Kelvin (K)

What Is Kelvin (K)?

Kelvin is a unit used to measure temperature in science and engineering. It starts at the coldest possible temperature called absolute zero. Unlike Celsius or Fahrenheit, the Kelvin scale has no negative values for thermal energy, so it is very useful for physics and chemistry.

Definition

Kelvin is the base unit of temperature in the International System of Units, also called SI. It is written with the symbol K. One kelvin is the same size step as one degree on the Celsius scale, but the starting point is different.

Today, the kelvin is defined using a constant from nature called the Boltzmann constant. This constant links temperature to energy at the tiny particle level.

• Symbol: K
• Quantity measured: thermodynamic temperature
• SI status: base unit for temperature

The official modern definition is based on fixing the value of the Boltzmann constant to exactly 1.380649 × 10−23 joule per kelvin. This means the kelvin is tied to a fundamental property of the universe, not to a man made material.

Important points in the Kelvin and Celsius scales link like this.

• Absolute zero. 0 K. −273.15 °C. This is the lowest possible temperature, where particles have minimum thermal motion.
• Freezing point of water at normal pressure. 273.15 K. 0 °C.
• Boiling point of water at normal pressure. 373.15 K. 100 °C.

History / Origin

The kelvin scale is named after the British physicist William Thomson, known as Lord Kelvin. In the 1800s he studied heat, energy and the ideas of thermodynamics. He saw that normal temperature scales like Celsius did not start at a true physical zero.

Lord Kelvin suggested a new temperature scale that would start at the lowest possible temperature. This point is where there is no more heat energy that can be removed from a substance. He called this point absolute zero.

Key steps in the history of the kelvin.

• Mid 1800s. Lord Kelvin develops the idea of an absolute temperature scale based on thermodynamics.
• Later 1800s. Scientists begin to use this absolute scale in advanced physics and engineering.
• 1954. The kelvin is adopted as the SI base unit for thermodynamic temperature.
• 1967. The unit name “kelvin” is set in lower case for the unit, and its symbol is set to capital K without a degree sign.
• 2019. The definition of the kelvin is updated and linked to the fixed value of the Boltzmann constant, instead of properties of water.

This change in 2019 made the kelvin more stable and precise, because it is now defined by a constant of nature that does not change with time or place.

Symbol & Abbreviation

The kelvin has a simple symbol and some special writing rules.

• Symbol. K
• Correct form. Always use a capital K and no degree sign. Write 300 K, not 300 °K and not 300 k.
• Name. The unit name is written as “kelvin” in lower case when spelled out in a sentence.
• Plural. When using the symbol, do not add s. Write 5 K, not 5 Ks. When writing the word, add s for plural, for example 5 kelvins.
• Spacing. Put a space between the number and the symbol. For example 77 K, 1000 K.

Because Kelvin is an SI base unit, it is used in many combined units too, such as joule per kelvin for heat capacity and watt per square meter per kelvin for heat transfer.

Current Use Around the World

Kelvin is the standard unit for temperature in science all over the world. It is used by physicists, chemists, engineers, and many other experts. While people still use Celsius or Fahrenheit in daily life, almost all serious scientific work uses kelvin for temperature.

Here are some common uses.

• Physics. Studying gases, particles, stars, black holes and the early universe. Temperatures in these areas are often extremely high or extremely low, and kelvin works well for calculations.
• Chemistry. Using gas laws, reaction rates and equilibrium laws. These formulas are written with temperature in kelvin, not Celsius or Fahrenheit.
• Engineering. Designing engines, power plants, refrigeration systems and electronics. Kelvin is used when working with heat flow and material properties.
• Astronomy and space science. Describing the temperature of stars, the cosmic microwave background and the cold of space. For example, the cosmic microwave background is about 2.7 K.
• Low temperature research. Studying superconductors, quantum effects and very cold gases near absolute zero. These labs often work at fractions of a kelvin.
• Climate and meteorology. Computer models of weather and climate often use kelvin internally, even if results are shown later in Celsius.

Even in countries that use Celsius or Fahrenheit for weather reports, scientists there still use kelvin for accurate work. This makes it easier to share data and research across the world.

Example Conversions

Kelvin uses the same size step as Celsius, but the starting points are different. You can change between kelvin, Celsius and Fahrenheit with simple formulas.

Kelvin to Celsius

Formula.

°C = K − 273.15

Examples.

