kelvin.

Absolute zero is 0 K, which is -273.15 °C, -459.67 °F and 0 °R. Nothing can be colder, so the converter refuses a value below it and says why. A temperature reading and a temperature difference also convert differently. A reading is scaled and shifted by the zero offset, but a difference is only scaled: NIST notes that an interval of one Celsius degree is 1.8 Fahrenheit degrees, so a rise of 10 °C is a rise of 18 °F. This tool converts readings.

The figure of 37 °C (98.6 °F) for body temperature goes back to measurements by the German physician Carl Wunderlich in the 19th century. It is a convention and an average, not a rule. Later studies of healthy adults found averages a little lower, about 98.2 °F (36.8 °C) in a widely cited 1992 study, and readings vary with the person, the time of day and where on the body they are taken. The table shows the number for reference only. This page gives no medical advice, so ask a doctor or pharmacist if you are worried about a temperature.

Worked examples

Celsius to Fahrenheit: 25 °C

25 x 9/5 + 32 = 45 + 32 = 77 °F. In kelvin it is 25 + 273.15 = 298.15 K.

Fahrenheit to Celsius: a 350 °F oven

(350 - 32) x 5/9 = 318 x 5/9 = 176.6666667 °C, rounded to 10 significant digits. Recipes usually print 175 °C or 180 °C. Going the other way, 180 °C is exactly 180 x 9/5 + 32 = 356 °F.

Celsius to kelvin: boiling and absolute zero

Water boils at about 100 °C at normal sea-level pressure, which is 373.15 K and 212 °F. Absolute zero, -273.15 °C, is exactly 0 K. Typing -300 °C is refused because it is below absolute zero.

Where Celsius and Fahrenheit meet

Set °C x 9/5 + 32 equal to °C. Then °C x 4/5 = -32, so °C = -40. At -40 the two scales show the same number, and the converter gives -40 °F = -40 °C exactly.

Reference points on all four temperature scales

Reference point°C°FK°R
Absolute zero-273.15-459.6700
The number that is the same in Celsius and Fahrenheit-40-40233.15419.67
Water freezes (normal sea-level pressure)032273.15491.67
Body temperature, the 37 °C convention (an average, not medical advice)3798.6310.15558.27
A common oven setting (recipes round the Fahrenheit figure to 350)180356453.15815.67
Water boils (normal sea-level pressure)100212373.15671.67

Frequently asked questions

What is the formula to convert Celsius to Fahrenheit?

°F = °C x 9/5 + 32, which is the same as °C x 1.8 + 32. For example 20 °C is 20 x 1.8 + 32 = 68 °F. The converter shows the formula for whichever pair of scales you choose.

What is the formula to convert Fahrenheit to Celsius?

°C = (°F - 32) x 5/9. Subtract 32 first, then multiply by 5/9. For example 98.6 °F is (98.6 - 32) x 5/9 = 66.6 x 5/9 = 37 °C.

At what temperature are Celsius and Fahrenheit the same?

At -40. Setting °C x 9/5 + 32 equal to °C gives °C x 4/5 = -32, so °C = -40. It is the only temperature where the two scales read the same number.

What is absolute zero?

Absolute zero is 0 K, the lowest temperature possible. It equals -273.15 °C, -459.67 °F and 0 °R. The converter refuses any value below it, because no real temperature can be colder.

Is 37 °C really the normal body temperature?

It is a convention and an average, not a fixed number. The 37 °C (98.6 °F) figure comes from 19th century measurements, and healthy people differ with age, time of day and where the temperature is taken. This page is for converting numbers and gives no medical advice. If you or someone you care for feels unwell, speak to a doctor or pharmacist.

At what temperature does water freeze and boil?

At normal sea-level pressure water freezes at 0 °C (32 °F) and boils at about 100 °C (212 °F). NIST gives the same two figures for the Celsius scale. Where the air pressure is lower, such as on high ground, water boils at a lower temperature.

Why is 180 °C written as 350 °F in recipes?

180 °C is exactly 356 °F. Recipe writers round oven settings to convenient numbers, so the Fahrenheit figure is usually printed as 350. Ovens are not that precise, but check the recipe if it matters.

Does a rise of 10 °C equal a rise of 10 °F?

No. A change of 1 °C is a change of 1.8 °F, so a rise of 10 °C is a rise of 18 °F. The offset of 32 only applies to a temperature reading, not to the size of a change.

Sources & references

Updated Oct 13, 2026