What is Kelvin? How to Convert Kelvin to Celsius and Fahrenheit

What is the Kelvin Scale?

Kelvin (K) is the SI base unit of thermodynamic temperature. Unlike Celsius and Fahrenheit, Kelvin is an absolute temperature scale — it starts at absolute zero, the coldest theoretically possible temperature, where all molecular motion stops.

The scale is named after William Thomson, 1st Baron Kelvin (Lord Kelvin), the 19th-century British physicist who established the concept of absolute zero. Notably, temperatures in Kelvin are written without a degree symbol — it's simply "300 K", not "300°K".

Key fact: 0 K = −273.15°C = −459.67°F (absolute zero)

Why Does Science Use Kelvin?

Kelvin is the preferred temperature scale in physics, chemistry, and engineering because it starts at absolute zero. This makes it essential for calculations involving gas laws, thermodynamics, and radiation:

• Ideal Gas Law: PV = nRT uses temperature in Kelvin because gas volume is proportional to absolute temperature, not to Celsius values.
• Blackbody radiation: Stefan-Boltzmann law (energy ∝ T⁴) requires Kelvin.
• Cryogenics: Liquid nitrogen is 77 K, liquid helium is 4.2 K — Celsius values would be negative and harder to compare.
• Colour temperature: Light bulbs and displays are rated in Kelvin (e.g., 5500K daylight, 2700K warm white).

Kelvin to Celsius Conversion

The Celsius and Kelvin scales use the same size degree — they just start at different points. The offset between them is exactly 273.15.

• K to °C: °C = K − 273.15
• °C to K: K = °C + 273.15

Examples:

• 0 K = −273.15°C (absolute zero)
• 273.15 K = 0°C (water freezes)
• 373.15 K = 100°C (water boils at sea level)
• 310 K = 36.85°C ≈ 37°C (human body temperature)
• 5778 K = 5504.85°C (surface temperature of the Sun)

Kelvin to Fahrenheit Conversion

To convert Kelvin directly to Fahrenheit, combine the K→°C and °C→°F steps:

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

Examples:

• 0 K = −459.67°F (absolute zero)
• 273.15 K = 32°F (water freezes)
• 373.15 K = 212°F (water boils)
• 310 K = 98.33°F (human body temperature)
• 294 K = 69.53°F (comfortable room temperature)

Rankine: The Absolute Scale for Fahrenheit Users

Just as Kelvin is the absolute version of Celsius, Rankine (°R) is the absolute version of Fahrenheit. The conversion is straightforward:

• °R = °F + 459.67
• °R = K × 9/5

Rankine is used in some US engineering contexts, particularly thermodynamics problems that prefer Fahrenheit-sized degrees. It's rarely seen outside American engineering textbooks.

Temperature Reference: All Four Scales

• Absolute zero: 0 K = −273.15°C = −459.67°F = 0°R
• Water freezes: 273.15 K = 0°C = 32°F = 491.67°R
• Room temperature: 293–295 K = 20–22°C = 68–72°F = 527–531°R
• Body temperature: 310 K = 36.85°C = 98.33°F = 558°R
• Water boils: 373.15 K = 100°C = 212°F = 671.67°R

Use our free temperature converter to convert between all four scales — Kelvin, Celsius, Fahrenheit, and Rankine — instantly.