Celsius to Kelvin Converter

The Celsius to Kelvin converter translates any temperature between degrees Celsius (°C) and the SI kelvin (K) with a single addition or subtraction. Type a value in either field and the other updates live, with a bonus Fahrenheit read-out shown alongside so you can pivot between the three most common temperature scales without leaving the page. The conversion is exact by international definition, there is no factor to remember, no rounding error, and no conversion table to look up.

This tool is useful for chemistry students working through thermodynamics homework with the ideal gas law (PV = nRT) in kelvin units, lab technicians converting between autoclave set-points printed in Celsius and equipment logs written in K, meteorologists comparing forecast-model temperatures with cryogenic-engineering data, physicists quoting absolute-zero experiments, and any everyday user who simply needs the K value for a thermometer reading in °C, or vice versa, without trusting the result of an online search engine's "approximate" widget.

How to Use the Celsius to Kelvin Converter

1. Type a number into Celsius (°C). The default value is 25 (room temperature), and the Kelvin (K) field will read 298.15 as soon as the page loads.
2. Or type a number into Kelvin (K). The Celsius field updates in the opposite direction.
3. Use the ⇄ Swap button between the two fields to flip which value sits in which box. The numbers themselves remain unchanged, only their labels swap.
4. Read the Forward formula / Inverse formula lines below the inputs to confirm the math being applied: K = °C + 273.15 and °C = K − 273.15.
5. Use the quick Celsius presets (absolute zero, dry ice, freezing, room temperature, boiling, Bunsen flame) or the quick Kelvin presets (absolute zero, liquid nitrogen, dry ice, freezing, ambient, boiling) to load a known reference temperature with one click.
6. Read the three result cards below the inputs for the current reading in °C, K, and °F.
7. Click Copy on any card, or Copy full result at the bottom, to paste the value into a lab notebook, email, or spreadsheet.
8. The Round-trip line beneath the cards shows °C → K → °C with the residual error. Because the offset is exact by definition, the residual is 0 to many decimal places, there is no drift.

The Conversion Formula

The Celsius and Kelvin scales are linearly related by a single additive offset. One kelvin is exactly the same size as one degree Celsius (a temperature difference of 1 K equals a temperature difference of 1 °C), but the two scales place their zero points 273.15 degrees apart. Specifically:

Celsius to Kelvin (forward): K = °C + 273.15

Kelvin to Celsius (inverse): °C = K − 273.15

The forward and inverse formulas are exactly the same identity rearranged: add 273.15 to convert in one direction, subtract 273.15 to convert in the other. There is no multiplication factor, no approximation, and no measurement uncertainty involved.

Why 273.15 is exact (and not 273 or 274)

The constant 273.15 dates from the 1954 General Conference on Weights and Measures, which redefined the kelvin in terms of the triple point of water, the single temperature and pressure at which pure water coexists as ice, liquid, and vapour in equilibrium. That triple point was assigned the exact value 273.16 K (i.e. 0.01 °C). Because the Celsius scale sets 0.01 °C as the location of the ice point, the difference of 0.01 °C shifts the offset to exactly 273.15 K = 0 °C. The 2019 SI redefinition did not change this offset: the kelvin is now defined by fixing the Boltzmann constant at exactly k = 1.380649 × 10⁻²³ J/K, but the relation 0 °C = 273.15 K is preserved (BIPM SI Brochure, 9th ed., Sec. 2.3.1).

So both K = °C + 273.15 and °C = K − 273.15 are identities that hold with infinite precision. The converter rounds to four decimal places only for display; the underlying arithmetic is exact.

Worked Examples

The five worked examples below match the values you can load with the quick presets on the converter.

Example 1, Room temperature: 25 °C = 298.15 K. Add 273.15 to 25: 25 + 273.15 = 298.15. This is the canonical "lab ambient" reference value used throughout chemistry, biology, and materials science. Many standard-state thermodynamic data (standard enthalpy, standard entropy, standard Gibbs energy) are quoted at exactly 298.15 K and 1 atm.

Example 2, Water freezes: 0 °C = 273.15 K. Add 273.15 to 0: 0 + 273.15 = 273.15. This is the freezing point of pure water at 1 atmosphere. It is 0.01 °C below the triple point (273.16 K), because the triple point of water is at lower pressure than 1 atm and pure water freezes a fraction of a degree higher than the ice point under standard pressure.

