Conversion from 5 Kelvins to Celsius

Information about the Kelvin (°K)

The kelvin is the base unit of thermodynamic temperature in the International System of Units. Its symbol is K, written without a degree sign and without the word degree, so temperatures are stated as 300 kelvins rather than 300 degrees kelvin.

Its zero is absolute zero, the temperature at which the thermal motion of particles reaches its quantum-mechanical minimum. Nothing can be colder, which makes the kelvin an absolute scale rather than a relative one. That property matters in physics because ratios become meaningful: a gas at 600 kelvins has twice the absolute temperature of one at 300, a statement that is simply false if the same temperatures are written in Celsius or Fahrenheit.

The size of the kelvin is identical to the size of the degree Celsius, so temperature differences are numerically the same in both. Only the offset differs, and it is exactly 273.15. William Thomson, later Lord Kelvin, proposed the absolute scale in 1848, reasoning from Carnot's work on heat engines that a thermodynamic temperature independent of any particular substance must exist.

Since 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380649 times ten to the power minus twenty-three joules per kelvin. The previous definition used the triple point of water, a real physical fixture that nonetheless depends on the isotopic composition of the sample. Cryogenics, astronomy, spectroscopy and colour temperature in photography and lighting all use the unit, which is why a warm white bulb is sold as 2700 K and daylight as 5500 K.

Whole disciplines live at particular points on the scale. The cosmic microwave background sits at 2.725 kelvin, the boiling point of liquid helium at 4.2, and superconductors of the older kind work below about 20, which is why they need helium rather than nitrogen. Liquid nitrogen boils at 77 kelvin and is cheap, so the discovery of materials superconducting above that mark in 1986 changed what was practical. At the other end, a filament lamp runs near 2800 kelvin, the surface of the Sun is 5800, and its core reaches fifteen million. Expressing all of this on one scale with a physical zero is what makes such comparisons meaningful.

Zero kelvin equals minus 273.15 degrees Celsius, and 273.15 kelvin equals 0 degrees Celsius.

Information about the Celsius (°C)

The degree Celsius is a unit of temperature on a scale where water freezes at 0 and boils at 100 under standard atmospheric pressure. Its symbol is the degree sign followed by C. It is the everyday temperature unit of almost every country in the world, and its degree is exactly the same size as the kelvin.

Anders Celsius proposed the scale in 1742 with the numbers the other way round: 0 for the boiling point of water and 100 for its freezing point. Whether Celsius himself, Carl Linnaeus or the instrument maker Daniel Ekström reversed it is disputed, but the inversion happened within a few years of his death and the scale has run upward ever since. It was called the centigrade scale for two centuries and renamed after Celsius in 1948, partly because centigrade already meant a hundredth of a right angle in French and Spanish surveying.

The modern definition no longer refers to water at all. The Celsius scale is defined from the kelvin, with zero degrees Celsius set at exactly 273.15 kelvin, and the kelvin itself fixed by the Boltzmann constant since 2019. The freezing and boiling points of water are now measured results rather than definitions, and neither falls at exactly 0 or 100 under all conditions.

The United States is the only major country that reports weather in Fahrenheit, and even there the sciences work in Celsius. Body temperature, cooking, weather and industrial process control all use the unit, and its two anchor points are memorable enough that most people can estimate an unfamiliar temperature without conversion.

A distinction of grammar carries a real meaning here. A temperature is written as a degree Celsius, but a difference between two temperatures is written as a kelvin, because the two scales share the same interval and differ only in where zero sits. Heat calculations therefore express specific heat capacity in joules per kilogram per kelvin even when the temperatures themselves are quoted in Celsius, and a rise of five degrees Celsius is a rise of five kelvin exactly. The convention looks pedantic until a calculation mixes an absolute temperature with a temperature difference, at which point keeping them apart is the only thing that prevents an offset of 273.15 from entering the arithmetic.

Zero degrees Celsius equals 273.15 kelvin or 32 degrees Fahrenheit, and a change of one degree Celsius equals a change of 1.8 degrees Fahrenheit.