Celsius to Kelvin Converter
Quick Conversion: K = C + 273.15
Celsius to Kelvin Converter
What Is Celsius?
The degree Celsius (°C) is the SI-derived unit of temperature, defined so that 0°C = 273.15 K exactly. The freezing point of water is 0°C and the boiling point is 100°C at standard pressure. It is the most widely used temperature scale worldwide.
What Is Kelvin?
The kelvin (K) is the SI base unit of thermodynamic temperature. Its zero point is absolute zero — the theoretical minimum where all molecular motion ceases. Since the 2019 SI redefinition, it is defined by fixing the Boltzmann constant at exactly 1.380 649 × 10⁻²³ J/K. One kelvin increment equals one Celsius degree.
Celsius to Kelvin Formula
The formula is exact:
K = °C + 273.15 exact
This is the simplest of all temperature conversions — no multiplication, just an addition. The offset 273.15 is the exact difference between the Celsius and Kelvin zero points.
Worked Example
Convert 20°C (room temperature) to Kelvin:
- Add: 20 + 273.15 = 293.15 K
So room temperature is about 293 K — a value that appears constantly in physics and chemistry textbooks.
Quick Estimation
Just add 273 (dropping the 0.15). Example: 25°C + 273 = 298 K (exact: 298.15 K). The error of 0.15 K is negligible for everyday purposes — only matters in precision lab work.
Reference Table: Scientific Milestones
| °C | K | Significance |
|---|---|---|
| −273.15 | 0 | Absolute zero — coldest possible temperature |
| −196 | 77.15 | Liquid nitrogen boils |
| −78.5 | 194.65 | Dry ice (CO₂) sublimes |
| 0 | 273.15 | Water freezes at standard pressure |
| 20 | 293.15 | Standard room temperature |
| 37 | 310.15 | Human body temperature |
| 100 | 373.15 | Water boils at standard pressure |
| 1,538 | 1,811 | Iron melts |
| 5,500 | 5,773 | Surface of the Sun |
Why 273.15?
The offset comes from the triple point of water — the exact temperature where ice, liquid water, and water vapor coexist in equilibrium. This was defined as exactly 273.16 K (0.01°C) by the BIPM. Since the Celsius scale sets water's freezing point at 0°C (not 0.01°C), the offset is 273.16 − 0.01 = 273.15 exactly.
When You Need This Conversion
- Chemistry and physics: Gas law calculations (PV = nRT) require temperature in kelvin. A lab at 25°C must use 298.15 K in the ideal gas equation.
- Engineering: Thermodynamic cycles, heat transfer coefficients, and material properties are expressed in kelvin. Standard conditions (STP) are 273.15 K and 1 atm.
- Astronomy: Stellar temperatures, cosmic microwave background (2.725 K), and planetary science all use kelvin. The Sun's surface: 5,778 K.
Frequently Asked Questions
What is 0°C in Kelvin?
0°C = 273.15 K. This is the freezing point of water and one of the most important reference values in science.
Is −273.15°C really the coldest possible temperature?
Yes. At 0 K (−273.15°C), particles have minimal possible energy — this is absolute zero. It can be approached but never reached, according to the third law of thermodynamics. Labs have achieved within nanokelvins of 0 K.
Why doesn't Kelvin use a degree symbol?
Because it is an SI base unit, not a derived unit like Celsius. The 13th CGPM (1967) removed the degree symbol — it is simply "kelvin" (K), not "degrees kelvin" (°K). This reflects its status as a fundamental measurement unit.
What is room temperature in Kelvin?
Room temperature (20–25°C) equals 293.15–298.15 K. In chemistry, "standard temperature" is typically defined as 25°C = 298.15 K.
Is a 1°C change the same as a 1 K change?
Yes — the degree sizes are identical. A temperature difference of 10°C equals exactly 10 K. The scales differ only in their zero points (0°C = 273.15 K), not in their increments.
Temperature conversion chart
| To Fahrenheit | To Celsius | To Kelvin | |
| From Fahrenheit (F) | F | (F - 32) × 5/9 | (F - 32) × 5/9 + 273.15 |
| From Celsius (C or o) | (C × 9/5) + 32 | C | C + 273.15 |
| From Kelvin (K) | (K - 273.15) × 9/5 + 32 | K - 273.15 | K |
Popular Temperature Conversions
Quick access to frequently searched temperature conversions:
Looking for the reverse? Convert Kelvin to Celsius
Related Temperature Converters
Explore other temperature conversion tools:
All unit conversions on CoolConversion use conversion factors defined or documented by internationally recognised standards bodies (such as ISO and NIST), including both SI and non-SI units.
Related in other categories:
- Joules to kWh — Temperature change requires energy (heat capacity)
