Celsius to Kelvin

1 Celsius (°C) = 274.15 Kelvin (K)

Amount

From

Result

274.15 K

1 °C = 274.15 K

How to Convert Celsius to Kelvin?

To convert Celsius to Kelvin, add 273.15 to the Celsius temperature. The formula is K = °C + 273.15. For example, 25°C equals 298.15 K. This conversion is fundamental in science, engineering, and any discipline that requires an absolute temperature scale. The Kelvin scale starts at absolute zero — the coldest temperature theoretically possible, where all molecular motion ceases — making it the standard for physics, chemistry, thermodynamics, and astronomy. Unlike Celsius and Fahrenheit, Kelvin does not use a degree symbol; temperatures are expressed simply as "kelvins" (e.g., 300 K, not 300°K). Because a one-unit change in Kelvin is identical in magnitude to a one-degree change in Celsius, the conversion between these two scales is remarkably straightforward — just a simple addition or subtraction of the constant 273.15.

How to Convert Celsius to Kelvin

Write down the temperature in degrees Celsius that you want to convert.
Add 273.15 to the Celsius value.
The result is the temperature in kelvins.
For most practical purposes, adding 273 is close enough — the 0.15 difference rarely matters outside of precise laboratory work.
Remember: Kelvin values are always positive for any temperature above absolute zero (-273.15°C). You should never get a negative Kelvin value in real-world scenarios.

Real-World Examples

Room temperature — A comfortable 22°C

22 + 273.15 = 295.15 K. In scientific papers, standard room temperature is often cited as 293.15 K (20°C) or 298.15 K (25°C).

Boiling point of water — 100°C at standard pressure

100 + 273.15 = 373.15 K. This is the boiling point of water at 1 atmosphere of pressure, a key reference in thermodynamics.

Freezing point of water — 0°C

0 + 273.15 = 273.15 K. The freezing point of water is one of the original calibration points for the Celsius scale and a commonly used reference in Kelvin.

Liquid nitrogen temperature — -196°C

-196 + 273.15 = 77.15 K. Liquid nitrogen boils at 77 K, which is why it is used extensively as a cryogenic coolant in laboratories and medicine.

Absolute zero — -273.15°C

-273.15 + 273.15 = 0 K. This is the lowest possible temperature, where particles have minimal vibrational motion. It has never been achieved in practice but has been approached to within fractions of a kelvin.

Surface of the Sun — approximately 5,500°C

5500 + 273.15 = 5773.15 K. Astronomers typically express stellar temperatures in kelvins. The Sun is classified as a G-type star with a surface temperature of roughly 5,778 K.

Quick Reference

Celsius (°C)	Kelvin (K)
1	274.15
2	275.15
5	278.15
10	283.15
25	298.15
50	323.15
100	373.15
500	773.15
1,000	1273.15

History of Celsius and Kelvin

The Kelvin scale is named after William Thomson, 1st Baron Kelvin (1824–1907), an Irish-Scottish mathematical physicist and engineer. In 1848, Thomson proposed an absolute thermometric scale based on the concept of absolute zero — the temperature at which a theoretical ideal gas would exert zero pressure. He recognized that there must be a lower limit to temperature, and he set this point as the zero of his scale. The Kelvin scale uses the same increment size as Celsius: a change of 1 kelvin is exactly the same as a change of 1 degree Celsius. The only difference is the starting point — 0 K corresponds to -273.15°C. This elegant relationship exists because the Kelvin scale was deliberately designed to complement Celsius. In 2019, the kelvin was redefined as part of a major revision of the International System of Units (SI). Rather than being defined by the triple point of water (273.16 K), it is now defined in terms of the Boltzmann constant, fixed at exactly 1.380649 × 10⁻²³ joules per kelvin. This change tied the kelvin to a fundamental physical constant rather than a physical artifact, making it more precise and universally reproducible. For everyday conversions, nothing changed — you still add 273.15 to convert from Celsius.

Common Mistakes to Avoid

Writing "°K" with a degree symbol. Kelvin is an absolute scale, and the correct notation is simply "K" (e.g., 300 K, not 300°K). The degree symbol is reserved for Celsius and Fahrenheit.
Subtracting 273.15 instead of adding it. The formula for Celsius to Kelvin is K = °C + 273.15. Subtracting gives the reverse conversion (Kelvin to Celsius).
Rounding 273.15 to 273 in situations that require precision. While the difference is only 0.15 K, it matters in laboratory calibration, chemical equilibrium calculations, and any context where tenths of a degree are significant.
Assuming negative Kelvin values are possible. The Kelvin scale starts at absolute zero (0 K). Any calculation that yields a negative Kelvin result contains an error.
Confusing Kelvin with Rankine. The Rankine scale is another absolute scale, but it uses Fahrenheit-sized increments. A temperature of 300 K is not the same as 300 R (300 K = 540 R).

Frequently Asked Questions

What is the formula to convert Celsius to Kelvin?

The formula is K = °C + 273.15. Simply add 273.15 to any Celsius temperature to get the equivalent in kelvins. This works because 0°C equals 273.15 K, and both scales use the same increment size.

Why do scientists use Kelvin instead of Celsius?

Many physical laws and equations — such as the ideal gas law (PV = nRT), Stefan-Boltzmann law, and Wien's displacement law — require an absolute temperature scale to work correctly. Using Celsius (where zero is arbitrary) would produce mathematical errors in these formulas. Kelvin's zero point corresponds to absolute zero, making ratios and proportions physically meaningful.

What is absolute zero in Celsius?

Absolute zero is -273.15°C (or 0 K). At this temperature, atoms and molecules have the least possible energy. It has never been reached in a laboratory, though scientists have cooled matter to within billionths of a kelvin of absolute zero.

Is a 1-degree change in Celsius the same as a 1-kelvin change?

Yes. The Celsius and Kelvin scales have the same increment size. A rise of 5°C is exactly the same as a rise of 5 K. The only difference between the scales is the zero point — 0°C = 273.15 K.

What is standard temperature and pressure (STP) in Kelvin?

The IUPAC definition of STP is 273.15 K (0°C) and exactly 100 kPa (1 bar) of pressure. An older convention used 273.15 K and 101.325 kPa (1 atm). In practice, many textbooks still refer to "room temperature" as 298.15 K (25°C) for standard thermodynamic calculations.

Do I need to use 273.15, or is 273 close enough?

For most everyday and educational purposes, adding 273 is close enough. The 0.15 K difference matters primarily in precision laboratory work, calibrating instruments, and performing thermodynamic calculations where errors compound. For quick conversions and general science homework, 273 is fine.

Quick Tip

The relationship between Celsius and Kelvin is the simplest of all temperature conversions — just add or subtract 273.15. Memorize a few key benchmarks: 0°C = 273.15 K (water freezes), 25°C = 298.15 K (standard lab temperature), 100°C = 373.15 K (water boils), and -273.15°C = 0 K (absolute zero). In scientific writing, always use "K" without a degree symbol, and remember that Kelvin is capitalized when referring to the unit name but the symbol "K" is always uppercase.