Convert K to °C kelvins ↔ degrees Celsius Free Online Secure
Type a temperature in kelvins and read it in degrees Celsius at once. The relation is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert K to °C (kelvins ↔ degrees Celsius)
| K | °C |
|---|---|
| 0 | -273.15 |
| 100 | -173.15 |
| 200 | -73.15 |
| 273.15 | 0 |
| 293.15 | 20 |
| 310.15 | 37 |
| 373.15 | 100 |
| 500 | 226.85 |
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 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.
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.
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.
Stars, planets and telescopes: kelvin into Celsius
The Sun's surface is 5772 K, a gas giant's cloud tops are 165 K and a space telescope's mirror is kept at 40 K. Observatory pages and textbooks quote kelvin throughout, yet most readers judge hot and cold in Celsius. To convert K to °C, subtract 273.15: 5772 K becomes 5498.85 °C, 165 K becomes −108.15 °C and 40 K becomes −233.15 °C. The calculator above subtracts as you type, and the sections below tie the figures to stars, planets, moons and instruments.
Expect the formula with worked examples, a short profile of each scale, a comparison, a long tour of astronomy and the slips to avoid. All arithmetic stays in your browser.
How to convert K to °C: subtract 273.15
The formula
The formula is °C = K − 273.15. So 300 K is 26.85 °C, 373.15 K is 100 °C and 4 K is −269.15 °C. To go back, add 273.15. For a quick estimate you can subtract 273, which leaves an error of only 0.15 degrees.
Worked examples
Venus has a surface near 737 K, which is 463.85 °C. Mars averages 210 K, or −63.15 °C. The Moon at midday reaches about 390 K, or 116.85 °C, and drops to 95 K at night, or −178.15 °C. Pluto sits near 44 K, which is −229.15 °C.
Why a plain subtraction
A kelvin and a Celsius degree are the same size. The scales differ only in where they start: kelvin at absolute zero, Celsius at the freezing point of water, which is 273.15 K higher.
A quick sanity test
Celsius is always the lower number. If a “convert K to °C” calculation result is larger than the starting value, you added by mistake.
The kelvin
The scale of astronomers
The kelvin is the SI unit of temperature, with zero at absolute zero. Astronomers use it because stellar physics and radiation laws are written in absolute temperature, and because a temperature in Celsius would lose meaning at millions of degrees.
Where it appears
Star catalogues, planetary fact sheets, telescope specifications and cosmology papers.
The Celsius scale
The scale of everyday experience
The Celsius scale sets 0 at the freezing point of water and 100 at its boiling point at sea level. It is how people judge hot and cold, so it is the scale that makes astronomical numbers feel real.
Where it appears
Press releases, school lessons, weather and popular science articles.
Kelvin against Celsius
The two scales have the same step, so a rise of 10 K is a rise of 10 °C. The values differ by 273.15, and that matters only for temperatures near everyday ranges. At the surface of a star the offset is a rounding error: 5772 K and 5499 °C are almost the same statement.
The subtraction is exact, so no rounding is involved in the conversion. The classic slip is to confuse a temperature with a difference.
Where the conversion shows up
The Sun and other stars
The Sun's photosphere is at 5772 K, or 5498.85 °C. Sunspots are cooler, near 3800 K, or 3527 °C. The core reaches about 15.7 million K. Cool red dwarfs sit near 3000 K, which is 2727 °C, while the hot star Rigel reaches 12,100 K, or 11,827 °C. A “convert K to °C” calculation for each star helps a reader grasp the scale without the offset mattering.
Planets in the Solar System
Mercury swings from about 700 K, or 427 °C, by day to 100 K, or −173 °C, by night. Venus is a steady 737 K, or 464 °C. Earth averages 288 K, or 15 °C. Mars is near 210 K, or −63 °C. At the one-bar level Jupiter is near 165 K, or −108 °C, Saturn 134 K, or −139 °C, Uranus 76 K, or −197 °C, and Neptune 72 K, or −201 °C.
Moons and dwarf planets
Titan has a surface near 94 K, or −179 °C. Europa's surface is near 100 K, or −173 °C, Enceladus about 75 K, or −198 °C, and Triton around 38 K, or −235 °C, among the coldest surfaces known in the Solar System. Pluto sits near 44 K, or −229 °C.
Spectral classes and colour
Stars are sorted by temperature into classes O, B, A, F, G, K and M. A G-type star such as the Sun is 5200 to 6000 K, or 4927 to 5727 °C. K-type stars are 3700 to 5200 K, M-type 2400 to 3700 K, and O-type stars exceed 30,000 K, or about 29,727 °C. Hotter stars look blue-white, cooler ones orange or red, which is why Betelgeuse looks red.
