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Convert Celsius to Kelvin instantly with our free online calculator. Essential for physics, chemistry, laboratory work, and any scientific context that requires the SI absolute temperature scale.
K = °C + 273.15
Conversion Formula
0 K
Absolute Zero (−273.15 °C)
273.15 K
Water Freezes (0 °C)
Formula: K = °C + 273.15 | °C = K − 273.15
| Celsius (°C) | Kelvin (K) | Significance |
|---|---|---|
| −273.15 °C | 0 K | Absolute zero |
| −196 °C | 77.15 K | Liquid nitrogen boiling point |
| −78.5 °C | 194.65 K | Dry ice sublimation point |
| −40 °C | 233.15 K | °C equals °F (−40 °F) |
| 0 °C | 273.15 K | Water freezing point |
| 20 °C | 293.15 K | Standard room temperature |
| 25 °C | 298.15 K | Standard conditions (STP reference) |
| 37 °C | 310.15 K | Human body temperature |
| 100 °C | 373.15 K | Water boiling point (1 atm) |
| 233 °C | 506.15 K | Tin melting point |
| 660 °C | 933.15 K | Aluminium melting point |
| 1064 °C | 1337.15 K | Gold melting point |
| 1538 °C | 1811.15 K | Iron melting point |
| 5500 °C | 5773.15 K | Surface of the Sun (approx.) |
Celsius (°C), originally called centigrade, is a temperature scale proposed by Swedish astronomer Anders Celsius in 1742. It defines 0 °C as the freezing point of pure water and 100 °C as the boiling point at standard atmospheric pressure (101.325 kPa). The Celsius scale is part of the metric system and is the most widely used temperature scale in the world for everyday measurement, weather forecasting, cooking, and industrial applications.
Kelvin (K) is the base unit of thermodynamic temperature in the International System of Units (SI). Named after the Irish-Scottish physicist Lord Kelvin (William Thomson), the scale starts at absolute zero — the point at which all thermal motion ceases (0 K = −273.15 °C). Unlike Celsius and Fahrenheit, Kelvin is an absolute scale, meaning it has a true physical zero. The SI convention omits the degree symbol: we write "300 K," not "300 °K."
Both scales use the same increment size: a rise of 1 K is identical to a rise of 1 °C. The only difference is the zero-point offset of 273.15. This makes conversion straightforward: K = °C + 273.15. Kelvin is indispensable in physics, chemistry, and engineering wherever formulas require an absolute temperature — for example, the ideal gas law (PV = nRT), blackbody radiation, and thermodynamic entropy calculations.
Historically, Celsius's original scale was inverted (100° for freezing, 0° for boiling). It was reversed after his death by Carl Linnaeus and others. Lord Kelvin proposed his absolute scale in 1848, grounding temperature measurement in fundamental thermodynamic principles rather than the properties of a single substance. The Kelvin was redefined in 2019 in terms of the Boltzmann constant, decoupling it from the triple point of water.
The formula to convert Celsius to Kelvin is: K = °C + 273.15. Simply add 273.15 to the Celsius temperature to obtain the Kelvin equivalent. To convert back, subtract 273.15 from the Kelvin value.
Question: A laboratory thermostat reads 25 °C. What is this in Kelvin?
Question: Liquid nitrogen boils at −196 °C. Express this in Kelvin.
Question: Water boils at 100 °C at sea level. What is this temperature in Kelvin?
For a quick mental estimate, add 273 instead of 273.15. This gives a result within 0.15 K of the exact value — more than accurate enough for most practical purposes. For example: 37 °C + 273 = 310 K (exact: 310.15 K). When high precision matters (analytical chemistry, calibration), always use the full 273.15 offset.
