Celsius to Kelvin Converter
Convert Celsius to Kelvin instantly. Formula: K = °C + 273.15. Full conversion table, absolute zero explained, and why scientists use Kelvin. Free.
Celsius to Kelvin Converter
To convert Celsius to Kelvin, add 273.15 to the Celsius temperature. The converter above handles both directions instantly — enter a value in either field to get the result. This page covers the formula, a full reference table, and explains why scientists use Kelvin instead of Celsius.
Celsius to Kelvin Formula
The formula for converting Celsius to Kelvin is:
K = °C + 273.15
For example, to convert 25°C to Kelvin: 25 + 273.15 = 298.15 K.
The constant 273.15 is not arbitrary. It is the gap between absolute zero (−273.15°C) and the Celsius zero point (freezing water). Both scales use the same degree size, so only the zero point differs.
Kelvin to Celsius Formula
To convert Kelvin back to Celsius, subtract 273.15:
°C = K − 273.15
For example, to convert 373.15 K to Celsius: 373.15 − 273.15 = 100°C (the boiling point of water at sea level).
Celsius to Kelvin Conversion Table
| Celsius (°C) | Kelvin (K) | Reference Point |
|---|---|---|
| −273.15°C | 0 K | Absolute zero |
| −200°C | 73.15 K | Liquid nitrogen range |
| −100°C | 173.15 K | Dry ice temperature |
| −40°C | 233.15 K | Celsius = Fahrenheit point |
| −10°C | 263.15 K | Cold winter temperature |
| 0°C | 273.15 K | Freezing point of water |
| 10°C | 283.15 K | Cool spring day |
| 20°C | 293.15 K | Room temperature |
| 25°C | 298.15 K | Standard lab temperature |
| 37°C | 310.15 K | Human body temperature |
| 100°C | 373.15 K | Boiling point of water |
| 200°C | 473.15 K | Oven baking temperature |
| 500°C | 773.15 K | Red-hot metal |
| 1,000°C | 1,273.15 K | Molten lava |
| 5,505°C | 5,778 K | Surface of the Sun |
What Is the Kelvin Scale?
The Kelvin scale is the International System of Units (SI) base unit of thermodynamic temperature. Unlike Celsius and Fahrenheit, Kelvin has no negative values — it begins at absolute zero (0 K), the coldest theoretically possible temperature. Each unit on the Kelvin scale (called a kelvin, symbol K) is the same size as one degree Celsius.
The scale is named after the British physicist William Thomson, 1st Baron Kelvin (1824–1907), who proposed an absolute temperature scale in 1848. The International Bureau of Weights and Measures (BIPM) redefined the kelvin in 2019. It is now fixed by the Boltzmann constant at exactly 1.380649 × 10²³ joules per kelvin.
What Is Absolute Zero?
Absolute zero (0 K = −273.15°C) is the theoretical temperature at which a system contains minimum possible thermal energy. At this point, molecular motion does not stop entirely, but reaches its quantum mechanical ground state. Absolute zero has never been achieved in practice, though laboratory experiments have reached temperatures within billionths of a degree above it.
The 273.15 offset comes directly from absolute zero. Since absolute zero is −273.15°C, adding 273.15 shifts the scale so 0 K marks the minimum possible temperature. This is why 0°C = 273.15 K and −273.15°C = 0 K.
Why Do Scientists Use Kelvin Instead of Celsius?
Kelvin is preferred in science and engineering for several important reasons:
- No negative values: Many physics equations require absolute temperature. Using Celsius can produce negative values that break mathematical relationships. The Ideal Gas Law (PV = nRT), for example, requires T in Kelvin.
- Direct proportionality: Doubling the Kelvin temperature doubles the thermal energy. This is not true for Celsius. 200 K is twice the thermal energy of 100 K, but 200°C is not twice the thermal energy of 100°C.
- SI standard: The Kelvin is the SI base unit for temperature. All scientific publications, physics formulas, and engineering standards use Kelvin for thermodynamic calculations.
- Consistent with thermodynamics: Laws such as the Stefan-Boltzmann law (radiation = σT&sup4;) and Wien's displacement law require Kelvin for correct results.
Celsius vs Kelvin — Key Differences
| Property | Celsius (°C) | Kelvin (K) |
|---|---|---|
| Zero point | Freezing point of water | Absolute zero |
| Degree size | Same as Kelvin | Same as Celsius |
| Negative values | Yes (below −273.15°C) | No (0 K is minimum) |
| Symbol | °C (degree sign used) | K (no degree sign) |
| Primary use | Weather, cooking, everyday life | Science, physics, engineering |
| Defined by | Water freezing and boiling points | Boltzmann constant (since 2019) |
Common Celsius to Kelvin Conversions
- 0°C = 273.15 K: The freezing point of pure water at sea level.
- 20°C = 293.15 K: Standard room temperature used in chemistry experiments.
- 25°C = 298.15 K: The reference temperature for thermodynamic data in chemistry (STP conditions).
- 37°C = 310.15 K: Normal human body temperature used in medical and biological research.
- 100°C = 373.15 K: The boiling point of water at standard atmospheric pressure (101.325 kPa).
- −40°C = 233.15 K: The only point where Celsius and Fahrenheit readings are equal (−40°F = −40°C).
How to Convert Celsius to Kelvin Without a Calculator
Add 273 to the Celsius value for a quick estimate. Using 273 instead of 273.15 introduces an error of less than 1% for temperatures above 0°C — accurate enough for most non-laboratory purposes. For example, 30°C + 273 = 303 K (exact: 303.15 K).
For scientific or engineering work, always use the precise constant: add 273.15. The difference of 0.15 K can matter in precise thermodynamic calculations, particularly near absolute zero or in cryogenic research.
Limitations of This Converter
- Kelvin has no negative values: Any Celsius temperature below −273.15°C would produce a negative Kelvin value, which is physically impossible. The converter will not produce results below 0 K.
- Not for Fahrenheit: This converter handles Celsius and Kelvin only. To convert Fahrenheit to Kelvin, first convert Fahrenheit to Celsius using (°F − 32) × 5/9, then add 273.15.
- Standard atmosphere assumed: The reference points in the table (boiling point, freezing point) apply at standard atmospheric pressure (101.325 kPa). At higher altitudes, water boils below 100°C.
For more temperature conversions, use DigiCalc's Fahrenheit to Celsius converter, the Celsius to Fahrenheit converter, or the full temperature conversion tool.