Celsius to Kelvin Converter

1. Type the temperature in degrees Celsius. Values down to -273.15 are accepted.
2. Set the decimal places to match your data. Keep as many decimals as the Celsius reading has.
3. Read the kelvin result. For a data table, paste the whole Celsius column into the bulk box.

In this guide, deg C means degrees Celsius. Kelvin is written K with no degree sign. The tool itself shows the degree sign for Celsius.

A Celsius to kelvin conversion moves a temperature from a scale that starts near the ice point of water to a scale that starts at absolute zero. The kelvin is the SI base unit of thermodynamic temperature.

The two scales share one degree size. NIST SP 330 states that the degree Celsius is by definition equal in magnitude to the kelvin. They differ only in where zero sits. SP 330 defines Celsius temperature as the thermodynamic temperature minus a reference of exactly 273.15 K. So the conversion is a single addition, with no multiplying at all.

Students and lab workers need it because thermometers and everyday data are in Celsius, while physics and chemistry formulas need an absolute temperature. NASA notes that the equation of state for a gas must use kelvin, not degrees Celsius, in the metric system.

This is not the tool for weather in US units. For that, use the Celsius to Fahrenheit converter.

The formula slides the zero point down by 273.15 degrees, from the Celsius zero to absolute zero, and leaves the degree size alone.

1. Write down the Celsius value with its sign. Example: 25.
2. Add 273.15. Example: 25 + 273.15 = 298.15 K.
3. Check by subtracting: 298.15 - 273.15 = 25 deg C.

NIST SP 811 prints the relation as T/K = t/deg C + 273.15. The number 273.15 is not a measured value with an error bar. It is a fixed part of the definition of the Celsius scale, so the conversion adds no uncertainty.

For negative Celsius values the addition becomes a subtraction in practice. -40 + 273.15 is 233.15 K. The result is never negative, because no temperature lies below -273.15 deg C.

A gas at 20 deg C in a chemistry problem. 20 + 273.15 = 293.15 K. This is the value that goes into PV = nRT. The ideal gas law calculator expects kelvin for the same reason.

Heating that gas to 40 deg C at constant pressure. 40 + 273.15 = 313.15 K. The volume grows by the ratio 313.15 / 293.15, which is 1.068, or 6.8 percent. Using the Celsius numbers would give 40 / 20 = 2, a doubling that does not happen.

A sealed tank warmed from 20 deg C to 80 deg C. The kelvin values are 293.15 and 353.15 K. At fixed volume the pressure rises by 353.15 / 293.15, about 20.5 percent.

A very cold sample at -268.95 deg C. -268.95 + 273.15 = 4.2 K. NIST notes that mercury loses all electrical resistance at 4.2 kelvins.

The triple point of water at 0.01 deg C. 0.01 + 273.15 = 273.16 K. From 1954 until the SI was revised, this point defined the kelvin.

Adding 273.15 is simple. These four points are where real work goes wrong.

Temperature or Temperature Difference

A reading of 30 deg C is 303.15 K. A rise of 30 deg C is a rise of 30 K. NIST SP 330 says a temperature difference has the same numerical value in either unit. Adding 273.15 to a difference is an error.

The 273 Shortcut

Many textbooks round the offset to 273. That makes every answer 0.15 K too low. At 298.15 K the error is about 0.05 percent, which is harmless in most homework but visible in careful lab work.

Significant Figures

Addition keeps decimal places, not significant figures. A thermometer read to the nearest whole degree at 25 deg C supports an answer of 298 K. Writing 298.15 K claims precision the reading does not have.

The Source Scale

A Fahrenheit value typed as Celsius gives a wrong kelvin result with no warning. 77 deg F is 298.15 K, but 77 deg C is 350.15 K. Use the "Convert from" box for Fahrenheit data.

Celsius and kelvin have equal steps and different starting points. The table shows what that means in practice.

The ratio row is the reason kelvin exists. 20 deg C is not twice as hot as 10 deg C. In kelvin the two are 293.15 and 283.15, a difference of about 3.5 percent.

Gas Law Problems

The Charles law, the combined gas law and the ideal gas law all need absolute temperature. Convert every Celsius reading before it enters the equation. Our article on the ideal gas law walks through a full problem.