| Description | °C | K |
|---|---|---|
| Absolute zero | −273.15 | 0 |
| Liquid helium boiling point | −268.93 | 4.22 |
| Liquid hydrogen boiling point | −252.87 | 20.28 |
| Liquid nitrogen boiling point | −195.79 | 77.36 |
| Liquid oxygen boiling point | −182.96 | 90.19 |
| Dry ice sublimation | −78.5 | 194.65 |
| Mercury freezing point | −38.83 | 234.32 |
| Description | °C | K |
|---|---|---|
| Water freezing point | 0 | 273.15 |
| Refrigerator temperature | 4 | 277.15 |
| Cool room | 15 | 288.15 |
| Standard room temperature | 20 | 293.15 |
| STP reference temperature | 25 | 298.15 |
| Human body temperature | 37 | 310.15 |
| Pasteurization temperature | 72 | 345.15 |
| Water boiling point (1 atm) | 100 | 373.15 |
| Description | °C | K |
|---|---|---|
| Lead melting point | 327 | 600.15 |
| Aluminium melting point | 660 | 933.15 |
| Glass softening point | 700 | 973.15 |
| Gold melting point | 1064 | 1337.15 |
| Iron melting point | 1538 | 1811.15 |
| Tungsten melting point | 3422 | 3695.15 |
| Surface of the Sun | ~5500 | ~5773 |
Most physics equations require absolute temperature. The ideal gas law (PV = nRT), Stefan-Boltzmann law, Carnot efficiency, and entropy calculations all demand Kelvin. Using Celsius in these formulas produces incorrect results because the zero point is arbitrary.
Reaction kinetics, equilibrium constants, and the Arrhenius equation all use absolute temperature. Chemists routinely convert between Celsius (displayed on lab thermometers) and Kelvin (required in calculations) dozens of times per experiment.
Stellar surface temperatures, cosmic microwave background radiation (2.725 K), and planetary atmospheres are all described in Kelvin. Astronomers use the Kelvin scale exclusively because it connects directly to the physics of radiation and thermal energy.
Cryogenic engineering, semiconductor fabrication, and superconductor research all operate in Kelvin. Material properties like thermal conductivity, specific heat, and phase transition points are catalogued and modeled using absolute temperature for consistency.
For scientific work, the 0.15 matters. The triple point of water is 273.16 K (0.01 °C), so dropping the decimal introduces measurable error in precision experiments.
If your result is negative, double-check your input. The lowest valid Kelvin value is 0 K (−273.15 °C). A negative Kelvin result means the Celsius input was below absolute zero, which is physically impossible.
A change of 10 °C equals a change of 10 K. You only need to convert when working with absolute values, not temperature differences.
Unlike Fahrenheit conversions (which involve multiplication and addition), Celsius-to-Kelvin is purely additive. There is no scaling factor — only a shift of 273.15.
The degree symbol is not used with Kelvin. Writing "°K" is incorrect per SI convention. The correct notation is simply "K" (e.g., 298.15 K).
Rankine is also an absolute scale, but it uses Fahrenheit-sized degrees. 0 K = 0 °R, but 1 K = 1.8 °R. Make sure you know which absolute scale your formula requires.
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. Both scales use the same degree increment size, so the conversion is a simple addition.
Kelvin is an absolute thermodynamic temperature scale, not a relative one like Celsius or Fahrenheit. Because it begins at absolute zero — the lowest physically possible temperature — it measures absolute temperature directly. The SI convention is to write "298 K" rather than "298 °K."
Absolute zero is 0 K (−273.15 °C), the theoretical temperature at which all particle motion ceases. It is the lowest limit of the thermodynamic temperature scale. While scientists have cooled matter to within billionths of a kelvin above absolute zero, the third law of thermodynamics states that reaching exactly 0 K is impossible.
Kelvin is required in scientific formulas such as the ideal gas law (PV = nRT), Stefan-Boltzmann radiation law, and Boltzmann distribution. It is standard in physics, chemistry, astronomy, and engineering. Celsius is more practical for everyday weather, cooking, and general temperature measurement.
No. Kelvin cannot be negative because it starts at absolute zero, the lowest possible temperature. Any Celsius temperature, even negative ones like −100 °C, maps to a positive Kelvin value (173.15 K in this case).
There is no temperature at which Celsius and Kelvin read the same number, because Kelvin is always 273.15 units higher than Celsius. Unlike Celsius and Fahrenheit (which intersect at −40°), these two scales are offset by a fixed constant and never cross.
The Celsius scale defines 0 °C as the freezing point of water and 100 °C as the boiling point at standard pressure. Absolute zero was experimentally determined to be −273.15 °C. The Kelvin scale simply shifts the Celsius scale so that 0 K aligns with absolute zero, which creates the fixed 273.15 offset.
Yes. One kelvin and one degree Celsius represent exactly the same temperature increment. A change of 10 K is identical to a change of 10 °C. The only difference between the two scales is the zero point: 0 K = −273.15 °C.
This calculator uses the internationally accepted offset of 273.15 between the Celsius and Kelvin scales. For metrological calibration, traceable standards, or legal compliance, always refer to your national metrology institute or the BIPM definition of the kelvin based on the Boltzmann constant.