Kelvin — Definition, Formula & Examples
Kelvin is the standard scientific unit of temperature that starts at absolute zero, the coldest possible temperature in the universe. Unlike Celsius or Fahrenheit, the Kelvin scale has no negative values because 0 K represents the complete absence of thermal energy.
The kelvin (symbol K) is the SI base unit of thermodynamic temperature. Its scale is defined so that 0 K equals absolute zero (−273.15 °C), and each increment of 1 K is equal in size to 1 °C. Temperatures in kelvin are written without a degree symbol.
Key Formula
Where:
- = Temperature in kelvin
- = Temperature in degrees Celsius
- = The offset between the Celsius and Kelvin scales
How It Works
To convert Celsius to Kelvin, you simply add 273.15. To go from Kelvin back to Celsius, subtract 273.15. The key idea is that a Kelvin degree and a Celsius degree are the same size — the scales are just shifted. Water freezes at 273.15 K and boils at 373.15 K. Scientists prefer Kelvin because many physics and chemistry formulas require a temperature scale that starts at true zero, where no further heat energy can be removed.
Worked Example
Problem: A pot of water is heated to 100 °C. What is this temperature in kelvin?
Write the formula: Use the Celsius-to-Kelvin conversion.
Substitute the value: Plug in 100 for degrees Celsius.
Calculate: Add the numbers together.
Answer: 100 °C equals 373.15 K.
Another Example
Problem: The surface of the Sun is about 5,778 K. What is that in degrees Celsius?
Rearrange the formula: Solve for Celsius by subtracting 273.15.
Substitute: Plug in 5,778 for K.
Calculate: Perform the subtraction.
Answer: The Sun's surface temperature is approximately 5,504.85 °C.
Visualization
Why It Matters
The Kelvin scale is essential in chemistry and physics courses whenever you work with gas laws, thermodynamics, or blackbody radiation, because those formulas break down if the temperature can go below zero. In careers like engineering, meteorology, and astrophysics, Kelvin is the default temperature unit for scientific calculations.
Common Mistakes
Mistake: Adding a degree symbol and writing °K instead of K.
Correction: Kelvin is an SI base unit, so no degree symbol is used. Write 300 K, not 300 °K.
Mistake: Subtracting 273.15 when converting Celsius to Kelvin instead of adding.
Correction: Kelvin values are always larger than Celsius values (for positive Celsius). Remember: K = °C + 273.15. Subtract only when going from Kelvin back to Celsius.