Thermodynamics and Efficiency

Formulas that divide one temperature by another only work in kelvin. A heat engine running between 25 deg C and 500 deg C runs between 298.15 K and 773.15 K.

Lab Reports and Data Tables

Many journals and courses ask for SI units. A column logged in Celsius converts in one paste with the bulk box.

When You Do Not Need It

Heat calculations that use a temperature change, such as mass times specific heat times the change, give the same answer in either unit. The heat energy calculator works on that change directly.

Exact values from K = deg C + 273.15.

Every 1 deg C step adds exactly 1 K, so any value between the rows is a simple addition.

The offset 273.15 is the distance from absolute zero to the zero of the Celsius scale, counted in kelvins.

NIST records that in 1848 Lord Kelvin published a paper on an absolute thermometric scale and placed absolute zero at -273 deg C. In 1954 the General Conference on Weights and Measures adopted the kelvin and fixed the triple point of water at 273.16 K. Celsius temperature is counted from a reference of 273.15 K, which NIST SP 330 describes as close to the ice point. That reference sits 0.01 K below the triple-point value.

The kelvin is now defined through the Boltzmann constant, fixed at 1.380 649 x 10^-23 joules per kelvin. NIST SP 330 explains that the triple point of water is therefore measured again, not fixed. The 273.15 offset between Celsius and kelvin stayed exact through that change, because it is part of the definition of Celsius temperature.

1. Subtracting 273.15

25 - 273.15 gives -248.15. A negative kelvin value is the sign of this error. Celsius to kelvin always adds.

2. Writing a Degree Sign with K

The unit is the kelvin, symbol K. It takes no degree sign and no word "degrees". Write 300 K.

3. Using 273.16

273.16 K is the old triple point of water, not the Celsius offset. Using it makes every answer 0.01 K too high.

4. Adding the Offset to a Difference

A 15 deg C rise is a 15 K rise. Writing 288.15 K for it confuses a change with a reading.

5. Leaving Celsius in a Ratio

Dividing 40 deg C by 20 deg C to compare two gas states gives 2. The correct ratio in kelvin is 1.068.

6. Multiplying as Well as Adding

A factor of 9/5 belongs to Fahrenheit and Rankine. Celsius to kelvin has no multiplier.

The conversion is exact, because 273.15 is fixed by the definition of the Celsius scale. Only the display is rounded.

0 deg C is 273.15 K. It is the zero of the Celsius scale, close to the temperature at which water freezes.

25 deg C is 298.15 K. Chemistry problems often use this value as a standard room condition.

A room at 20 deg C is 293.15 K. NIST describes room temperature as about 21 deg C, which is roughly 294 K.

The exact offset is 273.15. The value 273 is a rounded shortcut that leaves every answer 0.15 K low.

No. The kelvin scale starts at absolute zero, so 0 K is its lowest value. A negative result means the offset was subtracted or the input is wrong.

The kelvin is a unit in its own right, like the metre or the second. NIST notes that it is not expressed in degrees the way Celsius and Fahrenheit are.

Yes. Both units have the same size, so a temperature difference has the same number in each. Only readings need the 273.15 offset.

Type =A2+273.15 in a cell, with the Celsius value in A2, and fill the formula down the column.

-273.15 deg C is exactly 0 K. This is absolute zero, the lower limit of the thermodynamic temperature scale.

No. A change of 12 deg C equals a change of 12 K, so the heat calculation gives the same result in either unit.

No. The math runs in your browser. Saved results stay in your own browser storage until you clear them.

• Definition of the kelvin and of Celsius temperature, with the 273.15 K reference (NIST SP 330, The International System of Units, section 2).
• Conversion relation T/K = t/deg C + 273.15 (NIST SP 811, Appendix B.9).
• The kelvin as the SI unit of thermodynamic temperature (Bureau International des Poids et Mesures).
• Lord Kelvin, the 1848 absolute scale and the 1954 adoption of the kelvin (NIST, Kelvin: History).
• Absolute temperature in kelvin for the equation of state (NASA Glenn Research Center).