What this calculator does
Turn any Celsius temperature into kelvins, or kelvins back into Celsius, in one step. The kelvin is the science unit for temperature (the SI base unit), and its scale starts at absolute zero, the coldest possible state. Science and engineering need it because many laws, such as the ideal gas law and radiation formulas, only work with an absolute temperature. Celsius is the everyday metric scale, so lab notes, chemistry homework and data sheets often switch between the two.
The two scales use degrees of exactly the same size; they differ only in where zero sits. That makes the conversion a pure shift: add 273.15 to a Celsius reading to get kelvins, subtract 273.15 to go back. Enter a value and choose a direction.
Where it's used
Why this math matters outside the classroom:
- Chemistry labs. Gas law problems need absolute temperature, so a 25 °C room becomes 298.15 K.
- Cryogenics. Liquid nitrogen boils at about −196 °C, which is about 77 K.
- Absolute zero. Students see that −273.15 °C is 0 K, the lowest temperature possible.
- Astronomy. The Sun's surface is about 5,772 K, which is about 5,499 °C.
- Negative readings. A freezer at −18 °C is −18 + 273.15 = 255.15 K.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 7.NS.A.1. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the temperature in °C or K. Decimals and negative Celsius values are allowed.
- Choose Celsius to Kelvin or Kelvin to Celsius.
- Read the result and the one-line working. Values below absolute zero are rejected with an explanation.
What the inputs mean
- Temperature
- A reading in °C (at least −273.15) for °C → K, or in kelvins (at least 0) for K → °C.
- Direction
- Choose the scale you are starting from.
Formula and method
K = °C + 273.15 °C = K − 273.15
In the formulas, K is the temperature in kelvins and °C is the same temperature in degrees Celsius. Since the 2019 SI revision, the kelvin is fixed through the Boltzmann constant, a basic constant of physics. The Celsius scale is the kelvin scale shifted by exactly 273.15, with no multiplying involved.
The 0.15 matters in precise work. Rounding to 273 introduces a 0.15 K error, which is small for weather but noticeable in gas-law and calorimetry problems.
Worked example
A chemistry problem is set at 25 °C. What is that in kelvins?
- Add 273.15 to the Celsius value: 25 + 273.15 = 298.15.
Result: 25 °C is 298.15 K, the standard ambient temperature used in many thermodynamic tables.
Understanding the result
Mental shortcut: add 273 and remember the 0.15 if the answer is going into a formula. 20 °C is about 293 K (exactly 293.15 K).
Reference points: absolute zero is 0 K = −273.15 °C; liquid nitrogen boils near −196 °C, about 77 K; water freezes at 273.15 K and boils at about 373.15 K at sea-level pressure; 37 °C is 310.15 K.
A difference of 1 °C equals a difference of 1 K exactly, so temperature changes need no conversion at all. Only readings need the 273.15 shift.
Assumptions and limitations
- Values are temperature readings on the Celsius and Kelvin scales as defined by the SI.
- Readings below absolute zero are rejected, since no temperature can be lower than 0 K.
Frequently asked questions
Why is it 273.15 and not 273?
The kelvin scale was originally tied to the triple point of water at exactly 273.16 K, which is 0.01 °C. That placed 0 °C at 273.15 K, and the offset has been kept exact ever since.
Do you write "degrees kelvin"?
No. The unit is just the kelvin, symbol K, so 300 K is read as "300 kelvins" with no degree sign.
Can a kelvin temperature be negative?
No. 0 K is absolute zero, the lowest temperature possible, so kelvin readings are always zero or positive.
When must I use kelvins instead of Celsius?
Whenever a formula multiplies or divides by temperature, as in the ideal gas law PV = nRT (where T is the temperature). Plugging in Celsius would give nonsense, for example a zero or negative gas volume at 0 °C and below.
How this calculator is checked
Method: SI definition: t(°C) = T(K) − 273.15 exactly. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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