Kelvin to Celsius Converter

1. Type the temperature in kelvin. The lowest allowed value is 0 K, which is absolute zero.
2. Pick the decimal places. Two decimals keep the .15 that most answers end with.
3. Read the Celsius value. Paste a data column into the bulk box to convert a whole lab table.

On this page, deg C means degrees Celsius and deg F means degrees Fahrenheit. The kelvin is written K, with no degree sign.

A kelvin to Celsius conversion moves the zero point of a temperature from absolute zero to 273.15 K. The size of the unit does not change, so the conversion is a single subtraction.

The kelvin is the SI base unit of thermodynamic temperature. NIST SP 330 defines Celsius temperature as the difference between a thermodynamic temperature T and the reference temperature T0 = 273.15 K. It writes this as t = T - T0. The same document states that the degree Celsius is equal in size to the kelvin.

Science students meet this conversion in almost every chemistry and physics course. Gas law problems, heat problems and data tables use kelvin, while thermometers, weather reports and lab rooms use Celsius. The conversion links the two without changing any measured quantity.

A kelvin value is not a "degree kelvin". The correct form is 300 K, read as "three hundred kelvins".

The formula subtracts 273.15 from the kelvin value, and nothing else.

1. Write down the kelvin value. Example: 300 K.
2. Subtract 273.15. Example: 300 - 273.15 = 26.85 deg C.
3. Check by adding back: 26.85 + 273.15 = 300 K.

Why Is the Number Exactly 273.15?

The number 273.15 is exact because it is part of the definition of the Celsius scale, not a measured value. Before 2019 the kelvin was defined by setting the triple point of water at exactly 273.16 K. The Celsius zero was fixed 0.01 K below that, at 273.15 K, which is close to the ice point.

In 2019 the SI changed how the kelvin itself is defined. BIPM and NIST now define the kelvin by fixing the Boltzmann constant at exactly 1.380 649 x 10^-23 J/K. The 273.15 offset stayed as it was. NIST SP 330 notes that the triple point of water is now a measured value, 273.16 K with a relative standard uncertainty of 3.7 x 10^-7.

A gas law answer of 298.15 K. 298.15 - 273.15 = 25 deg C. This is the standard lab temperature printed in many chemistry data tables.

A reaction flask at 350 K. 350 - 273.15 = 76.85 deg C. The water bath must be set close to 77 deg C.

Absolute zero, 0 K. 0 - 273.15 = -273.15 deg C. No Celsius reading can be lower than this.

The triple point of water, 273.16 K. 273.16 - 273.15 = 0.01 deg C. It is one hundredth of a degree above the Celsius zero.

A cold winter value of 250 K. 250 - 273.15 = -23.15 deg C. Any kelvin value under 273.15 gives a negative Celsius answer.

A temperature rise, not a temperature. A sample warms from 290 K to 305 K. The rise is 15 K, which is 15 deg C. Do not subtract 273.15 from a difference.

To go the other way, the Celsius to kelvin converter adds 273.15.

The subtraction is exact, so these points decide whether your answer is right.

Rounding the Offset to 273

Using 273 in place of 273.15 makes every answer 0.15 deg C too high. That is fine for a rough check and wrong for a graded answer. For 300 K the shortcut gives 27 deg C, not 26.85 deg C.

Significant Figures

Subtracting an exact number keeps the decimal places of your input. A reading of 295 K has no decimals, so report 22 deg C. A reading of 295.4 K gives 22.25 deg C. The reading is only good to a tenth of a kelvin, so the answer is only good to a tenth of a degree.

Temperature or Temperature Difference

A temperature needs the offset. A temperature difference does not. NIST SP 330 states that a difference has the same number in kelvins and in degrees Celsius.

The Scale Behind the Thermometer

Lab thermometers report values on the International Temperature Scale of 1990. NIST SP 330 calls these close approximations to true thermodynamic temperature. The gap is far smaller than classroom equipment can show.