Example 3, Water boils: 100 °C = 373.15 K. Add 273.15 to 100: 100 + 273.15 = 373.15. This is the boiling point of pure water at exactly 1 standard atmosphere (101,325 Pa). At higher altitudes the boiling point drops, at Denver (≈ 1,609 m) water boils at about 95 °C = 368.15 K.

Example 4, Absolute zero: −273.15 °C = 0 K. Add 273.15 to −273.15: −273.15 + 273.15 = 0. Absolute zero is the lower bound of the thermodynamic temperature scale: the temperature at which the entropy of a perfect crystal reaches its minimum. It is unattainable; the lowest temperature reached in a lab (by laser cooling and evaporative cooling of dilute gases) is in the picokelvin range.

Example 5, Inverse (round-trip): 298.15 K = 25 °C. Subtract 273.15 from 298.15: 298.15 − 273.15 = 25. This inverts Example 1 exactly, with a residual of 0 (no rounding error). The same round-trip works for any value: type 100 K, get −173.15 °C; type that back as °C, you get 100 K; and so on indefinitely.

Where It Shows Up

Celsius and Kelvin are used together across nearly every science and engineering domain that involves temperature.

• Chemistry laboratories quote equilibrium constants, rate constants, dissociation constants, and standard-state thermodynamic data at 25 °C (= 298.15 K), but the equations they sit inside, the Arrhenius equation, the Nernst equation, the van 't Hoff equation, use the absolute temperature in K. Doing a rate-constant calculation at "298" without the 273.15 offset is one of the most common mistakes in lab chemistry.
• The ideal gas law PV = nRT requires T in kelvin. If you substitute 25 (°C) instead of 298.15 (K) into a PV = nRT calculation, the predicted pressure or volume will be off by about 10%, which is usually a failing experiment.
• Cryogenics, the engineering of extremely low temperatures, works entirely in kelvin. Liquid nitrogen boils at 77.36 K, liquid helium at 4.22 K, dilution refrigerators reach a few millikelvin, and the coldest man-made temperatures (in Bose-Einstein condensates) are in the picokelvin range. Celsius is meaningless at those temperatures because it goes negative without bound; kelvin starts at the only physically meaningful zero.
• Meteorology and atmospheric science quote surface temperatures in °C (or °F in the US) for the daily forecast but use kelvin in every physics equation, saturation vapour pressure, adiabatic lapse rate, Clausius-Clapeyron slope, radiative transfer. Forecast-model output files often store temperatures in K even when the consumer-facing product strips the offset.
• Astronomy always uses kelvin for stellar colour temperature, black-body spectra, cosmic microwave background temperature (2.725 K), and effective temperatures of planets. The Sun's photosphere is 5,778 K (= 5,505 °C), Mars's surface averages 210 K (= −63 °C), and the cosmic microwave background is 2.725 K (= −270.425 °C).
• Materials science uses kelvin for the Curie temperature of ferromagnets (iron: 1,043 K), the superconducting transition temperature of low- and high-temperature superconductors (niobium: 9.26 K; YBCO: ≈ 92 K), and the Debye temperature that separates classical and quantum mechanical heat capacity in a solid.
• Cooking and food safety quote the safe hold temperature of hot food as 63 °C in the UK or 145 °F in the US, which converts to 336.15 K, useful if you ever see a sous-vide controller programmed in kelvin instead of Celsius.

Common Mistakes

The Celsius-Kelvin conversion is short, but the short list of gotchas still trips people up.

