Conversion from Fahrenheits to Kelvins

Information about the Fahrenheit (°F)

The degree Fahrenheit is a unit of temperature on a scale where water freezes at 32 and boils at 212 under standard atmospheric pressure. Its symbol is the degree sign followed by F. It is the everyday temperature unit of the United States and a handful of small territories, and it is used almost nowhere else.

Daniel Gabriel Fahrenheit devised the scale in 1724 using three reference points. Zero was the temperature of a freezing mixture of ice, water and ammonium chloride, the coldest he could reliably reproduce. Thirty-two was the freezing point of plain water, and ninety-six was intended as human body temperature, though later measurement put normal body temperature nearer 98.6. The odd-looking numbers are the residue of that construction rather than arbitrary choices.

The degree is smaller than the Celsius degree, five Fahrenheit degrees to every nine Celsius, which gives the scale finer resolution in whole numbers. Defenders argue this suits weather reporting, where the range from zero to a hundred covers most of the inhabited world's climate. Critics point out that the same resolution is available in Celsius by using one decimal place, and that the scale's reference points relate to nothing anyone encounters.

American usage is nearly total in daily life: forecasts, thermostats, ovens and body temperature are all in Fahrenheit. American science, medicine and engineering are not, which produces the routine domestic conversion of a laboratory result in Celsius into a clinical note in Fahrenheit. The two scales cross at minus forty, where both read the same number.

Its finer graduation is the one technical argument in its favour. Because a Fahrenheit degree is five ninths of a Celsius degree, ordinary weather can be described in whole numbers across a wider range without fractions, and the span from a cold night to a hot afternoon covers roughly a hundred units rather than forty. Defenders of the scale add that it is calibrated to human comfort rather than to water: zero and a hundred bracket the range a person is likely to meet outdoors, whereas nought and a hundred Celsius bracket the range a kettle meets. Neither argument has moved any country to adopt it, but both explain why American resistance to changing has been durable.

Thirty-two degrees Fahrenheit equals 0 degrees Celsius, and a change of one degree Fahrenheit equals a change of five ninths of a degree 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.