Temperature Conversion: Celsius, Fahrenheit, and Kelvin

Whether you are reading a weather forecast from another country, adjusting an oven for a European recipe, or studying thermodynamics in a physics course, understanding temperature conversion is an essential everyday skill. The three major temperature scales — Celsius, Fahrenheit, and Kelvin — each serve different purposes, and knowing how to move between them quickly saves time and prevents costly errors.

This guide breaks down the science behind each scale, provides all six conversion formulas you will ever need, and includes a quick reference table so you can convert temperatures at a glance.

Understanding the Three Temperature Scales

Each temperature scale was designed with a specific purpose and reference points. Understanding their origins helps you remember why conversions work the way they do.

Celsius (C) was developed by Swedish astronomer Anders Celsius in 1742. It sets 0 degrees at the freezing point of water and 100 degrees at the boiling point of water at standard atmospheric pressure. This clean 0-to-100 range makes it intuitive for everyday weather and cooking measurements, and it is the standard in most countries worldwide.

Fahrenheit (F) was created by Daniel Gabriel Fahrenheit in 1724. It places the freezing point of water at 32 degrees and the boiling point at 212 degrees. While this may seem arbitrary, Fahrenheit designed his scale so that 0 degrees represented the coldest temperature he could reliably produce in his laboratory using a salt-ice-water mixture, and 96 degrees approximated human body temperature. The United States, the Bahamas, the Cayman Islands, and a few other territories still use Fahrenheit for daily life.

Kelvin (K) is the SI unit of temperature used in science and engineering worldwide. Proposed by Lord Kelvin in the 19th century, it begins at absolute zero — the point where all thermal molecular motion stops. Zero Kelvin equals -273.15 degrees Celsius. Importantly, Kelvin uses no degree symbol; you write 300 K, not 300 degrees K. Each Kelvin increment is exactly the same size as one Celsius degree, so converting between them requires only adding or subtracting 273.15.

Temperature Conversion Formulas

There are six possible conversions between the three scales. Here are all the formulas you need:

The key numbers to memorize are 9/5 (or 1.8) for the Celsius-Fahrenheit ratio, 32 for the Fahrenheit offset, and 273.15 for the Celsius-Kelvin offset. With these three values, you can derive any conversion.

Step-by-Step Conversion Examples

Working through calculations manually reinforces the formulas. Here are detailed step-by-step conversions for each direction.

Convert 25 degrees Celsius to Fahrenheit: Multiply 25 by 9/5 to get 45, then add 32. The result is 77 degrees Fahrenheit. A comfortable room temperature in Celsius translates to a familiar indoor reading in Fahrenheit.

Convert 98.6 degrees Fahrenheit to Celsius: Subtract 32 from 98.6 to get 66.6, then multiply by 5/9 to get 37 degrees Celsius. Normal human body temperature reads 37 in Celsius — a number worth memorizing for health-related conversions.

Convert 100 degrees Celsius to Kelvin: Add 273.15 to 100. The result is 373.15 K. This is the boiling point of water on the Kelvin scale.

Practical Examples

Here is how temperature conversion applies in real-world scenarios with fictional but realistic situations.

Quick Reference Conversion Table

Use this table for instant lookups of common temperature values across all three scales.

Tips for Mastering Temperature Conversions

These strategies help you convert temperatures faster and with greater confidence in everyday situations.

• Memorize key anchor points. If you remember that 0 C = 32 F, 100 C = 212 F, and 37 C = 98.6 F, you can estimate most everyday conversions by interpolating between these values.
• Use the quick mental math shortcut. To go from Celsius to Fahrenheit quickly, double the Celsius value and add 30. This gives an approximate answer: 20 C becomes (20 x 2) + 30 = 70 F (actual: 68 F). Good enough for weather and travel.
• Remember that Fahrenheit degrees are smaller. One Celsius degree equals 1.8 Fahrenheit degrees. This is why the Fahrenheit scale has a wider numerical range for the same physical temperature span.
• Think in ranges for cooking. Low oven heat is 300 to 325 F (150 to 163 C), moderate is 350 to 375 F (177 to 191 C), and high is 400 to 450 F (204 to 232 C). Memorizing these ranges is more practical than converting every recipe precisely.
• Know when to use Kelvin. Kelvin is needed for scientific formulas involving ratios of temperatures, such as the ideal gas law (PV = nRT) or Stefan-Boltzmann law. If a formula involves temperature in the denominator or as an exponent, Kelvin is almost always required.

Common Mistakes to Avoid

• Forgetting to add 32 when converting to Fahrenheit. Many people multiply by 9/5 but forget the +32 offset, producing a result that is 32 degrees too low.
• Using the wrong order of operations. When converting Fahrenheit to Celsius, you must subtract 32 first, then multiply by 5/9. Reversing the order gives an incorrect result.
• Writing degree symbols with Kelvin. Kelvin is not written with a degree symbol. It is 300 K, not 300 degrees K. This is a common error in scientific writing.
• Confusing body temperature scales. A fever of 101 F is concerning but not dangerous, while 101 C would be fatal. Always verify which scale a medical reading uses before reacting.
• Rounding too aggressively. For cooking, rounding to the nearest 5 degrees is fine. For scientific work, maintaining at least two decimal places in Kelvin conversions prevents significant accumulated error.

Last updated: February 23, 2026