Convert °C to K degrees Celsius ↔ kelvins Free Online Secure
Type a temperature in degrees Celsius and read it in kelvins at once. The relation is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
Nothing uploaded
Common values — Convert °C to K (degrees Celsius ↔ kelvins)
| °C | K |
|---|---|
| -40 | 233.15 |
| -10 | 263.15 |
| 0 | 273.15 |
| 10 | 283.15 |
| 20 | 293.15 |
| 25 | 298.15 |
| 37 | 310.15 |
| 100 | 373.15 |
Where these units come from
Fahrenheit set his zero in 1724 at the coldest thing he could make reliably — a brine of ice, water and salt — and put body heat near 96, so that his scale could be divided and divided again. Celsius published his in 1742 the other way up, with 0 for boiling and 100 for freezing; it was turned around after his death, which is why he is remembered for a scale he never used. Kelvin is the only one of the four that begins where there is nothing left to take away: absolute zero. Since 2019 it is defined by fixing the Boltzmann constant rather than by the triple point of water, so the kelvin no longer depends on a particular substance. Rankine is Fahrenheit's degree counted from absolute zero, and Réaumur, with 80 degrees between freezing and boiling, survives mostly in old recipes and in a few cheese dairies.
Where you meet them
Celsius runs weather, cooking and medicine almost everywhere; Fahrenheit does the same work in the United States and keeps a foothold in oven settings and in American recipes. Kelvin belongs to physics, to the colour temperature of lamps and screens, and to anything cold enough that negative numbers would be a nuisance. Rankine appears in American thermodynamics, and Réaumur mainly in historical texts — which is exactly why an old recipe can mislead someone reading it as Celsius.
Questions people ask
Why can't I just double the number to double the temperature?
Because three of these scales have arbitrary zeros. Twenty degrees Celsius is not twice as hot as ten; it is ten degrees further along a scale whose zero was chosen for the convenience of water. Only kelvin and Rankine begin at absolute zero, where there is genuinely nothing left, so only on those two does doubling the figure mean doubling the thermal energy. That is also why every conversion here adds or subtracts as well as multiplying.
Why does the kelvin have no degree sign?
Because since 1967 it is a unit in its own right, written K and read as kelvin, not degrees Kelvin. Zero kelvin is absolute zero, which is −273.15 °C, and a step of one kelvin is exactly the same size as a step of one degree Celsius. That makes conversion a matter of adding or subtracting 273.15.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
Lab sheets, climate reports and data sheets: Celsius into kelvin
A chemistry problem gives the temperature as 25 °C but the formula asks for kelvin. A climate report speaks of 1.5 degrees of warming, a data sheet quotes the melting point of gallium as 302.9 K, and a Linux terminal prints a processor temperature in thousandths of a degree. Anyone who studies or works with science meets both scales on the same page. To convert °C to K, add 273.15: 0 °C is 273.15 K, 25 °C is 298.15 K and 100 °C is 373.15 K. The calculator above adds as you type, and the sections below show what the figures mean for exams, instruments, climate science and computing.
You will find the formula with worked examples, a short profile of each scale, a comparison, a long set of practical cases and the slips to avoid. The arithmetic runs in your browser, and what you type is not sent anywhere.
How to convert °C to K: add 273.15
The formula
The formula is K = °C + 273.15. So 20 °C is 293.15 K, 37 °C is 310.15 K and −78.5 °C, the sublimation point of dry ice, is 194.65 K. To go back, subtract 273.15. In quick work you may add 273, and in careful work you keep the .15.
Worked examples
Room temperature of 22 °C is 295.15 K. The boiling point of ethanol, 78.4 °C, is 351.55 K. Mercury freezes at −38.83 °C, which is 234.32 K. Liquid nitrogen at −196 °C is 77.15 K, and the lowest possible temperature, −273.15 °C, is 0 K.
Why only an addition
A kelvin and a degree Celsius are the same size. The only difference between the scales is where zero sits: kelvin starts at absolute zero and Celsius at the freezing point of water, 273.15 K higher.
