Temperature Converter (°C, °F, K)
Convert temperatures between Celsius, Fahrenheit, Kelvin and Rankine instantly.
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Frequently asked questions
Is any data sent to a server when I use this tool?
Who invented the Celsius and Fahrenheit scales?
When do scientists use Kelvin instead of Celsius?
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Why does Fahrenheit use such odd numbers like 32 and 212?
What is absolute zero and can it be reached?
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About Temperature Converter (°C, °F, K)
Temperature is the measure of the average kinetic energy of the particles in a substance, and human civilization has developed several different scales to quantify it, each with its own historical origin and domain of use. The Fahrenheit scale, proposed by German physicist Daniel Gabriel Fahrenheit in 1724, was the first widely adopted standardized temperature scale. Fahrenheit based his scale on three reference points: the freezing point of a saltwater brine solution (0°F), the freezing point of pure water (32°F), and an approximation of human body temperature (96°F, later revised to 98.6°F). The scale was convenient for meteorology and medicine in the 18th century but its reference points are now considered arbitrary. The Celsius scale, introduced by Swedish astronomer Anders Celsius in 1742 and later reversed to its modern form by Carl Linnaeus, is anchored to the phase transitions of pure water: 0°C for freezing and 100°C for boiling at standard pressure. This elegant simplicity made Celsius the scientific and international standard.
The Kelvin scale, named after the Irish-Scottish physicist William Thomson (Lord Kelvin) who proposed it in 1848, is the foundation of thermodynamics. Unlike Celsius and Fahrenheit, which use arbitrary reference points, Kelvin starts at absolute zero — the theoretical temperature at which all molecular motion ceases. Absolute zero is 0 K, equivalent to -273.15°C or -459.67°F. Because Kelvin starts at the physical minimum, it is used exclusively in scientific contexts: gas law calculations, blackbody radiation, cryogenics, astrophysics, and any application where a ratio of temperatures is meaningful. The Rankine scale is an imperial equivalent of Kelvin, starting at the same absolute zero but using Fahrenheit-sized degrees, primarily found in US aerospace and thermodynamics engineering.
This tool converts temperatures between all four scales simultaneously. Type any value and the remaining three update instantly. All conversions are exact and run entirely in your browser — no server is involved. The tool also warns you when an input falls below absolute zero, since no physical temperature can be lower than 0 K.
A common confusion for students is that the Kelvin scale has no degree symbol — you write 300 K, not 300°K, because Kelvin is an absolute scale with a true zero, similar to how we write 5 kg not 5°kg. Another pitfall is that the size of one Kelvin degree equals one Celsius degree, so temperature differences are identical in both scales (a change of 10°C is also a change of 10 K), but a temperature in Celsius will always be 273.15 lower than its Kelvin equivalent. Fahrenheit temperature differences require an additional scaling factor of 9/5 relative to Celsius.
Brine, Body Heat, and Absolute Zero: The Surprising Origins of Temperature Scales
Daniel Gabriel Fahrenheit was a skilled instrument maker, and the scale bearing his name was a breakthrough in the early 18th century simply because it was reproducible. Before standardized scales, each thermometer maker used their own reference points, and thermometers from different manufacturers were useless for scientific comparison. Fahrenheit chose three points he could reliably recreate in his workshop: the temperature of an ice-brine-salt mixture (0°F), the freezing point of plain water (32°F), and the temperature of the human body (initially calibrated at 96°F). His mercury thermometers became the most accurate instruments of their era and were widely adopted in England and the Netherlands. The scale was only later adjusted to place the boiling point of water exactly at 212°F, which moved body temperature slightly to the famous 98.6°F.
Lord Kelvin — born William Thomson in Belfast in 1824 — arrived at the concept of an absolute temperature scale through thermodynamic theory. In 1848, building on the work of Carnot and Clausius, he proposed a scale where zero corresponded to the complete absence of heat energy, a point that could be calculated theoretically even if never reached physically. The Kelvin scale was defined in 1954 using the triple point of water (the unique temperature and pressure at which water, ice, and water vapour coexist) as the single fixed reference point, set at exactly 273.16 K. In 2019, the definition was revised once more — Kelvin is now defined by fixing the value of the Boltzmann constant, linking temperature directly to the fundamental physics of energy at the molecular level.
The Rankine scale, invented by Scottish engineer William John Macquorn Rankine in 1859, combines the absolute zero starting point of Kelvin with the degree size of Fahrenheit. It remains in use in certain branches of US engineering, particularly in thermodynamic calculations for aerospace propulsion, where engineers trained in imperial units still find it more natural than Kelvin. A temperature of 0°F corresponds to 459.67 degrees Rankine, and standard room temperature of 68°F equals 527.67°R. Outside of US aerospace and HVAC engineering, Rankine is rarely encountered today.