What Is the Kelvin Scale?
The Kelvin scale is the fundamental unit of thermodynamic temperature in the International System of Units (SI). Named after the British physicist William Thomson, 1st Baron Kelvin, who proposed it in 1848, the Kelvin scale begins at absolute zero — the coldest possible temperature in the universe, where all molecular motion theoretically ceases. This makes it an absolute temperature scale, unlike Celsius or Fahrenheit, which are relative scales based on the properties of water.
Unlike Celsius (°C) and Fahrenheit (°F), the Kelvin unit is not expressed with a degree symbol. You write 300 K, not 300°K. This is because Kelvin is a true SI base unit, on par with the metre and the kilogram. The scale is used universally in science and engineering, from astrophysics to chemistry to materials science.
The size of one Kelvin is identical to the size of one degree Celsius. This means the difference between two temperatures is the same whether you express it in Kelvin or Celsius. The only thing that changes is the zero point: Celsius sets 0° at the freezing point of water, while Kelvin sets 0 at absolute zero (−273.15°C).
The Kelvin to Celsius Conversion Formula
Converting Kelvin to Celsius is one of the most straightforward temperature conversions because both scales use identical intervals. The formula is:
°C = K − 273.15
To go the other way — Celsius to Kelvin — simply add 273.15:
K = °C + 273.15
The constant 273.15 is derived from the definition of 0°C (the triple point of water minus a small correction). Some textbooks round this to 273, but for precision work, 273.15 should always be used.
Worked Examples
Let's walk through some practical conversions:
- 0 K to Celsius: 0 − 273.15 = −273.15°C (absolute zero)
- 273.15 K to Celsius: 273.15 − 273.15 = 0°C (water's freezing point)
- 300 K to Celsius: 300 − 273.15 = 26.85°C (warm room temperature)
- 310.15 K to Celsius: 310.15 − 273.15 = 37°C (human body temperature)
- 373.15 K to Celsius: 373.15 − 273.15 = 100°C (boiling water at sea level)
How to Also Convert to Fahrenheit
To convert Kelvin to Fahrenheit, you first convert to Celsius and then to Fahrenheit, or use the direct formula:
°F = (K − 273.15) × 9/5 + 32
Alternatively, if you have the Celsius value, simply apply: °F = (°C × 9/5) + 32. Our calculator handles all three scales simultaneously, so you can enter any value and instantly see the equivalents in all three temperature units.
Why Is Kelvin Used in Science?
Scientists and engineers prefer the Kelvin scale because it is an absolute scale — there are no negative temperatures in Kelvin (you cannot go below 0 K). This makes mathematical relationships much cleaner. For example, gas laws like the Ideal Gas Law (PV = nRT) require temperature in Kelvin to work correctly. If you used Celsius or Fahrenheit in such equations, you would get physically meaningless negative results.
Kelvin is also essential in:
- Astronomy and astrophysics: Star surface temperatures are given in Kelvin (the Sun's surface is about 5778 K)
- Cryogenics: Liquid helium boils at 4.2 K, liquid nitrogen at 77 K
- Thermodynamics: All heat engine calculations use Kelvin
- Photography and lighting: "Colour temperature" of light sources is measured in Kelvin (e.g. daylight ≈ 6500 K, warm tungsten ≈ 3200 K)
- Materials science: Superconductors operate at very low Kelvin temperatures
History of the Kelvin Scale
William Thomson (later Lord Kelvin) published his proposal for an absolute temperature scale in 1848, building on earlier work by Nicolas Léonard Sadi Carnot on heat engines. Thomson recognised that there must be a lower bound to temperature — a point at which a substance has no thermal energy. He calculated this at approximately −273°C, setting it as the zero point of his new scale.
The exact value of absolute zero was refined over the following decades. In 1954, the 10th General Conference on Weights and Measures (CGPM) formally defined the Kelvin in terms of the triple point of water (273.16 K, or 0.01°C). In 2019, a significant redefinition occurred: the Kelvin is now defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K. This makes the scale even more precisely anchored to fundamental physics rather than the properties of water.
Kelvin vs Celsius vs Fahrenheit: Key Differences
Understanding the three main temperature scales and their differences helps clarify when each is appropriate:
Kelvin is the SI base unit, used in science. It starts at absolute zero, has no negative values, and uses the same interval size as Celsius. Written as K (no degree symbol).