A quick sanity test
A temperature in kelvin can never be negative. If a “convert °C to K” calculation gives a negative number, either the sign was lost or the starting value was below absolute zero, which cannot occur.
The Celsius scale
The everyday scale
The Celsius scale puts 0 at the freezing point of water and 100 at its boiling point at standard atmospheric pressure. It is the everyday scale of nearly every country, and the one on most laboratory thermometers.
Where it appears
Weather forecasts, ovens, medical thermometers and school experiments.
The kelvin
The absolute scale
The kelvin is the SI base unit of temperature. Its zero is absolute zero, and since 2019 it is defined by fixing the Boltzmann constant. It is written K, without a degree sign, and called simply kelvin, never degree kelvin.
Where it appears
Physics, chemistry, thermodynamics, engineering data sheets and climate models.
Celsius against kelvin
The two scales have identical steps, so a change of 1 °C is a change of 1 K. A warming of 1.5 °C is therefore a warming of 1.5 K. The values themselves differ by 273.15, so 15 °C is 288.15 K. The only place where the offset matters is when you state an absolute temperature.
The addition is exact, so there is no rounding in the conversion. The common slip is to treat an interval as a position on the scale.
Where the conversion shows up
Standard conditions in chemistry
IUPAC defines standard temperature and pressure as 273.15 K, which is 0 °C, and 1 bar. The common SATP convention is 298.15 K, which is 25 °C. One mole of an ideal gas occupies 22.41 litres at 273.15 K and 1 atm, and about 24.5 litres at 298.15 K. Exam questions often supply a temperature in Celsius and expect you to run a “convert °C to K” calculation before plugging into a formula.
Heating water
The specific heat of water is 4.184 joules per gram per kelvin. Heating one kilogram from 20 °C to 100 °C takes a rise of 80 K, so 80 × 4.184 = 334.7 kJ, which is about 0.093 kWh. The temperature rise is the same number in kelvin and Celsius, so no 273.15 is needed for the difference.
Climate and temperature anomalies
When scientists say the world has warmed by about 1.1 to 1.2 K since pre-industrial times, they mean the same thing as 1.1 to 1.2 °C. The Paris target of 1.5 °C is a warming of 1.5 K. The absolute global mean surface temperature is near 288 K, which is 15 °C, but anomalies are quoted without the offset.
Reference conditions on data sheets
Data sheets for gases, liquids and materials state the reference temperature. NIST often uses 20 °C, or 293.15 K, while chemistry tables use 25 °C, or 298.15 K, and physics tables 273.15 K. Comparing values from two sources without noticing the reference conditions leads to small but real discrepancies, so a quick convert °C to K on each source clears them up.
Batteries and cold
A lithium-ion cell is usually rated for operation between −20 and 60 °C, which is 253.15 to 333.15 K. Capacity falls sharply in the cold, and charging below 0 °C, or 273.15 K, can damage the cell. Engineers who model the battery use kelvin in the Arrhenius equations that describe how its chemistry slows in winter.
Spacecraft
The surface of a satellite in low Earth orbit can swing between about −150 and +120 °C as it passes between sunlight and shadow, which is 123 to 393 K. Designers use radiators, heaters and coatings to keep electronics near 273 to 313 K. All thermal models for spacecraft are written in kelvin, and the summary tables often add a Celsius column.
Medicine and cooling therapy
After cardiac arrest, patients may be cooled to 32 to 34 °C, which is 305.15 to 307.15 K, to protect the brain. Normal body temperature is 37 °C, or 310.15 K. Hospitals record in Celsius, while researchers who model heat transfer in tissue convert to kelvin, so the “convert °C to K” calculation step recurs in clinical studies.
Deep ocean and ice
The deep ocean sits at about 2 to 4 °C, which is 275 to 277 K, a difference of only 2 K between the cold and warm limits. Ice in Antarctica can be at −50 °C, or 223 K, near the surface. Oceanographers talk about changes of tenths of a degree, and these have the same value in either scale.
Thermal conductivity and U-values
Thermal conductivity is given in watts per metre per kelvin. A mineral wool of 0.035 W/(m·K) conducts 0.035 watts through a metre of thickness for each kelvin of temperature difference between the faces. Because the unit contains a kelvin in the denominator, it is also per degree Celsius, and no conversion is needed for the difference.