Kelvin and Celsius share one unit size and differ only in where zero sits.

US readers who need the everyday scale can use the kelvin to Fahrenheit converter.

Reporting a Gas Law Answer

Gas laws need kelvin, because volume and pressure scale with absolute temperature. A gas warmed from 300 K to 600 K doubles in volume at constant pressure. The same two states read 26.85 deg C and 326.85 deg C, which is not a doubling. Solve in kelvin with the ideal gas law calculator, then convert the final answer to Celsius.

Reading a Data Table

Textbook tables list melting points, boiling points and reaction conditions in kelvin. Converting to Celsius tells you which hot plate, bath or freezer setting matches the table.

Checking a Heat Calculation

Heat formulas use a temperature change. A change of 25 K equals a change of 25 deg C, so both units give one answer. The heat energy calculator shows that step.

Sense-Checking Homework

A Celsius value is easier to judge. An answer of 310.15 K means little at first sight. Its Celsius form, 37 deg C, is clearly close to normal body temperature.

Values from deg C = K - 273.15, exact to 2 decimals, with Fahrenheit for reference.

Kelvin values that end in .15 give whole Celsius numbers. Whole kelvin values give Celsius numbers that end in .85 above the ice point and in .15 below it.

1. Adding 273.15

Adding converts Celsius to kelvin. For 300 K it gives 573.15, which is far too hot. The Celsius number is always the smaller one.

2. Dropping the .15

Subtracting 273 leaves an error of 0.15 deg C in every answer.

3. Writing Degrees Kelvin

The unit is the kelvin and the symbol is K. A degree sign belongs with Celsius and Fahrenheit only.

4. Converting a Difference

A 10 K rise is a 10 deg C rise. Subtracting 273.15 from it gives a meaningless -263.15.

5. Using Celsius Inside a Gas Law

Ratios such as V1/T1 = V2/T2 fail with Celsius numbers. Convert the answer at the end, never the inputs.

6. Typing a Celsius Value as Kelvin

A value such as 25 is almost always 25 deg C. Entered as kelvin it returns -248.15 deg C.

The result is exact, because 273.15 is a defined number. The only rounding is the display setting you choose.

0 K is -273.15 deg C. This is absolute zero, the lowest temperature on any scale, and it is also -459.67 deg F.

The exact number is 273.15. Many school problems allow 273, which leaves every answer 0.15 deg C too high. Use 273.15 unless your teacher says otherwise.

No. A value of -273.15 deg C equals 0 K, and nothing can be colder than absolute zero. The converter blocks lower values.

The kelvin is a unit in its own right, like the meter or the second. NIST notes it is used by itself, so the correct form is 300 K, not 300 degrees K.

The kelvin is defined by fixing the Boltzmann constant at exactly 1.380 649 x 10^-23 joules per kelvin. Before 2019 it was tied to the triple point of water.

No. The offset is part of the definition of the Celsius scale and stayed exact. Only the way the kelvin itself is realised changed.

Yes. The two units are equal in size, so any rise or fall has the same number in both. Only the zero point differs.

Never. The kelvin number is always 273.15 larger than the Celsius number, so the two can never match.

Use =A2-273.15 with the kelvin value in cell A2, then fill the formula down the column.

NIST lists room temperature as about 20 deg C. That is 293.15 K, often rounded to 293 K in class problems.

No. Your numbers never leave the page. Anything you press Save on is kept in this browser only.

• Celsius temperature defined as t/deg C = T/K - 273.15, and the kelvin fixed by the Boltzmann constant (NIST SP 330, The International System of Units, section 2).
• Degree Celsius to kelvin, T/K = t/deg C + 273.15 (NIST SP 811, Appendix B.8).
• SI base unit kelvin, defined through the Boltzmann constant (BIPM, International Bureau of Weights and Measures).
• Exact temperature conversion table and the triple point cell at 273.16 K (NIST Office of Weights and Measures).