• Using 273 instead of 273.15. The most common error. The two scales are offset by exactly 273.15, not 273. Writing K = °C + 273 introduces a 0.15 K shortfall and ruins any calculation that depends on the absolute temperature (the most important of which is the exponent in the Arrhenius, Nernst, and van 't Hoff equations, where a 0.15 K error changes a rate constant or equilibrium constant by a measurable percentage). Always use 273.15.
• Adding 273 to a Kelvin value and expecting to get back to Celsius. That step is wrong by sign as well as by 0.15. Going back from K to °C you must subtract 273.15, not add 273. Subtracting is the direction that brings a high absolute reading down to a smaller Celsius value (K is larger than °C by exactly 273.15 because the scales share the same degree size).
• Confusing zero-point with degree size. A temperature difference of 1 K equals a temperature difference of 1 °C exactly. Only the zero points differ. If a reaction's enthalpy is reported as "ΔH = +25 kJ mol⁻¹ at 25 °C", that is the same as "ΔH = +25 kJ mol⁻¹ at 298.15 K", with no scaling needed. The trap is to think the 273.15 offset also changes the size of the degree.
• Citing the triple point instead of the freezing point. A common misconception is that 0 °C = 273.16 K because the triple point of water is 273.16 K. The triple point is 0.01 °C (one hundredth of a degree above ice-point), so 0 °C is 273.15 K, not 273.16 K. The triple point is used as the calibration definition for the kelvin, not as the freezing point.
• Mixing up "absolute" Celsius and Celsius proper. The absolute-thermodynamic scale after the offset is the kelvin, not "Celsius absolute" or any "°C above absolute zero" form. Writing "A = 30 °C absolute" is non-standard and misleading. The correct SI unit for absolute temperature is the kelvin, period.
• Forgetting the small print for Fahrenheit. Fahrenheit is not part of this converter's primary cross-update, it is a bonus read-out. To read Fahrenheit from a Celsius value, multiply by 9/5 and add 32; from a Kelvin value, subtract 273.15 first then multiply by 9/5 and add 32. The converter performs the arithmetic, but if you derive a quick estimate by hand, remember there are two formulas not one.

Frequently Asked Questions

What is the exact formula to convert Celsius to Kelvin?

K = °C + 273.15. The constant 273.15 is exact by international definition; do not round to 273. For example, 25 °C + 273.15 = 298.15 K, the standard ambient reference temperature used throughout chemistry. The conversion has no measurement uncertainty because the offset is defined, not measured.

What is the exact formula to convert Kelvin to Celsius?

°C = K − 273.15. Subtraction, not addition, is the correct direction (kelvin values are larger than Celsius values by 273.15). Example: 298.15 K − 273.15 = 25 °C. Going the wrong way is the single most common mistake in undergraduate thermodynamics, a "K − 273" shortcut gives 25.15 °C, which is off by 0.15 degrees and silently breaks any subsequent calculation that needs absolute temperature.

Why is the offset 273.15 and not 273?

The 273.15 comes from the 1954 General Conference on Weights and Measures, which fixed the triple point of water, the one temperature and pressure where pure water is in equilibrium as ice, liquid, and vapour, at exactly 273.16 K. Because the triple point sits at 0.01 °C on the Celsius scale, the offset between the two zero points works out to 273.16 − 0.01 = 273.15 exactly. Rounding it to 273 introduces a 0.15 K (≈ 0.04%) error, which is too large for any thermodynamics calculation that uses an exponential dependence on 1/T.

Is 0 °C the same as 273 K?

Almost, but not exactly. 0 °C = 273.15 K by definition, not 273 K and not 273.16 K (which is the triple point, a different reference). The .15 in 273.15 is a deliberate part of the value: the Celsius scale places its zero at the ice point of water, and the kelvin places its zero at the point 273.15 degrees below that. Both scales share the same unit size, a 1 °C interval is the same physical temperature interval as a 1 K interval, only their zero points differ.

What is absolute zero in Celsius?

Absolute zero is exactly −273.15 °C, by the same definition that makes 0 K = −273.15 °C exactly. It is the lowest possible thermodynamic temperature: the temperature at which a perfect crystal has zero entropy. Absolute zero is unattainable, but experiments have reached temperatures in the picokelvin range in Bose-Einstein condensates of dilute rubidium and sodium gases. On the Celsius scale, absolute zero is just the same value minus the 273.15 K offset, written as a negative number.

Does the 2019 SI redefinition change the Kelvin offset?

No. Since 20 May 2019, the kelvin is defined by fixing the Boltzmann constant at exactly k = 1.380649 × 10⁻²³ J/K. The magnitude of the kelvin did not change, and neither did its offset from the Celsius scale: 0 °C is still 273.15 K exactly. What did change is that "273.15 K is 0.01 °C below the triple point of water" no longer requires water to define it, the kelvin can now in principle be realized in any apparatus that can measure thermal energy per particle in joules (acoustic gas thermometry, Johnson-noise thermometry, Doppler-broadening thermometry).