Comets and the outer Solar System
A comet far from the Sun is a frozen body at 30 to 50 K, or −243 to −223 °C. As it nears the Sun, ices sublimate when the surface warms past about 150 K, or −123 °C for carbon dioxide-rich ices and 200 K, or −73 °C, for water ice. Cometary models track the surface temperature in kelvin.
Rocket engines and re-entry
Combustion in a rocket engine reaches about 3500 K, or 3227 °C. A spacecraft entering the atmosphere from low orbit faces surface temperatures near 1900 K, or 1627 °C, on the leading edges. Heat shield designers state limits in kelvin, and a Celsius value suits a general audience, so the “convert K to °C” calculation step appears in every mission summary.
Black body radiation
Every object with a temperature emits light. A body at 300 K, or 27 °C, glows in the infrared, one at 3000 K, or 2727 °C, glows orange, and one at 6000 K, or 5727 °C, looks white. Planck's law needs temperature in kelvin, so any calculation of the colour of a star starts with a conversion.
Solar eclipses and the corona
The Sun's corona is visible during a total eclipse and has a temperature of one to three million kelvin, much hotter than the 5772 K surface. The reason for this contrast is still debated. At millions of degrees, 273.15 is a rounding error, so papers speak in kelvin or in megakelvin.
Ice on distant worlds
Water ice on Europa at 100 K, or −173 °C, is as hard as rock. Methane and nitrogen ices on Pluto at 44 K, or −229 °C, form mountains and glaciers. The Voyager and New Horizons missions measured temperatures with infrared instruments, and the findings appear in kelvin in technical papers.
Handling a catalogue of temperatures
Star and planet catalogues often come as tables in kelvin. A new column with =A2-273.15 gives Celsius, and the original stays untouched for reference. Because the range runs from a few kelvin to many thousands, apply scientific notation to the large entries and two decimals to the small ones. Add the unit to every header so the sheet can be shared safely.
Rounding and precision
Astronomical temperatures are rarely known to better than a few per cent. Writing 5498.85 °C for the Sun suggests a precision that the data do not have. Round to the precision of the source, such as 5500 °C, and use scientific notation for millions of kelvin.
A mental shortcut
Subtract 273 and carry on. For 300 K, 27 °C is a good estimate. For 100 K, −173 °C. For anything above a few thousand kelvin, the offset can be ignored, and the Celsius value is essentially the same number.
Landmarks to memorise
273.15 K is the freezing point of water, 288 K is Earth's mean, 373.15 K is boiling, 5772 K is the Sun's surface, 77 K is liquid nitrogen and 2.7 K is the cosmic background. If you remember these six, you can place any figure in an astronomy article on a mental map.
Reference ladder
2.725 K is −270.4 °C. 20 K is −253.15 °C. 77 K is −196.15 °C. 273.15 K is 0 °C. 288 K is 14.85 °C. 373.15 K is 100 °C. 5772 K is 5498.85 °C. A ladder like this makes quick checks easy.
White dwarfs and neutron stars
A young white dwarf like Sirius B has a surface near 25,000 K, or 24,727 °C, and cools slowly over billions of years. A newborn neutron star has a surface near a million kelvin, about 1,000,000 °C. Differences of 273.15 vanish at these values, and professional papers do not bother with Celsius at all.
Checking by reversing
To check any convert K to °C result, add 273.15 and see whether you return to the start. If 300 K became 26.85 °C, then 26.85 plus 273.15 is 300. A second test is the direction: the Celsius number is always smaller than the kelvin number.
Temperature differences
When a probe's instrument warms by 5 K, it warms by 5 °C. A difference in kelvin equals the same difference in Celsius. Never subtract 273.15 from a difference. Mission engineers quote temperature ranges, such as 60 K between day and night on a surface, and these ranges are identical in both scales.
Writing the unit properly
Write the kelvin as K with a space after the number and no degree sign: 5772 K. Write Celsius as °C with a space: 5499 °C. Science writers sometimes put °K by habit, which the SI system does not allow, and editors flag it as an error.
The edge of absolute zero
No object can reach 0 K, but laboratories have come within billionths of a kelvin. On Earth, the coldest natural place is Antarctica at about 184 K, which is −89 °C. Space itself, between galaxies, is warmed to 2.7 K by the cosmic background, so even the emptiest space is above absolute zero.
Reading a fact sheet
NASA planetary fact sheets list mean temperatures in kelvin, usually next to Celsius. The numbers are averages over the surface or at a standard pressure, not what a thermometer on the ground would read at a given moment. When you convert with a “convert K to °C” calculation, remember the averages hide large swings in day and night.
