The celsius to kelvin formula is $K = °C + 273.15$.
So $0°C$ is $273.15$ K, $25°C$ is $298.15$ K, and $100°C$ is $373.15$ K.
[CALLOUT: Quick Answer]
Result: $K = °C + 273.15$ Notation: Add $273.15$ to the celsius value; the degree size is identical on both scales. Method shown: Direct addition (one step). Worked check: $37°C + 273.15 = 310.15$ K (human body temperature).
Celsius to Kelvin Conversion Table
A scan-friendly reference for the temperatures you meet most.
Celsius (°C) | Kelvin (K) | Context |
|---|---|---|
$-273.15$ | $0$ | Absolute zero |
$-196.15$ | $77$ | Liquid nitrogen boils |
$-78.15$ | $195$ | Dry ice sublimes |
$-18$ | $255.15$ | Home freezer |
$0$ | $273.15$ | Water freezes |
$20$ | $293.15$ | Room temperature |
$37$ | $310.15$ | Human body |
$100$ | $373.15$ | Water boils (1 atm) |
$5504.85$ | $5778$ | Sun's surface |
Where the Celsius to Kelvin Formula Appears
Scientists convert celsius to kelvin whenever a law of physics demands an absolute scale. The gas laws are the clearest case: $PV = nRT$ only holds when $T$ is in kelvin, because doubling the kelvin temperature genuinely doubles the average kinetic energy — something doubling celsius cannot claim, since $0°C$ is not "no heat."
So a chemistry student measuring a gas at $25°C$ converts to $298.15$ K before plugging into the ideal gas law, and a thermodynamics engineer does the same before any entropy or efficiency calculation.
What Is the Celsius to Kelvin Conversion?
Celsius and kelvin measure the same quantity with steps of the same size: a rise of one degree celsius is a rise of one kelvin. They differ only in where zero sits.
Celsius puts zero at the freezing point of water. The kelvin scale puts zero at absolute zero — the lowest temperature physically possible, where molecular motion ceases. Those zeros are $273.15$ units apart, which is why converting is a single addition with no stretching of the scale.
How to Convert Celsius to Kelvin
Why add $273.15$ instead of scaling the number? Because both scales count in identically sized degrees. You only shift the zero point; you never resize the ruler.
Method 1: Direct addition
Take the celsius value and add $273.15$.
$$K = °C + 273.15$$
Convert $25°C$:
$$K = 25 + 273.15 = 298.15 \text{ K}$$
Final answer: $25°C = 298.15$ K.
Method 2: Reverse the formula (kelvin back to celsius)
To check the result or convert the other way, subtract $273.15$:
$$°C = K - 273.15$$
Take that $298.15$ K back to celsius: $298.15 - 273.15 = 25°C$. The round trip closes, confirming the conversion.
Examples of Celsius to Kelvin Formula
Example 1
Convert $37°C$ (body temperature) to kelvin.
$$K = 37 + 273.15 = 310.15 \text{ K}$$
Final answer: $310.15$ K.
Example 2
Convert $-40°C$ (a deep winter low) to kelvin.
Wrong attempt. With a negative input it is tempting to subtract: $-40 - 273.15 = -313.15$ K. A negative kelvin is impossible — kelvin starts at zero — so the negative sign on the operation is the giveaway that the direction is wrong.
Correct.
$$K = -40 + 273.15 = 233.15 \text{ K}$$
Final answer: $233.15$ K. Even a cold $-40°C$ is a comfortably positive kelvin reading.
Example 3
Convert $100°C$ (water boiling at 1 atm) to kelvin.
$$K = 100 + 273.15 = 373.15 \text{ K}$$
Final answer: $373.15$ K.
Example 4
Convert $-196.15°C$ (liquid nitrogen) to kelvin.
$$K = -196.15 + 273.15 = 77 \text{ K}$$
Final answer: $77$ K — a clean integer, which is why $77$ K is the figure cryogenics labs quote.
Example 5
Standard room temperature in chemistry is $25°C$. Express it in kelvin for a gas-law calculation.
$$K = 25 + 273.15 = 298.15 \text{ K}$$
Final answer: $298.15$ K, the value tabulated as "standard" conditions in most textbooks.
Example 6
Convert $-273.15°C$ to kelvin.
$$K = -273.15 + 273.15 = 0 \text{ K}$$
Final answer: $0$ K — absolute zero, the lowest the kelvin scale reaches.
Common Mistakes With the Celsius to Kelvin Formula
Mistake 1: Subtracting instead of adding
Where it slips in: Conversions with a negative celsius input, where subtracting "feels" right.
Don't do this: Write $K = °C - 273.15$.
The correct way: Climbing from celsius up to kelvin always adds $273.15$.
Mistake 2: Using 273 instead of 273.15
Where it slips in: Quick estimates where the $0.15$ seems trivial.
Don't do this: Treat $0°C$ as exactly $273$ K for graded or lab work.
The correct way: $0°C$ is exactly $273.15$ K. Use $273$ only for a rough mental figure; carry $273.15$ into anything reported to decimals.
Mistake 3: Forgetting kelvin has no degree symbol
Where it slips in: Writing the converted value.
Don't do this: Write the answer as $298.15°$K.
Conclusion
The celsius to kelvin formula is $K = °C + 273.15$ — one addition, because both scales share degree size.
Reverse it with $°C = K - 273.15$ to go from kelvin back to celsius.
Kelvin can never be negative; a negative result means you subtracted when you should have added.
Use $273.15$, not $273$, whenever the answer carries decimals.
$0°C = 273.15$ K (water freezes) and $-273.15°C = 0$ K (absolute zero).
Practice These Three Before Moving On
Work through these, then check against the table above.
Convert $20°C$ to kelvin (a comfortable room).
Convert $-78.15°C$ to kelvin (dry ice).
A gas is heated from $15°C$ to $85°C$. Convert both readings to kelvin, then state the temperature change in kelvin.
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