What Is Kelvin (K)?
Kelvin is the main temperature unit used in science. It starts at absolute zero, which is the coldest temperature that can exist, and goes up as things get hotter.
Unlike Celsius or Fahrenheit, Kelvin does not use a degree symbol. You simply write K after the number. For example, 300 K.
Definition
Kelvin is the base unit of temperature in the International System of Units SI. It measures the average energy of particles in a substance. More particle motion means a higher Kelvin temperature.
Kelvin is defined using water and a special point called the triple point in older definitions and today using fixed physical constants. For everyday use, it is easier to remember this link.
- 0 degrees Celsius equals 273.15 K
- 0 K is absolute zero, where particle motion is as low as possible
- A change of 1 K is the same size as a change of 1 degree Celsius
So if the temperature rises by 10 degrees Celsius, it also rises by 10 K.
History / Origin
The Kelvin scale is named after William Thomson, better known as Lord Kelvin, a 19th century physicist from Scotland. He worked on the idea of an absolute temperature scale that does not depend on where water freezes or boils.
Scientists discovered that there is a lowest possible temperature, where particles have almost no motion. They called this point absolute zero. Lord Kelvin built a temperature scale that starts at this point.
Later, when the metric system grew into the modern SI system, Kelvin was chosen as the official base unit for temperature in science and engineering.
Symbol & Abbreviation
The unit uses a single capital letter.
- Name of unit. kelvin
- Symbol. K
Important points.
- Write K with a capital K and no degree sign. 300 K not 300 deg K and not 300 degrees Kelvin
- The word kelvin with a small k is used in writing. For example. The temperature is 300 kelvin
- The plural is kelvins, but in science people often just say kelvin
Current Use Around the World
Kelvin is used all over the world in science and technology because it is part of the SI system. It helps scientists share results clearly without confusion between Celsius and Fahrenheit.
Main uses.
- Physics and chemistry. For gas laws, particle motion, and thermodynamics
- Astronomy. To state the temperature of stars, planets, and space
- Engineering. For heat transfer, engines, and materials that face very high or very low temperatures
- Electronics. For sensors, superconductors, and semiconductor work
- Lighting and photography. Color temperature of light bulbs and screens is given in kelvin, like 2700 K for warm light and 6500 K for cool daylight
In daily life, most people still use Celsius or Fahrenheit for weather and cooking, but Kelvin is the common language for scientific work worldwide.
Example Conversions
To move between Kelvin and Celsius, you add or subtract 273.15.
- Celsius to Kelvin. K = degrees Celsius + 273.15
- Kelvin to Celsius. degrees Celsius = K minus 273.15
To move between Kelvin and Fahrenheit, you use Celsius as a middle step.
- Kelvin to Fahrenheit. degrees Fahrenheit = (K minus 273.15) × 9/5 + 32
- Fahrenheit to Kelvin. K = (degrees Fahrenheit minus 32) × 5/9 + 273.15
Some simple examples.
- 0 degrees Celsius = 273.15 K
- 100 degrees Celsius = 373.15 K
- 25 degrees Celsius room temperature around 298.15 K
- 0 K = minus 273.15 degrees Celsius
- 300 K ≈ 26.85 degrees Celsius
- 300 K ≈ 80.33 degrees Fahrenheit
Related Units
Kelvin is closely linked to other temperature units.
- Degree Celsius degrees C. Common in most countries for weather and everyday life. Same step size as Kelvin, just shifted by 273.15
- Degree Fahrenheit degrees F. Common in the United States for weather, cooking, and homes
- Rankine R or deg R. An absolute temperature scale like Kelvin, but with the Fahrenheit step size. Mostly used in some areas of engineering
Because Kelvin and Celsius share the same step size, changes in temperature are easy to compare between them.
FAQs
Why is Kelvin used instead of Celsius in science?
Kelvin starts at absolute zero and measures the true energy of particles. Equations in physics and chemistry work more cleanly with Kelvin, so it is preferred for scientific work.
Why is there no degree symbol with Kelvin?
Kelvin is defined as a base SI unit, not as degrees above a reference point like water freezing. For this reason, the correct way is to write K without the degree sign.
Can Kelvin temperatures be negative?
On the normal absolute temperature scale, Kelvin values are zero or positive. 0 K is the lowest. There are special advanced physics ideas that talk about negative temperatures, but in everyday science you never go below 0 K.
What is absolute zero in Kelvin?
Absolute zero is exactly 0 K. It is the point where particle motion is at a minimum. It is equal to about minus 273.15 degrees Celsius.
Is 1 K the same size as 1 degree Celsius?
Yes. A change of 1 K is the same size as a change of 1 degree Celsius. The only difference is where the zero point is placed on the scale.
Where do we see Kelvin in real life?
You may see Kelvin on LED light bulb boxes as color temperature, such as 3000 K warm white or 5000 K cool white. You also see it in science textbooks and lab work.
How hot is room temperature in Kelvin?
Room temperature is usually around 20 to 25 degrees Celsius. That is about 293 to 298 K.
How do I quickly estimate Celsius from Kelvin?
Subtract 273 from the Kelvin temperature. For example. 300 K minus 273 is about 27 degrees Celsius. This is close enough for many simple estimates.
