Temperature Converter

How to use the Temperature Converter

1. Type a temperature. Negative values are fine, for example −18.
2. Choose the scale you are converting from and to: Celsius, Fahrenheit, Kelvin or Rankine.
3. Read the converted value and the formula that produced it.
4. Use the reference table for oven, body and weather temperatures, and click a row to load it.

Features

• Exact formulas, not rounded multipliers
• The formula is shown with every result
• Celsius, Fahrenheit, Kelvin and Rankine
• Warns you if a value is below absolute zero
• Reference points for cooking, health and weather
• All four scales shown at once, click to copy

Why temperature needs formulas

Most units convert by multiplying: a meter is always 3.28 feet. Temperature scales are different because they start counting from different zero points. Zero degrees Celsius is the freezing point of water, zero Fahrenheit is far colder, and zero kelvin is absolute zero, the point where nothing can get colder. Converting therefore needs a scale factor and an offset, which is why “multiply by 1.8” on its own gives the wrong answer.

A short history of the four scales

Fahrenheit was proposed by the German-Dutch instrument maker Daniel Gabriel Fahrenheit in 1724. On his scale water freezes at 32° and boils at 212°, 180 degrees apart. The United States and a handful of other countries still use it for weather and cooking.

Celsius comes from the Swedish astronomer Anders Celsius, whose 1742 scale originally ran backward, with 0 for boiling and 100 for freezing. It was flipped soon after his death and is now used for everyday temperatures in nearly every country.

Kelvin is named after William Thomson, Lord Kelvin, who argued for an absolute scale in 1848. It uses Celsius-sized steps but starts at absolute zero, so 0 K = −273.15 °C. The kelvin is the SI base unit of temperature and is written without a degree sign. Rankine does the same for Fahrenheit-sized steps and still appears in some American engineering thermodynamics tables.

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
• Fahrenheit to Rankine: °R = °F + 459.67

Worked examples

A warm day, 25 °C: 25 × 9/5 = 45, plus 32 = 77 °F.

An American oven at 350 °F: 350 − 32 = 318, times 5/9 = 176.7 °C. European recipes round this to 180 °C, or about 160 °C for a fan oven.

Liquid nitrogen at 77 K: 77 − 273.15 = −196.15 °C.

Reference temperatures

Mistakes to avoid

The quick rule “double it and add 30” gets you close to the Fahrenheit value for mild weather, but it drifts by several degrees in very hot or cold conditions, so use the exact formula for cooking and medicine. Be careful with temperature differences, too: a rise of 10 °C is a rise of 18 °F, not 50 °F, because the +32 offset only applies to actual readings. Oven settings often change with the oven type as well as the scale; the Cooking Converter helps with the rest of the recipe.

Frequently asked questions