Celsius (also called centigrade) is used in everyday life across the UK and most of the world. It sets 0° at the freezing point of water and 100° at the boiling point. Written as °C.
Fahrenheit is primarily used in the United States and a few other territories. It sets 32° at the freezing point of water and 212° at the boiling point, giving it a wider degree span for everyday temperatures. Written as °F.
Colour Temperature in Photography and Lighting
One everyday application of Kelvin that many people encounter without realising is colour temperature in photography, videography, and interior lighting. When a camera or phone asks you to set "white balance," the units are Kelvin. Lower Kelvin values (2700–3000 K) produce warm, yellowish light similar to candlelight or incandescent bulbs. Higher values (5500–6500 K) produce cool, blue-white daylight. Midday sunlight is approximately 5500 K, overcast sky can reach 7000 K or above.
LED bulbs purchased in UK shops are rated in Kelvin. A "warm white" bulb is typically 2700–3000 K, a "cool white" is 4000 K, and a "daylight" bulb is 6500 K. Understanding this Kelvin scale helps you choose the right lighting for your home — warm tones for living rooms and bedrooms, cooler tones for kitchens, bathrooms, and offices where task visibility is important.
Absolute Zero: The Coldest Possible Temperature
Absolute zero (0 K = −273.15°C = −459.67°F) is the theoretical lowest possible temperature. At this point, particles in a substance have minimum internal energy — they retain only zero-point energy as required by quantum mechanics. Absolute zero has never been achieved in practice, but scientists have come extraordinarily close. In 2021, a team at MIT cooled sodium-lithium molecules to 100 nanokelvin (0.0000001 K above absolute zero) — the coldest molecules ever observed.
The Third Law of Thermodynamics states that it is impossible to reach absolute zero in a finite number of steps. This is a fundamental constraint in the universe. Nevertheless, temperatures within billionths of a degree of absolute zero are routinely achieved in physics laboratories to study quantum phenomena, Bose-Einstein condensates, and superconductivity.
Frequently Asked Questions
What is the formula to convert Kelvin to Celsius?
The formula is °C = K − 273.15. Simply subtract 273.15 from your Kelvin value to get the equivalent in Celsius. For example, 400 K − 273.15 = 126.85°C. The formula is straightforward because Kelvin and Celsius use the same scale interval, differing only in their zero points.
What is 0 Kelvin in Celsius?
0 Kelvin equals −273.15°C. This is known as absolute zero — the coldest theoretically possible temperature in the universe. No substance can be cooled below this temperature, and it represents the point at which particles have minimum possible kinetic energy.
What is the difference between Kelvin and Celsius?
Kelvin and Celsius use the same scale interval (1 K difference = 1°C difference), but their zero points differ by 273.15. Celsius sets 0° at water's freezing point; Kelvin sets 0 at absolute zero. Kelvin has no negative values, making it essential for scientific calculations.
How do you convert Celsius to Kelvin?
Use the formula K = °C + 273.15. Add 273.15 to any Celsius temperature to get Kelvin. Room temperature of 20°C becomes 293.15 K. Boiling water at 100°C becomes 373.15 K.
Why does the Kelvin scale not use a degree symbol?
Kelvin is an SI base unit, not a derived unit. Unlike "degrees Celsius" (°C) or "degrees Fahrenheit" (°F), a Kelvin is simply a Kelvin — written as K, not °K. This was standardised by the CGPM in 1967 to reflect Kelvin's status as a fundamental measurement unit.
What temperature is 273.15 K in Celsius?
273.15 K = 0°C, the freezing point of pure water at standard atmospheric pressure. This is the defining offset between the Kelvin and Celsius scales. It is also close to (but not the same as) the triple point of water at 273.16 K, which is the reference point used in the pre-2019 definition of the Kelvin.
What are some common Kelvin temperatures to know?
Key reference points: 0 K (absolute zero, −273.15°C), 77 K (liquid nitrogen boiling, −196.15°C), 273.15 K (water freezes, 0°C), 293–295 K (room temperature, 20–22°C), 310 K (body temperature, ~37°C), 373.15 K (water boils, 100°C), 5778 K (Sun's surface temperature).