What Is Kelvin? The Absolute Temperature Scale

Definition: The SI Unit of Temperature

The kelvin (symbol: K) is the base unit of temperature in the International System of Units (SI). Unlike Celsius and Fahrenheit, the Kelvin scale starts at absolute zero — the point where all thermal motion of particles ceases in classical physics. This makes it an absolute temperature scale, meaning 0 K represents the theoretical minimum temperature in the universe.

Since 2019, the kelvin has been defined by fixing the numerical value of the Boltzmann constant at exactly 1.380649 × 10²³ J/K. This ties the kelvin directly to fundamental physics rather than to properties of a specific substance like water.

Absolute Zero: The Starting Point

Absolute zero is 0 K, which equals −273.15°C or −459.67°F. At this temperature, atoms and molecules have the lowest possible energy. While absolute zero can be approached in laboratory conditions (scientists have cooled substances to within billionths of a kelvin above 0 K), it can never truly be reached, as stated by the third law of thermodynamics.

How Kelvin Relates to Celsius and Fahrenheit

The Kelvin and Celsius scales have the same increment size — a change of 1 K is identical to a change of 1°C. The only difference is the starting point. Converting between them is simple:

• Kelvin to Celsius: °C = K − 273.15
• Celsius to Kelvin: K = °C + 273.15
• Kelvin to Fahrenheit: °F = (K − 273.15) × 9/5 + 32
• Fahrenheit to Kelvin: K = (°F − 32) × 5/9 + 273.15

Use our temperature converter to convert between all three scales instantly.

Key Temperature Reference Points

Named After Lord Kelvin

The scale is named after William Thomson, 1st Baron Kelvin (1824–1907), a Belfast-born physicist and engineer who worked at the University of Glasgow. In 1848, Thomson proposed an absolute temperature scale based on the efficiency of heat engines, building on the work of Sadi Carnot. His insight was that there must be a natural zero point where no more heat energy can be extracted from a system. Note that unlike Celsius and Fahrenheit, temperatures in kelvin are written without the degree symbol — it is “300 K,” not “300°K.”

Where Kelvin Is Used

Scientific Research

Virtually all scientific disciplines use kelvin for temperature measurements. Chemistry, physics, astronomy, and materials science all rely on the absolute scale. Gas law calculations (such as the ideal gas law PV = nRT) require temperature in kelvin to produce correct results. Using Celsius in these equations would yield nonsensical answers because Celsius has an arbitrary zero point.

Color Temperature in Lighting and Photography

The kelvin scale is widely used to describe the color temperature of light sources. A candle flame is roughly 1,800 K (warm, orange light). A standard incandescent bulb is about 2,700 K. Neutral white light is around 4,000 K, and daylight ranges from 5,500 to 6,500 K (cool, bluish white). Photographers and filmmakers use color temperature to set white balance and achieve the desired mood.

Cryogenics and Superconductivity

At extremely low temperatures — typically below 10 K — certain materials become superconductors, losing all electrical resistance. MRI machines use superconducting magnets cooled with liquid helium (boiling point: 4.22 K). The cosmic microwave background radiation has a temperature of about 2.725 K, the remnant heat of the Big Bang.

Astronomy

Stars are classified by their surface temperature in kelvin. Red dwarfs are around 3,000 K, our Sun is about 5,778 K, and blue supergiants can exceed 30,000 K. The kelvin scale is essential for understanding stellar evolution and the electromagnetic spectrum of celestial objects.

Frequently Asked Questions

What is 0 Kelvin?

0 Kelvin is absolute zero, the lowest theoretically possible temperature. It equals −273.15°C or −459.67°F. At this point, particles have the minimum possible thermal energy.

How do you convert Kelvin to Celsius?

Subtract 273.15 from the Kelvin value: °C = K − 273.15. For example, 300 K = 26.85°C.

Why does the Kelvin scale not use degrees?

The Kelvin scale is an absolute thermodynamic scale, so it uses “kelvins” (K) rather than “degrees.” This convention distinguishes it from relative scales like Celsius and Fahrenheit.

What is color temperature in Kelvin?

Color temperature describes the hue of light: warm yellowish light is around 2,700 K, neutral white is about 4,000 K, and cool daylight is 5,500–6,500 K. It is used in photography, film, and lighting design.

Quick Reference Section

• Kelvin to Celsius: °C = K − 273.15
• Celsius to Kelvin: K = °C + 273.15
• Absolute zero: 0 K = −273.15°C = −459.67°F
• Room temperature: ~293 K = ~20°C = ~68°F

Convert between Kelvin, Celsius, and Fahrenheit instantly with our free temperature converter.