Brown dwarfs
Brown dwarfs are objects too small to ignite hydrogen fusion. The warmest are near 2000 K, or 1727 °C, and the coolest known are only a few hundred kelvin, such as 300 to 400 K, which is 27 to 127 °C, comparable to a hot day or boiling water. They glow in the infrared and are found by telescopes sensitive to heat. A “convert K to °C” calculation puts a distant object's temperature in everyday terms.
Planetary atmospheres and clouds
Jupiter's cloud tops, at the one-bar level, are near 165 K, or −108 °C, and made of ammonia ice. Deeper, the temperature climbs by about 2 K per kilometre, so at the level where water clouds may form it is close to 273 K, or 0 °C. Saturn and Neptune show similar patterns, and atmospheric models for gas giants are written in kelvin.
The Voyager and Pioneer probes
Deep-space probes carry radioisotope heaters and thermal blankets to keep electronics warm in the dark. Voyager's instruments, far from the Sun, are kept above 250 K, or −23 °C, although the space around them is only a few kelvin. As power declines, mission teams switch off heaters, and instrument temperatures fall by a few kelvin each year, a change recorded in kelvin and Celsius alike.
The habitable zone
Liquid water exists between 273.15 K and 373.15 K at one atmosphere, that is between 0 °C and 100 °C. A planet's equilibrium temperature depends on its distance from the star and its reflectivity. Earth would be near 255 K, or −18 °C, without a greenhouse effect, and the real 288 K, or 15 °C, comes from the 33 K that greenhouse gases add.
Hot Jupiters
Some exoplanets orbit so close to their stars that their daytime temperatures reach 1500 to 3000 K, which is 1227 to 2727 °C. Iron melts at 1811 K, or 1538 °C, so metals would be liquid or gaseous there. Astronomers measure these temperatures indirectly, from the light that planets give off, and quote them in kelvin.
The James Webb Space Telescope
Webb keeps its mirror and instruments near 40 K, which is −233 °C, behind a five-layer sunshield. The mid-infrared instrument is cooled further, to about 7 K, or −266 °C, using a cryocooler. The cold is needed so the telescope's own heat does not drown the faint infrared light from distant objects. Engineering reports use kelvin, while news stories give Celsius.
Molecular clouds
The clouds where stars form are among the coldest places in the galaxy, at 10 to 20 K, which is −263 to −253 °C. At that temperature gases clump, and gravity can take over. A radio astronomer who compares such a cloud with the Earth's surface at 288 K sees a gap of over 270 K, which a “convert K to °C” calculation helps to visualise.
The cosmic microwave background
The leftover radiation from the early Universe has a temperature of 2.725 K, or −270.425 °C. It was far hotter when emitted, about 3000 K, or 2727 °C, when atoms first formed. Expansion has stretched the light and cooled it by a factor of about 1100 since then. The figure is quoted in kelvin in every textbook.
The Moon's extremes
The lunar equator reaches around 390 K, or 117 °C, at noon and falls to about 95 K, or −178 °C, at night, since there is no atmosphere to hold heat. In permanently shadowed craters near the poles, temperatures can sink below 40 K, which is −233 °C, cold enough to trap water ice for billions of years.
Mars missions
Mars rovers record air temperatures that swing from about 293 K, or 20 °C, at midday in summer near the equator to 150 K, or −123 °C, on a polar night. A typical daily range is 70 to 100 K. Mission reports use kelvin, but press summaries translate to Celsius, and a quick convert K to °C lets you check their numbers.
More values for convert K to °C
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 K = -546.3 °C
- -100 K = -373.15 °C
- -20 K = -293.15 °C
- 10 K = -263.15 °C
- 15 K = -258.15 °C
- 25 K = -248.15 °C
- 30 K = -243.15 °C
- 50 K = -223.15 °C
Common slips when you convert K to °C
Adding instead of subtracting
Celsius is the smaller number. If 300 K became 573.15, you added.
Subtracting from a difference
A range of 10 K is a range of 10 °C. Do not subtract 273.15 from it.
Writing °K
The kelvin takes no degree sign. Write 300 K.
Quick questions
What is the method for a “convert K to °C” calculation?
Take away 273.15.
What is 300 K in degrees Celsius?
26.85 °C.
What is 273.15 K in degrees Celsius?
0 °C.
What is 0 K in degrees Celsius?
−273.15 °C.
Is my figure stored or sent?
No, it stays on your device.
Where to go next
Other temperature converters are listed here: Convert K to °F, Convert K to °R, Convert °Ré to °C, Convert °F to °C, Convert °C to °Ré, Convert km to mi, Convert mi to km, Convert L to gal, Convert gal to L. If you need the opposite direction, use Convert °C to K.
The takeaway: a “convert K to °C” calculation subtracts 273.15, and the result should carry its unit. The figure you enter is processed on your device only.