How to Convert Temperature: Celsius, Fahrenheit, and Kelvin
Learn how to convert between Celsius, Fahrenheit, and Kelvin with clear formulas, worked examples, and practical tips for everyday and scientific use.
The Three Major Temperature Scales
There are three temperature scales in widespread use today. Celsius (also called centigrade) is the standard in most of the world and in all scientific contexts. Fahrenheit is used primarily in the United States for weather, cooking, and everyday measurements. Kelvin is the SI base unit for thermodynamic temperature and is used extensively in physics, chemistry, and engineering. Each scale measures the same physical quantity, thermal energy, but uses different reference points and degree sizes. Understanding how they relate to each other is essential for interpreting weather reports, following recipes from other countries, and doing scientific calculations.
Celsius to Fahrenheit
To convert Celsius to Fahrenheit, multiply the Celsius temperature by 9/5 (or 1.8) and then add 32. The formula is F = (C times 9/5) + 32. For example, 25 degrees Celsius becomes (25 times 1.8) + 32 = 45 + 32 = 77 degrees Fahrenheit. This formula works because the Fahrenheit scale is offset by 32 degrees (its freezing point) and uses smaller degrees than Celsius. Each Celsius degree spans 1.8 Fahrenheit degrees. A quick mental shortcut: double the Celsius value and add 30 to get a rough Fahrenheit estimate, which is accurate enough for casual use.
Fahrenheit to Celsius
To convert Fahrenheit to Celsius, subtract 32 from the Fahrenheit temperature and then multiply by 5/9. The formula is C = (F - 32) times 5/9. For example, 98.6 degrees Fahrenheit (normal body temperature) becomes (98.6 - 32) times 5/9 = 66.6 times 0.5556 = 37 degrees Celsius. This is simply the algebraic inverse of the Celsius-to-Fahrenheit formula. A useful mental shortcut is to subtract 30 and divide by 2, which gives a rough approximation. The two scales intersect at exactly -40 degrees, meaning -40 Fahrenheit equals -40 Celsius.
Celsius to Kelvin and Back
Converting between Celsius and Kelvin is the simplest temperature conversion because the degree sizes are identical. To go from Celsius to Kelvin, add 273.15. To go from Kelvin to Celsius, subtract 273.15. For example, 100 degrees Celsius equals 373.15 Kelvin. The Kelvin scale starts at absolute zero, the lowest possible temperature where all molecular motion ceases, which is -273.15 degrees Celsius. There are no negative values on the Kelvin scale. Scientists prefer Kelvin because it provides a true ratio scale: 200 K genuinely has twice the thermal energy of 100 K, a property that neither Celsius nor Fahrenheit can claim.
Fahrenheit to Kelvin
To convert Fahrenheit directly to Kelvin, first convert to Celsius using C = (F - 32) times 5/9, then add 273.15. The combined formula is K = (F - 32) times 5/9 + 273.15. For example, 212 degrees Fahrenheit converts to (212 - 32) times 5/9 + 273.15 = 100 + 273.15 = 373.15 K. While this two-step process may seem cumbersome, it reflects the fact that Kelvin and Celsius share the same degree size, making Celsius the natural bridge. In practice, most people convert Fahrenheit to Celsius first and then shift to Kelvin if needed for scientific work.
Common Reference Points
Memorizing a few key temperatures in all three scales makes estimation much easier. Water freezes at 0 degrees Celsius, 32 degrees Fahrenheit, and 273.15 Kelvin. Water boils at 100 degrees Celsius, 212 degrees Fahrenheit, and 373.15 Kelvin. Normal human body temperature is approximately 37 degrees Celsius or 98.6 degrees Fahrenheit. Room temperature is roughly 20 to 22 degrees Celsius, which is 68 to 72 degrees Fahrenheit. Absolute zero is -273.15 degrees Celsius, -459.67 degrees Fahrenheit, and 0 Kelvin. Having these benchmarks in mind lets you sanity-check any conversion result quickly.
Practical Tips for Everyday Conversions
When cooking with a recipe from another country, pay close attention to oven temperatures. A recipe calling for 180 degrees Celsius corresponds to 356 degrees Fahrenheit, which you would typically round to 350 degrees Fahrenheit on a standard oven dial. For weather, remember that 0 Celsius is freezing and 30 Celsius is hot, while in Fahrenheit those benchmarks are 32 and 86. If you travel internationally, knowing that a comfortable room temperature is about 20 to 22 Celsius helps you set hotel thermostats correctly. For scientific laboratory work, always use Kelvin to avoid errors in thermodynamic equations that require absolute temperature values.
Try These Calculators
Put what you learned into practice with these free calculators.
Related Guides
Metric vs Imperial: A Complete Guide to the Two Measurement Systems
Understand the differences between the metric and imperial measurement systems. Learn their history, how they compare, and when to use each system for length, weight, volume, and temperature.
Understanding Weight Conversions: Grams, Kilograms, Pounds, and Ounces
Master weight conversions between grams, kilograms, pounds, ounces, and stones. Includes formulas, practical examples, and tips for cooking, shipping, and health.
How to Convert Length Units: Inches, Feet, Meters, and More
Learn how to convert between inches, feet, yards, miles, millimeters, centimeters, meters, and kilometers with clear formulas, examples, and practical tips.