To convert kelvin to Celsius, subtract 273.15. A temperature of 300 K is 26.85 °C because 300 − 273.15 = 26.85. To convert Celsius to kelvin, add 273.15. The two scales have equally sized steps, but their zero points are different.
Kelvin is an absolute temperature scale: its zero is absolute zero. That makes it useful for comparing thermodynamic temperatures and interpreting scientific formulas. Before doing arithmetic, identify whether your number describes a temperature, a temperature change, or the color appearance of light. The same letter K can appear in contexts where those distinctions matter.
What does kelvin measure?
The kelvin is the SI base unit of thermodynamic temperature. Its symbol is an uppercase K, without a degree sign. Write 300 K, with a space between the value and symbol. In running text, the unit name is lowercase: one kelvin, or several kelvins. The symbols °C and °F, by contrast, include degree signs.
The modern definition fixes the value of the Boltzmann constant, connecting temperature with an energy scale. NIST’s temperature reference explains this definition and identifies 0 K as absolute zero. You do not need the constant to make ordinary conversions; the exact Celsius offset is enough.
Absolute zero corresponds to −273.15 °C or −459.67 °F. A negative Celsius value can therefore describe an ordinary positive kelvin temperature. For example, −20 °C equals 253.15 K. The minus sign says that the value is below the Celsius origin, not below absolute zero.
Why is the offset exactly 273.15?
Celsius temperature is defined relative to thermodynamic temperature with an exact offset of 273.15 K. Imagine two number lines whose tick marks have identical spacing, with their zeros placed at different positions. Changing scales shifts the number; it does not stretch the distance between neighboring marks.
That is why converting kelvin and Celsius requires addition or subtraction alone. A temperature of 273.15 K corresponds to 0 °C, and 274.15 K corresponds to 1 °C. The relationship concerns the scales themselves. It does not depend on the object, material, or location whose temperature is being described.
Kelvin, Celsius and Fahrenheit formulas
In the formulas below, K, C and F stand for the numerical temperature values expressed in kelvins, degrees Celsius and degrees Fahrenheit. Parentheses specify which operation happens first.
Kelvin to Celsius: C = K − 273.15.
Celsius to kelvin: K = C + 273.15.
Kelvin to Fahrenheit: F = (K − 273.15) × 9/5 + 32.
Fahrenheit to kelvin: K = (F − 32) × 5/9 + 273.15.
For 310 K, first subtract 273.15 to get 36.85 °C. Multiply 36.85 by 1.8 and add 32 to get 98.33 °F. Reversing the calculation gives (98.33 − 32) ÷ 1.8 + 273.15 = 310 K.
A compact alternative is F = 1.8K − 459.67. Both versions produce the same result. The longer form makes the intermediate Celsius value visible, which can help catch a misplaced sign. For the two degree scales alone, see our Celsius and Fahrenheit conversion guide.
Reference values for checking arithmetic
| Kelvin (K) | Celsius (°C) | Fahrenheit (°F) |
|---|---|---|
| 0 | −273.15 | −459.67 |
| 250 | −23.15 | −9.67 |
| 273.15 | 0 | 32 |
| 293.15 | 20 | 68 |
| 300 | 26.85 | 80.33 |
| 400 | 126.85 | 260.33 |
The entries are conversions of the stated numbers, not claims about the exact temperature of familiar objects. For a quick check, every kelvin value should be 273.15 greater than its Celsius value. That numerical gap stays constant throughout the table.
Temperature changes do not need the offset
A temperature value locates a point on a scale. A temperature interval measures the distance between two points. NIST’s SI guidance allows temperature intervals to be expressed in either kelvins or degrees Celsius, with the same numerical value.
Consider a change from 280 K to 295 K. The difference is 15 K. Those endpoints are 6.85 °C and 21.85 °C, whose difference is 15 °C. Subtracting the endpoints cancels the offset. A rise of 15 K is therefore a rise of 15 °C or 27 °F, not 288.15 °C.
For differences, use ΔC = ΔK and ΔF = 1.8 × ΔK. The delta symbol means “change in.” A fall of 5 K can be written as a signed change of −5 K; that negative interval does not mean the final temperature is below absolute zero.
Why ratios require an absolute scale
For a ratio of thermodynamic temperatures, convert both values to kelvins first. A value of 400 K is twice 200 K. Their Celsius equivalents, 126.85 °C and −73.15 °C, cannot be divided to obtain that same physical ratio because Celsius has a shifted origin.
Similarly, 40 °C is not twice the thermodynamic temperature of 20 °C. The appropriate ratio is 313.15 ÷ 293.15, approximately 1.068. Avoid translating this into “twice as hot” in everyday language: a temperature ratio does not, by itself, describe perceived warmth or the total energy stored in two objects.
Color temperature describes light appearance
A lamp marked 3000 K is using kelvins to describe its light’s color appearance. It does not mean the lamp housing has a physical temperature of 3000 K. Correlated color temperature compares the light with a reference radiator, as the Department of Energy explains.
Lower color-temperature numbers generally describe warmer-looking, yellowish white light; higher numbers describe cooler-looking, bluish white light. Those everyday words concern appearance. Subtracting 273.15 from a lamp’s color-temperature label does not reveal its surface temperature.
Keep the units and precision attached
Use the full 273.15 offset during calculation, then round the final answer as needed. An exact conversion does not improve an approximate starting measurement. Our rounding and precision guide explains that distinction. Always retain the unit, and label a difference explicitly so it cannot be mistaken for a temperature value.
For further reading, browse temperature and thermodynamics books on Amazon.
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