Temperature Converter (Celsius, Fahrenheit, Kelvin)

Why temperature is different from other conversions

Length, weight, and volume all share a natural zero: zero meters is the same amount of nothing as zero feet. Temperature scales do not. Zero degrees Celsius and zero degrees Fahrenheit describe two completely different temperatures, because each scale picked its own reference point.

That means temperature conversion cannot be a simple multiplication. It needs an offset as well, and in the Celsius-to-Fahrenheit case, both an offset and a scale factor:

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

A practical consequence: a difference in temperature converts differently from a temperature itself. A rise of 10 °C is a rise of 18 °F (multiply by 9/5, no offset), but a temperature of 10 °C is 50 °F (multiply and then add 32). Mixing these up is a common source of error in specifications and engineering documents.

The three scales

Celsius sets 0 at the freezing point of water and 100 at its boiling point, at standard atmospheric pressure. This makes it convenient for weather and everyday life, and it is the standard in almost every country.

Fahrenheit places the freezing point at 32 and boiling at 212, giving 180 degrees between them, so each Fahrenheit degree is a smaller step than a Celsius degree. It remains in everyday use in the United States. Its finer granularity is occasionally cited as an advantage for describing weather without decimals.

Kelvin uses the same step size as Celsius but starts at absolute zero, the point where thermal motion is at its minimum. That makes it an absolute scale, which is why it is the SI unit and why physics formulas involving temperature use kelvin rather than Celsius. Note that kelvin is written without a degree symbol: 300 K, not 300 °K.

Reference table of everyday temperatures

Practical examples

Reading a foreign weather forecast

For rough mental conversion from Fahrenheit to Celsius, subtract 30 and halve the result. For 75 °F: (75 − 30) ÷ 2 = 22.5, and the exact answer is 23.9 °C. The shortcut is accurate to within about 2 degrees across normal weather temperatures, enough to decide what to wear.

Going the other way, double the Celsius value and add 30. For 20 °C: 20 × 2 + 30 = 70, exact answer 68 °F.

Oven temperatures in recipes

Recipe temperatures are worth converting precisely, since baking is sensitive to a 10-degree error. Common equivalents: 160 °C = 320 °F, 180 °C = 356 °F (usually written as 350 °F in US recipes), 200 °C = 392 °F (written as 400 °F), 220 °C = 428 °F (written as 425 °F). US recipes round to convenient multiples of 25, so exact conversion is not always what the recipe author intended.

Scientific and engineering work

Gas laws, thermodynamics, and blackbody radiation formulas all require absolute temperature, meaning kelvin. Substituting Celsius into these equations produces nonsense, since the formulas assume that doubling the value doubles the thermal energy, which is only true on an absolute scale. Convert to kelvin first, then calculate.

Body temperature and fever

Normal body temperature is around 37 °C (98.6 °F), though it varies by person, time of day, and measurement site. A fever is usually defined from 38 °C (100.4 °F) upward. Because the Fahrenheit scale has finer steps, small changes look larger in Fahrenheit, one degree Celsius is 1.8 Fahrenheit degrees.

Frequently asked questions

What temperature is the same in Celsius and Fahrenheit?

−40 degrees. It is the single point where the two scales cross, which follows directly from solving °C = °C × 9/5 + 32.

How do I convert Celsius to Fahrenheit?

Multiply by 9/5 (that is, 1.8) and add 32. For 25 °C: 25 × 1.8 = 45, plus 32 gives 77 °F.

Why can Kelvin not be negative?

Kelvin is measured from absolute zero, the theoretical minimum of thermal motion. There is nothing colder, so a negative kelvin value has no physical meaning. This converter rejects such inputs rather than displaying an impossible result.

Is it 300 K or 300 °K?

300 K. The degree symbol was dropped from kelvin in 1967 because kelvin is an absolute unit rather than a scale of degrees relative to a reference point.

Why does a 10-degree temperature difference convert differently from a 10-degree temperature?

Because the offset only applies to absolute temperatures, not to differences. A change of 10 °C equals a change of 18 °F (10 × 1.8). A temperature of 10 °C equals 50 °F (10 × 1.8 + 32). Always check whether a figure is a temperature or a temperature difference.

Which countries use Fahrenheit?

The United States and its territories, plus a small number of others such as the Cayman Islands and Liberia. Essentially everywhere else uses Celsius for daily life and kelvin for scientific work.

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