What is 1 °C in Kelvin terms, is it the same as 1 K?

A temperature difference of 1 °C equals a temperature difference of 1 K exactly. The degree on the Celsius scale and the kelvin are the same size; only their zero points differ by 273.15. So if a thermostat set-point rises by 5 °C, it has risen by 5 K, the underlying physical temperature change is identical, and any subsequent temperature-difference calculation (rate constant change, thermal expansion, vapour pressure change) will give the same answer in either unit.

Why do chemists and physicists use kelvin instead of Celsius?

Because thermodynamic equations, the ideal gas law PV = nRT, the Arrhenius equation k = A exp(−Ea / RT), the Nernst equation E = E° − (RT / nF) ln Q, the Planck black-body distribution, the Boltzmann distribution, are linear in T (or exponential in 1/T), and they only make sense when T is referenced from absolute zero. Celsius puts zero at the freezing point of water, which is meaningless as a fundamental reference: it would make the same physical law depend on whether you are using Celsius or Fahrenheit arithmetic. Kelvin is the only temperature unit that gives a temperature of 0 at the lowest achievable temperature, which is why it is the SI base unit for thermodynamic temperature.

How do I convert Kelvin to Fahrenheit?

Subtract 273.15 first to get Celsius, then multiply by 9/5 and add 32: °F = (K − 273.15) × 9/5 + 32. Example: 298.15 K − 273.15 = 25 °C; 25 × 9/5 = 45; 45 + 32 = 77 °F. The converter computes that as a bonus read-out alongside the primary °C ↔ K cross-update. Note that the conversion is no longer exact in the second step, the Fahrenheit degree is exactly 5/9 of a Celsius degree (or kelvin) by definition, but the offset of 32 is exact by definition too, so the only "rounding" ever introduced is when the decimal places are truncated for display.

What is the boiling point of water in kelvin?

The boiling point of pure water at exactly 1 standard atmosphere (101,325 Pa) is 100 °C = 373.15 K. The conversion is just 100 + 273.15 = 373.15. At reduced pressure (higher altitude) water boils at a lower temperature, at Denver (≈ 1,609 m, pressure ≈ 83,500 Pa), water boils at about 95 °C = 368.15 K. At pressures above 1 atm, water boils at a higher temperature, a pressure cooker at 2 atm boils water at about 120 °C = 393.15 K.

Is the conversion always exact?

Yes. K = °C + 273.15 and °C = K − 273.15 are identities, not approximations. The 273.15 offset is fixed by international definition, and the degree size is the same on both scales. Any rounding error you see comes from how many decimal places you display, not from the conversion itself. The converter rounds to four decimal places for readability; the underlying arithmetic is exact to the limits of IEEE 754 double-precision arithmetic (about 15 significant digits).

References

• BIPM SI Brochure, 9th edition (2019). The official SI reference published by the Bureau International des Poids et Mesures, defining the kelvin since 20 May 2019 via a fixed value of the Boltzmann constant (k = 1.380649 × 10⁻²³ J/K exactly) and documenting the relationship 0 °C = 273.15 K exactly in Sec. 2.3.1.
• NIST Special Publication 811 (2008, with later amendments). "Guide for the Use of the International System of Units (SI)." The US authoritative reference for SI unit definitions, conventions, and conversion factors; reproduces the BIPM definitions and gives the canonical Celsius ↔ Kelvin offset.
• IUPAC Quantities, Units and Symbols in Physical Chemistry ("Green Book"), 3rd edition (2007). The International Union of Pure and Applied Chemistry reference for the use of SI units in physical chemistry, including the recommended convention for quoting temperatures and standard states at 298.15 K.
• NIST Reference Points for Thermometry. The fixed-point cells used by national metrology institutes to realize the kelvin, including the triple point of water at 273.16 K and the freezing points of tin, zinc, aluminium, silver, gold, and copper.
• BIPM / CCT Guide on the Realization of the Kelvin. Operational guide from the BIPM Consultative Committee for Thermometry describing how the kelvin is experimentally realized in modern metrology laboratories via acoustic gas thermometry, Johnson-noise thermometry, and Doppler-broadening thermometry.