Temperature Unit Converter

Temperature Scales and Conversion Between Them

Temperature is one of the fundamental physical quantities describing the thermal state of matter. Over the years, different temperature scales have been developed to suit the needs of science, industry, and daily life. The most commonly used scales are: Kelvin (K), Celsius (°C), Fahrenheit (°F), and Rankine (°R). Below is a description of each scale and how to convert between them.

Kelvin Scale (K)

The Kelvin scale is an absolute scale primarily used in scientific contexts. Its zero point, known as absolute zero (0 K), represents the lowest possible temperature.

• 0 K = -273.15°C
• No negative values – all temperatures are positive

History and definition:
The Kelvin scale was proposed by Lord William Thomson (later Lord Kelvin) in 1848. It is based on the Celsius scale but starts at absolute zero. One Kelvin degree is equal in magnitude to one Celsius degree. Today, the definition is based on the Boltzmann constant: one Kelvin corresponds to a change in thermal energy per particle as defined by this constant.

Celsius Scale (°C)

The most widely used temperature scale in everyday life. Its zero point (0°C) corresponds to the freezing point of water, and 100°C to the boiling point of water.

• 0°C = 273.15 K

History and definition:
The Celsius scale was introduced by Anders Celsius in 1742. Originally, 0°C corresponded to the boiling point of water and 100°C to its freezing point – the scale was reversed after his death. Today, the Celsius scale is defined in terms of the Kelvin scale: one Celsius degree equals one Kelvin degree.

Fahrenheit Scale (°F)

The Fahrenheit scale is mainly used in the United States. Water freezes at 32°F and boils at 212°F.

• 0°C = 32°F
• 100°C = 212°F

History and definition:
The Fahrenheit scale was developed by Daniel Gabriel Fahrenheit in 1724. The zero point was based on a mixture of ice, water, and salt, while 96°F was originally intended to represent body temperature (now considered 98.6°F). Modern definitions use the freezing and boiling points of water: 180 Fahrenheit degrees span the range between 32°F and 212°F, so one Fahrenheit degree equals 5/9 of a Celsius degree.

Rankine Scale (°R)

An absolute scale expressed in Fahrenheit units. It is primarily used in engineering in English-speaking countries.

• 0°R = 0 K = -459.67°F

History and definition:
The Rankine scale was developed by William Rankine in 1859. It is analogous to the Kelvin scale but uses Fahrenheit degrees. One Rankine degree is equal to one Fahrenheit degree, measured from absolute zero. Modern definitions are linked to the Kelvin scale and the Boltzmann constant, with a conversion factor of 1.8.

Conversions Between Scales

• Celsius ↔ Kelvin
  K = °C + 273.15
°C = K − 273.15
• Celsius ↔ Fahrenheit
  °F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
• Kelvin ↔ Rankine
  °R = K × 1.8
K = °R ÷ 1.8
• Fahrenheit ↔ Rankine
  °R = °F + 459.67
°F = °R − 459.67
• Celsius ↔ Rankine
  °R = (°C + 273.15) × 1.8
°C = (°R ÷ 1.8) − 273.15