Spreadsheet formulas
Put Celsius values in column A and use =A2+273.15 in column B. For the reverse, use =B2-273.15. For temperature differences, copy the value unchanged. Format with two decimals, and write K in the header with no degree sign. Keep the source column visible so that a reviewer can check the offset.
Rounding and significant figures
273.15 has five significant figures, but a typical thermometer gives only three. Converting 21.4 °C gives 294.55 K, and you should round it to 294.6 K. Do not report more decimals than the measurement supports, and keep the unit symbol after a space.
A mental shortcut
Add 273 and, if it matters, 0.15. For 20 °C that is 293, or precisely 293.15. For −10 °C it is 263, or 263.15. For everyday and school-level questions the shortcut is accurate, and for a metrology report you keep the full 273.15.
Landmarks to memorise
0 °C is 273.15 K, 20 °C is 293.15 K, 25 °C is 298.15 K, 37 °C is 310.15 K and 100 °C is 373.15 K. Add the absolute zero at −273.15 °C and you have six anchors that cover most of the questions in an introductory course.
Reference ladder
−273.15 °C is 0 K. −196 °C is 77.15 K. −78.5 °C is 194.65 K. −40 °C is 233.15 K. 0 °C is 273.15 K. 100 °C is 373.15 K. 1,000 °C is 1,273.15 K. A ladder like this helps check a table at a glance.
The temperature fixed points
The international temperature scale is built on fixed points: the triple point of argon at 83.8058 K, mercury at 234.3156 K, water at 273.16 K, gallium melting at 302.9146 K, indium at 429.7485 K, tin at 505.078 K and zinc at 692.677 K. In Celsius, the tin point is 231.928 °C and zinc 419.527 °C. Calibration laboratories report in either scale.
Checking by reversing
To check any convert °C to K result, subtract 273.15 and see whether you return to the start. If 25 °C became 298.15 K, then 298.15 minus 273.15 is 25. A second test is the sign: the kelvin value is always positive, and always larger than the Celsius value.
Writing the unit correctly
Write 300 K with a space and no degree sign. The SI brochure calls the unit kelvin, with a lower-case k when it is spelled out. Celsius takes the degree sign and a capital C, as in 25 °C. Mixing the forms, such as °K, is a common marking error in student work.
Temperature differences in practice
When a process heats a liquid by 30 °C, it heats it by 30 K, and the heat required is mass times specific heat times 30. Adding 273.15 to a difference is a frequent error in homework: 30 °C of heating is not 303.15 K of heating.
Radiative heat transfer
Radiated power per unit area is proportional to T⁴, with T in kelvin. A surface at 300 K emits 459 W/m², and one at 330 K emits 672 W/m², about 46 per cent more for a 10 per cent rise in absolute temperature. If the numbers were in Celsius, a rise from 27 °C to 57 °C would look like a doubling, which it is not.
Exam strategy
In a timed exam, convert first. Write the temperature in kelvin at the top of the working, check it against the units in the formula, and only then calculate. A “convert °C to K” calculation done at the start prevents a wrong answer that no later step can rescue, and examiners often award marks for the correct conversion.
Why lakes freeze from the top
Fresh water is densest at 3.98 °C, which is 277.13 K. Colder water is lighter and rises, so ice forms at the surface while the deep layer stays near 4 °C. Limnologists track lake temperature profiles in Celsius, and the density formulas they use take the temperature in either scale, but equations of state are often written in kelvin, which makes a “convert °C to K” calculation step part of the modelling.
Autoclaves and sterilisation
Hospital autoclaves sterilise instruments at 121 °C, or 394.15 K, for at least 15 minutes, or at 134 °C, or 407.15 K, for shorter cycles. A pressure cooker at home reaches about 115 to 120 °C, or 388 to 393 K. Sterilisation standards specify the temperature in Celsius, while the kinetic models of microbial death use kelvin, so the two scales sit in the same validation file.
