Temperature Scales — Celsius and Kelvin

Temperature is a measure of how hot or cold something is. Scientists use the Kelvin scale, which starts at absolute zero, while everyday life uses Celsius. Converting between them requires adding or subtracting 273.

What is Temperature?

Temperature measures the average kinetic energy of particles in a substance. The hotter the substance, the faster its particles move.

Measured with a thermometer
Common units: Celsius (°C) and Kelvin (K)

The Celsius Scale

The Celsius scale is based on the properties of water:

Point	Temperature (°C)
Water freezes	0 °C
Water boils	100 °C
Body temperature	~37 °C
Room temperature	~20 °C

The Kelvin Scale

The Kelvin scale starts at absolute zero — the coldest possible temperature, where particle movement effectively stops.

Absolute zero: 0 K = −273 °C
No negative temperatures in Kelvin
Each 1 K change = each 1 °C change (same step size)

Converting Between Celsius and Kelvin

$K = °C + 273$ $°C = K - 273$

Celsius ↔ Kelvin Conversion

K = °C + 273

Add 273 to convert Celsius to Kelvin. Subtract 273 to convert Kelvin to Celsius.

Temperature Conversion

Convert 27 °C to Kelvin.

$K = 27 + 273 = 300 \text{ K}$

Convert 400 K to Celsius.

$°C = 400 - 273 = 127 \text{ °C}$

Note

Gas laws (like Boyle's Law) always use Kelvin temperatures in calculations, not Celsius.

Real world: Doctor checking for a fever

Normal body temperature is 37 °C = 310 K. A patient with a temperature of 39.5 °C has a significant fever. In Kelvin: K = 39.5 + 273 = 312.5 K. Although the Kelvin difference is small, even 2–3 °C above normal body temperature is medically serious.

Real world: Freezer and food safety

A home freezer is set to −18 °C. Converting to Kelvin: K = −18 + 273 = 255 K. At this temperature, bacterial growth effectively stops, keeping food safe. The Kelvin scale makes it clear that −18 °C is still well above absolute zero (0 K), meaning particles are still moving — just very slowly.

Real world: Baking from an American recipe

American recipes often give oven temperatures in Fahrenheit (°F). A recipe calling for 350 °F is approximately 177 °C. Scientists always work in Celsius or Kelvin — converting 177 °C to Kelvin gives 450 K, which is used in calculations involving heat energy and gas laws.

Tip Where do you see this in real life? Weather forecasts, medical thermometers, oven settings, and scientific research all use temperature scales — make sure you know which scale is being used!

Practice Problems

Convert 100 °C to Kelvin.

Show answer

K = 100 + 273 = 373 K

Convert 0 K to Celsius. What does this temperature represent?

Show answer

°C = 0 − 273 = −273 °C. This is absolute zero — the coldest possible temperature.

Which is hotter: 300 K or 50 °C?

Show answer

300 K = 300 − 273 = 27 °C. So 50 °C is hotter than 300 K.