The Celsius to Kelvin Calculator converts a Celsius temperature into Kelvin, the SI base unit for temperature used throughout physics and chemistry. Enter a value in Celsius, and the tool adds 273.15, showing the substitution so the result can be verified without a separate reference chart.
Kelvin and Celsius share the same degree size: one Kelvin is the same amount of temperature change as one degree Celsius. The two scales differ only in where zero sits. That makes Celsius-to-Kelvin the simplest of the classic temperature conversions, a single addition rather than a scale-and-shift pair.
Convert Celsius to Kelvin
To convert Celsius to Kelvin, add 273.15 to the Celsius value.
K = °C + 273.15Worked example: convert 25 °C to Kelvin.
1. Add 273.15. 25 + 273.15 = 298.15 K.
So a comfortable 25 °C room sits at 298.15 K. Because Kelvin has no negative values in normal physical use, the calculator will flag an entry below −273.15 °C, since that would correspond to a Kelvin reading below zero, which is not physically meaningful.
Understand why Kelvin starts at absolute zero
Kelvin is an absolute temperature scale: zero Kelvin is absolute zero, the theoretical point at which particles have minimum thermal motion. Celsius, by contrast, sets its zero at the freezing point of water, a convenient but arbitrary reference for everyday use.
The offset between the two scales, 273.15, is exactly the gap between absolute zero and water's freezing point on the Celsius scale.
Because the degree sizes match, no multiplication is needed, unlike the Celsius-to-Fahrenheit conversion. Only the starting point differs, so a single addition handles the full conversion in either direction.
Check common reference points
| Celsius | Kelvin | Context |
|---|---|---|
| −273.15 °C | 0 K | Absolute zero |
| 0 °C | 273.15 K | Water freezes |
| 20 °C | 293.15 K | Mild room temperature |
| 37 °C | 310.15 K | Average body temperature |
| 100 °C | 373.15 K | Water boils (sea level) |
Each row follows the same rule: add 273.15 to the Celsius figure. For 37 °C, 37 + 273.15 = 310.15 K, the Kelvin equivalent of average human body temperature.
Convert Kelvin back to Celsius
The Celsius to Kelvin Calculator also runs the reverse direction. To convert Kelvin to Celsius, subtract 273.15.
°C = K − 273.15Worked example: convert 300 K to Celsius.
1. Subtract 273.15. 300 − 273.15 = 26.85 °C.
Because the relationship is a pure offset with no scaling factor, this reverse step is just as direct as the forward one.
Use Kelvin correctly in scientific work
Kelvin values are written without a degree symbol: 298.15 K, not 298.15 °K. That convention reflects Kelvin's status as an absolute unit rather than a relative degree scale.
Scientific formulas that involve gas laws, thermodynamics or radiation nearly always require Kelvin rather than Celsius, because those equations depend on temperature measured from true zero rather than from an arbitrary freezing point.
A common mistake is plugging a Celsius value directly into a formula that expects Kelvin. Since the offset is 273.15, that error becomes significant at everyday temperatures: using 25 instead of 298.15 in a gas-law calculation would be wrong by more than a factor of ten.
Convert more everyday Celsius readings
The same addition applies outside the freezing-to-boiling range shown in the table. An oven set to 180 °C for baking converts to 180 + 273.15 = 453.15 K. A household freezer held at −18 °C converts to −18 + 273.15 = 255.15 K. A fever reading of 38.5 °C converts to 38.5 + 273.15 = 311.65 K.
Each of these follows the identical rule used throughout this page: add 273.15, regardless of whether the starting value is negative, at room temperature, or well above body temperature. That consistency is what makes the Celsius-to-Kelvin relationship the simplest conversion in the whole temperature family.
Understand how Kelvin behaves at very low temperatures
Kelvin readings stay meaningful even far below everyday experience. Liquid nitrogen boils at about 77 K under normal atmospheric pressure, which converts back to Celsius as 77 − 273.15 = −196.15 °C. That single subtraction is the same reverse formula used earlier on this page, just applied at the cold end of the scale instead of near room temperature.
The same rule holds no matter how far the reading sits from everyday conditions, since the 273.15 offset never changes with temperature.
Frequently asked questions
What is the formula to convert Celsius to Kelvin?
Add 273.15 to the Celsius value: K = °C + 273.15. For example, 25 °C converts to 25 + 273.15 = 298.15 K.
How do you convert Kelvin back to Celsius?
Subtract 273.15 from the Kelvin value: °C = K − 273.15. For example, 300 K converts to 300 − 273.15 = 26.85 °C.
What is 0 °C in Kelvin?
0 °C equals 273.15 K, the freezing point of water expressed on the Kelvin scale.
What is absolute zero in Celsius?
Absolute zero is −273.15 °C, which corresponds to exactly 0 K, the lowest temperature physically possible.
Why doesn't Kelvin use a degree symbol?
Kelvin is treated as an absolute unit rather than a relative degree scale, so values are written as "298.15 K" rather than "298.15 °K," following SI convention.
Can a Kelvin value be negative?
No. Kelvin measures temperature from absolute zero, so negative Kelvin values are not physically meaningful in classical thermodynamics. Any Celsius entry below −273.15 will be flagged.
Does this calculator also handle Rankine and Fahrenheit?
Yes. Selecting a different target scale runs the same temperature engine used across the site, including the full Temperature Conversion Calculator that covers all four scales in one place.
Summary
Celsius converts to Kelvin with a single addition of 273.15, since the two scales share the same degree size and differ only in their zero point. So 25 °C becomes 298.15 K and 0 °C becomes 273.15 K. The reverse direction subtracts 273.15.
Kelvin's absolute-zero reference makes it the required scale for most scientific formulas, where a Celsius value substituted by mistake produces meaningfully wrong results.