Heat pumps and refrigerators
The ideal efficiency of a refrigerator depends on the absolute temperatures. A freezer at −18 °C, or 255.15 K, in a kitchen at 22 °C, or 295.15 K, has a theoretical coefficient of performance of 255.15 divided by 40, which is 6.4. Real machines reach about one third of that. The “convert °C to K” calculation step turns kitchen figures into the quantities that thermodynamics needs.
Melting and boiling in everyday substances
Gallium melts at 302.91 K, which is 29.76 °C, so it liquefies in a warm hand. Ethanol boils at 351.5 K, or 78.4 °C, and acetone at 329.2 K, or 56 °C. Mercury boils at 629.9 K, or 356.7 °C. A chemistry student building a table of boiling points will convert many values between scales.
Polymers and glass transitions
A glass transition temperature describes where a polymer changes from hard to rubbery. PET sits near 348 K, which is about 75 °C, PVC near 353 K, or 80 °C, and polystyrene near 373 K, or 100 °C. Plastics engineers quote these in either scale, and a bottle that softens in a hot dishwasher shows why it matters.
The lapse rate
The standard atmosphere cools by 6.5 K for each kilometre of altitude. A mountain pass at 3,000 m is therefore about 19.5 K colder than the valley, which is also 19.5 °C colder, because it is a difference. If the valley is at 25 °C, the pass is near 5.5 °C, and in kelvin 298.15 K falls to 278.65 K.
The speed of sound
The speed of sound in dry air is about 331.3 times the square root of T divided by 273.15, with T in kelvin. At 20 °C, or 293.15 K, the speed is 343.2 m/s. At −20 °C, or 253.15 K, it falls to 319 m/s. Acoustics calculations fail if Celsius is inserted directly, so a “convert °C to K” calculation is the first line of the working.
Wien's law and thermal cameras
Wien's law says that the peak wavelength of thermal radiation is 2898 micrometre-kelvins divided by the temperature in kelvin. A human body at 310 K, which is 37 °C, peaks near 9.3 micrometres, in the far infrared, which is why thermal cameras work in that band. The Sun at 5772 K peaks near 502 nanometres, in the green.
Computer temperatures
On Linux, the temperature files under /sys/class/thermal report millidegrees Celsius, so a reading of 45000 means 45 °C, or 318.15 K. A processor with a maximum junction temperature of 100 °C is limited to 373.15 K. Monitoring tools show Celsius, while some firmware tables and kernel documents use decikelvins, so a conversion script is common.
Data centres and cooling
Recommended inlet air for data centres is 18 to 27 °C, which is 291.15 to 300.15 K. A temperature rise across a server of 12 K moves the air from 24 °C to 36 °C. Airflow calculations use the difference, so they work in either scale, but the density of air in the same calculation depends on absolute temperature in kelvin.
More values for convert °C to K
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- -273.15 °C = 0 K
- -100 °C = 173.15 K
- -20 °C = 253.15 K
- 10 °C = 283.15 K
- 15 °C = 288.15 K
- 25 °C = 298.15 K
- 30 °C = 303.15 K
- 50 °C = 323.15 K
Common slips when you convert °C to K
Forgetting the conversion in a gas law
Gas laws need absolute temperature. Using 25 instead of 298.15 gives an answer that is off by a factor of twelve.
Adding to a difference
A change of 10 °C is a change of 10 K. Do not add 273.15 to it.
Writing °K
The kelvin takes no degree sign. Write 300 K.
Short answers
How do I convert °C to K?
Add 273.15.
What is 25 °C in kelvin?
298.15 K.
What is 0 °C in kelvin?
273.15 K.
What is absolute zero in Celsius?
−273.15 °C.
Does my number leave my device?
No, the calculation runs in your browser.
More pages and a wrap-up
For other temperature conversions see Convert °C to °F, Convert °C to °Ré, Convert °F to K, Convert °R to K, Convert K to °F, Convert km to mi, Convert mi to km, Convert L to gal, Convert gal to L. The reverse direction is Convert K to °C.
In short, a “convert °C to K” calculation means adding 273.15 and writing the unit beside the answer. Your number is computed locally, with no account and no upload.