• 0 K = −273.15 °C. This is absolute zero.
• 273.15 K = 0 °C. This is the freezing point of water.
• 293.15 K ≈ 20 °C. A cool room.
• 298.15 K ≈ 25 °C. A comfortable room.
• 373.15 K = 100 °C. Boiling point of water at normal pressure.

Celsius to Kelvin

Formula.

K = °C + 273.15

Examples.

• −40 °C = 233.15 K
• 0 °C = 273.15 K
• 25 °C = 298.15 K
• 100 °C = 373.15 K

Kelvin to Fahrenheit

Formula.

°F = (K − 273.15) × 9/5 + 32

Examples.

• 273.15 K = 32 °F
• 293.15 K ≈ 68 °F
• 310.15 K ≈ 99 °F

Fahrenheit to Kelvin

Formula.

K = (°F − 32) × 5/9 + 273.15

Examples.

• 32 °F = 273.15 K
• 68 °F ≈ 293.15 K
• 98.6 °F ≈ 310.15 K

Related Units

Kelvin is closely related to other temperature units. Knowing how they link together helps you understand when to use each one.

• Celsius, °C. Common in everyday life and weather reports in most of the world. It sets 0 °C at the freezing point of water and 100 °C at the boiling point of water at normal pressure. One degree Celsius is the same size as one kelvin.
• Fahrenheit, °F. Often used in the United States for daily weather, cooking and household thermometers. Its scale has different step sizes. 1 °F is smaller than 1 K or 1 °C.
• Rankine, R or °R. An absolute temperature scale mainly used in some branches of engineering, especially in the past, and mostly in the United States. Rankine uses Fahrenheit sized degrees but starts at absolute zero like kelvin.
• Degree Celsius, as SI derived unit. While the kelvin is the SI base unit for temperature, degree Celsius is an SI accepted unit. It is often used for practical work, but formulas in physics and chemistry usually expect kelvin.

Conversion hints.

• Difference of 1 K = Difference of 1 °C
• Difference of 1 K ≈ Difference of 1.8 °F
• 0 K = −273.15 °C ≈ −459.67 °F

FAQs

Why is there no degree sign with kelvin?

Kelvin is written without a degree sign because it is an absolute scale that measures thermodynamic temperature directly. The SI system decided that absolute temperature units, like kelvin, do not use the degree symbol. So we write 300 K, not 300 °K.

Can kelvin values be negative?

On the normal thermodynamic kelvin scale, temperatures cannot go below 0 K. Zero kelvin is absolute zero, the lowest possible temperature. You can have numbers between 0 and 1 K, but never less than 0 K on this scale. In some advanced physics topics, people talk about special systems with “negative temperatures” in a different sense, but that is not used in ordinary thermodynamics.

What is room temperature in kelvin?

People often think of room temperature as about 20 to 25 °C. In kelvin this is about 293 to 298 K. For many calculations, scientists use 298.15 K which equals 25 °C as a standard room temperature.

Why do scientists prefer kelvin over Celsius?

Scientists like kelvin because many physics and chemistry formulas only work correctly if you use an absolute temperature scale that starts at zero energy. Celsius can have negative values, which can break these formulas. In kelvin, zero always means no thermal energy that can be removed, and higher values are directly proportional to the energy per particle. This makes math and theory simpler and more accurate.

Do people use kelvin in daily life?

Most people do not use kelvin for everyday things like cooking or reading the weather. They use Celsius in most countries or Fahrenheit in the United States. Kelvin is mainly for science, engineering and advanced technology. However, you might see kelvin used for the color of light bulbs or screens, where it shows the color temperature.

How is kelvin used in color temperature?

Color temperature is a way to describe how warm or cool a light looks. It uses kelvin. Lower values, like 2700 K, look warm and yellowish, like a soft lamp. Higher values, like 6500 K, look cool and bluish, like noon daylight. This does not mean the bulb itself is that hot. It means the light looks similar to an ideal object heated to that temperature.

Is the size of 1 kelvin the same as 1 degree Celsius?

Yes. A change of 1 K is exactly the same size as a change of 1 °C. The only difference is where zero is placed. Zero kelvin is absolute zero. Zero Celsius is the freezing point of water. Because of this, temperature differences are the same values in kelvin and in Celsius.

What is absolute zero in simple words?

Absolute zero is the lowest possible temperature. At this point, particles have as little motion from heat as nature allows. It is 0 K, which equals −273.15 °C, which is about −459.67 °F. It is not possible to cool anything below absolute zero.