Celsius to Kelvin Converter
A physics undergraduate finishing the second hour of a thermodynamics problem set keeps adding 273.15 to every Celsius temperature in the textbook before plugging into the ideal gas law because Kelvin is the only temperature unit the equation will accept. Celsius-to-Kelvin is the workhorse conversion of the entire physical sciences — every gas-law calculation, every entropy computation, every thermal radiation problem, every astrophysics paper depends on the absolute scale that Lord Kelvin proposed in 1848. Unlike Celsius-to-Fahrenheit, the Celsius-to-Kelvin formula is a simple addition with no scaling: K = C + 273.15. The two scales share the same degree size because Kelvin was deliberately defined to use Celsius increments, and they differ only in where they place the zero — Celsius at water's freezing point, Kelvin at the absolute coldest possible temperature where molecular motion theoretically stops.
Calculator
(1 × 1) + 273.15 = 274.1500
Formula
Add 273.15 to Celsius to get Kelvin. So 0 °C equals 273.15 K, 100 °C equals 373.15 K, and absolute zero is exactly 0 K (which equals -273.15 °C). For most everyday use the offset is rounded to 273, making the math even simpler at the cost of about one part in a thousand precision. Note that Kelvin uses no degree symbol — temperatures are written as 273 K rather than 273 °K — because Kelvin is not a relative scale but an absolute one. Power users memorize that room temperature near 20 °C corresponds to 293.15 K, body temperature at 37 °C corresponds to 310.15 K, and water boiling at 100 °C corresponds to 373.15 K.
Where You'll Use This
Physical chemistry calculations are the dominant professional use of this conversion. Ideal gas law (PV = nRT) and every other equation derived from kinetic theory require absolute temperature, so any laboratory measurement taken in Celsius must be converted to Kelvin before plugging into the formula. Thermodynamic process design in chemical engineering — heat exchangers, distillation columns, refrigeration cycles — uses Kelvin throughout the calculation pipeline. Astronomy and astrophysics use Kelvin exclusively because stellar surface temperatures range from a few thousand Kelvin (red dwarfs) to tens of thousands of Kelvin (blue giants), and the Celsius equivalents are unhelpfully large negative numbers at the cool end and unwieldy at the hot end. Cryogenics laboratories working with liquid nitrogen (77 K), liquid helium (4.2 K), and superconducting magnets use Kelvin as the working unit because their target temperatures are within a hundred degrees of absolute zero. Even color temperature in photography and stage lighting (warm white at 2700 K, daylight at 5500 K, blue overcast at 7500 K) uses Kelvin as the standard descriptor across the entire industry.
Reference Table
| From (Celsius) | To (Kelvin) |
|---|---|
| -273 | 0.15 |
| -200 | 73.15 |
| -100 | 173.15 |
| -50 | 223.15 |
| -40 | 233.15 |
| -20 | 253.15 |
| 0 | 273.15 |
| 5 | 278.15 |
| 10 | 283.15 |
| 20 | 293.15 |
| 25 | 298.15 |
| 37 | 310.15 |
| 50 | 323.15 |
| 75 | 348.15 |
| 100 | 373.15 |
| 150 | 423.15 |
| 200 | 473.15 |
| 300 | 573.15 |
| 500 | 773.15 |
| 750 | 1023.15 |
| 1000 | 1273.15 |
| 1500 | 1773.15 |
| 2000 | 2273.15 |
| 3000 | 3273.15 |
| 5000 | 5273.15 |
A Bit of History
William Thomson, later Lord Kelvin, proposed the absolute temperature scale in 1848 as part of his work on Carnot's thermodynamic cycle, recognizing that the second law of thermodynamics implied a true zero temperature below which no real substance could go. The 273.15 offset between Celsius zero and absolute zero was determined experimentally over the following decades by extrapolating gas pressure-versus-temperature curves to zero pressure, with the modern figure pinned to extreme precision by the 2019 SI redefinition that fixed the Boltzmann constant. The Kelvin became one of the seven SI base units in 1960 and has been the standard scientific temperature unit ever since, even as Celsius continues to dominate everyday weather and cooking applications.
FAQ
What is 0 °C in Kelvin?
Zero degrees Celsius equals 273.15 Kelvin — the freezing point of water in absolute temperature units. Equivalently, 273.15 K is the baseline above which liquid water coexists with ice at standard atmospheric pressure.
How do I convert Celsius to Kelvin?
Add 273.15 to the Celsius value. Twenty-five degrees Celsius (room temperature) becomes 298.15 K. The conversion is a simple offset because both scales use degrees of the same size — Kelvin just shifts the zero down to absolute zero.
Why does Kelvin not have a degree symbol?
Because Kelvin is an absolute scale measured from absolute zero rather than a relative scale measured from an arbitrary reference. Scientific convention writes 273 K rather than 273 °K, distinguishing it from Celsius and Fahrenheit. The omission of the degree symbol is the typographical signal that this is an SI base unit.
What is absolute zero in Kelvin?
Absolute zero is 0 K (zero Kelvin), or equivalently -273.15 °C. At this temperature classical molecular motion theoretically stops. Real systems can approach but never quite reach absolute zero — current laboratory records sit in the picokelvin range, a few trillionths of a degree above zero.