What is the formula to convert Celsius to Fahrenheit?

Multiply the Celsius value by 9/5 (i.e. 1.8) then add 32. In symbols: °F = (°C × 9/5) + 32. For example, 20 °C = (20 × 1.8) + 32 = 36 + 32 = 68 °F.

How do I convert Fahrenheit back to Celsius?

Subtract 32 from the Fahrenheit value, then multiply by 5/9. Formula: °C = (°F − 32) × 5/9. For example, 98.6 °F = (98.6 − 32) × 5/9 = 66.6 × 0.5556 = 37 °C — normal body temperature.

How do you convert Celsius to Kelvin?

Simply add 273.15. The Kelvin scale has the same degree size as Celsius but starts at absolute zero, so 0 °C = 273.15 K, 100 °C = 373.15 K, and −273.15 °C = 0 K.

What temperature is the same in both Celsius and Fahrenheit?

Setting °F = °C in the formula gives (x × 9/5) + 32 = x, which solves to x = −40. So −40 °C equals exactly −40 °F — the only crossover point on the two scales.

What is normal human body temperature in Fahrenheit?

37 °C is the standard clinical normal (oral), which converts to exactly 98.6 °F using the formula 37 × 1.8 + 32. Bear in mind individual body temperature varies by roughly ±0.5 °C around that figure.

What is the Rankine scale and when is it used?

Rankine (°Ra) is an absolute temperature scale used mainly in US engineering thermodynamics. It has the same degree size as Fahrenheit and starts at absolute zero: 0 K = 0 °Ra, 273.15 K = 491.67 °Ra, 373.15 K = 671.67 °Ra.

Celsius to Fahrenheit Converter

Temperature scales can trip up even experienced professionals — a recipe in Celsius looks wrong when your oven dial shows only Fahrenheit, a physics problem in Kelvin needs converting for a lab thermometer, and engineering specs in Rankine confuse everyone who grew up with metric. This converter handles all five scales simultaneously so you type once and read all the answers at once.

How the conversion works

All five scales are linked through Celsius as the pivot:

Celsius → Fahrenheit: multiply by 9/5, then add 32. °F = (°C × 9/5) + 32
Fahrenheit → Celsius: subtract 32, then multiply by 5/9. °C = (°F − 32) × 5/9
Celsius → Kelvin: add 273.15 (exact by SI definition). K = °C + 273.15
Celsius → Rankine: add 273.15 to get Kelvin, then multiply by 9/5. °Ra = (°C + 273.15) × 9/5
Celsius → Réaumur: multiply by 4/5. °Ré = °C × 4/5

When you type into any field the tool first converts that value to Celsius using the inverse of the relevant formula, then projects outward to the remaining four scales in a single calculation pass. This means no rounding accumulates: the output is always one-step from the source value.

Worked example

Suppose you want to know what 180 °C (a moderate oven) is in Fahrenheit, Kelvin, Rankine and Réaumur.

Celsius to Fahrenheit: (180 × 9/5) + 32 = 324 + 32 = 356 °F
Celsius to Kelvin: 180 + 273.15 = 453.15 K
Celsius to Rankine: (180 + 273.15) × 9/5 = 453.15 × 1.8 = 815.67 °Ra
Celsius to Réaumur: 180 × 4/5 = 144 °Ré

You can verify each result by typing 180 into the Celsius field and checking the other boxes.

Why so many scales?

Fahrenheit was the dominant scale in English-speaking countries before metrication and remains the everyday standard in the United States. Kelvin is the SI base unit of thermodynamic temperature and is used throughout science because it starts at absolute zero, making equations involving energy proportional. Rankine fills the same role in the US customary engineering system — thermodynamic calculations but in Fahrenheit-sized degrees. Réaumur is largely historical (18th-century Europe) but still appears in some food and cheesemaking literature.

Formula note

The 9/5 factor between Fahrenheit and Celsius degree sizes arises because Anders Celsius defined his scale with 100 degrees between the freezing and boiling points of water, while Daniel Fahrenheit defined his with 180 degrees over the same interval (32 °F to 212 °F). The ratio 180/100 = 9/5 = 1.8 is exact. The offset of 32 is simply because Fahrenheit placed the freezing point at 32 rather than 0.