Temperature Converter

Type a temperature, pick the scale it is in, and pick the scale you want it in. The converter handles Celsius, Fahrenheit and Kelvin, recalculates as you type, and rejects anything below absolute zero, because no such temperature exists. The result is shown to two decimals; the arithmetic underneath keeps full precision.

Why temperature needs an offset, not just a factor

Converting meters to feet is a single multiplication because both units agree on where zero is. Temperature scales do not. Zero Celsius is the freezing point of water, zero Fahrenheit was the coldest brine mixture Daniel Fahrenheit could reliably reproduce, and zero kelvin is absolute zero. Because the zeros sit in different places, every conversion is a shift plus a scale, never a scale alone. This converter routes everything through Kelvin:

K = C + 273.15

K = (F + 459.67) × 5 ÷ 9

Converting out of Kelvin runs the matching formula in reverse. With the default inputs — 20 °C into Fahrenheit — the two steps are 20 + 273.15 = 293.15 K, then 293.15 × 9 ÷ 5 − 459.67 = 527.67 − 459.67 = 68 °F. Going through a common floor is not the only method — you can jump straight between Celsius and Fahrenheit with F = 1.8C + 32 — but a single pivot means the converter carries just two formulas instead of six, and the pivot it uses, Kelvin, is the only one of the three whose zero is a physical floor rather than a chosen mark.

Three scales, three different zeros

Daniel Gabriel Fahrenheit (1686–1736), born in Danzig to a family of German origin and working most of his life in the Dutch Republic, built the first dependable mercury-in-glass thermometers and published the scale that carries his name in 1724, in a set of Latin papers for the Royal Society. He borrowed the structure from an earlier scale by the Danish astronomer Ole Rømer, whom he visited in 1708, then stretched the numbers. Zero marked the lowest temperature he could hold steady in the workshop, a slurry of water, ice and ammonium chloride. He set the freezing of plain water at 32 and roughly 96 for body heat, which leaves exactly 64 degrees between the two, and 64 can be bisected six times over, so the intermediate marks could be scratched onto a tube by eye. Later standardization pinned the scale to water freezing at 32 and boiling at 212, an interval of 180 degrees, which shifted normal body temperature to the familiar 98.6.

Anders Celsius (1701–1744), professor of astronomy at Uppsala from 1730, described a hundred-step scale in 1742 to the Royal Society of Sciences there, and ran it upside down by modern habit: 0 at the boiling point of water, 100 at the freezing point. Within a few years the orientation was reversed. The French physicist Jean-Pierre Christin published a right-way-up version at Lyon in 1743, and Carl Linnaeus had a thermometer built by the Swedish instrument maker Daniel Ekström reading 0 for melting ice and 100 for boiling water. The scale went by "centigrade," from the Latin for a hundred steps, for roughly two centuries. The name Celsius became official in 1948, when the 9th General Conference on Weights and Measures listed "degree Celsius" and the symbol °C among the standard units. One reason was a clash of names: in French angular measure, a centigrade was already a hundredth of a gradian, the gradian itself being a hundredth of a right angle.

Kelvin came out of thermodynamics rather than the weather. William Thomson, later Lord Kelvin, argued in his 1848 paper "On an Absolute Thermometric Scale" for a scale independent of any particular substance, built on the physics of heat engines rather than the quirks of mercury or alcohol. Extrapolating how gases lose pressure as they cool, he placed the true zero near −273 °C; the accepted figure today is −273.15 °C. Guillaume Amontons had glimpsed the same floor as early as 1703, reasoning from an air thermometer that pressure would fall to nothing somewhere around −240 °C. Absolute zero is the state in which a system holds the least internal energy it possibly can, thermal motion at its minimum, though quantum mechanics leaves a residual zero-point energy even there. That hard floor is why the converter refuses any value below 0 K and returns an error instead of a number.

Anchor points worth knowing

The −40 crossover is algebra rather than coincidence: a Fahrenheit degree is smaller than a Celsius degree, but the Fahrenheit scale starts 32 units higher, and the two effects cancel at exactly one point. Set C equal to F in F = 1.8C + 32 and −40 is the only solution.

From the triple point to the Boltzmann constant

Kelvin's scale still needed a fixed number to hang on. From 1954, when the 10th General Conference on Weights and Measures settled the question, that anchor was the triple point of water, the single temperature and pressure at which ice, liquid and vapour coexist, defined as exactly 273.16 K. In 1967 the 13th General Conference dropped the "degree" and the ° symbol, renaming the unit simply the kelvin, symbol K, and keeping it as a base unit of the metric system. That is why the correct form is 300 K, not 300 °K, and why you say three hundred kelvins the way you would say three hundred meters.

Then the definition changed once more. Rather than tie the kelvin to a property of water, the 26th General Conference, meeting in November 2018, fixed the Boltzmann constant, the bridge between temperature and energy, at exactly 1.380649 × 10⁻²³ joules per kelvin, and the new wording took effect on 20 May 2019. The size of one kelvin now follows from a constant of nature instead of a sample of water. For everyday conversion nothing shifted, because the numbers were chosen so 0 °C stays at 273.15 K, but the definition underneath is now reproducible in any laboratory without a flask of anything.

A mental shortcut and how wrong it gets

For weather, double the Celsius number and add 30. The shortcut is exact at 10 °C, where the rule and the real formula both give 50 °F, and it drifts by 0.2 °F for each degree away from there. At 30 °C it says 90 °F against a true 86 °F; at −10 °C it says 10 °F against a true 14 °F. Anywhere between −10 and 30 °C you stay within 4 °F, which settles a jacket decision and should never set an oven.

Where each scale is still used

The split for weather and cooking is mostly geographic. Fahrenheit survives chiefly in the United States, where forecasts, ovens and thermostats all use it, along with its territories, the Cayman Islands and the freely associated Pacific states of Palau, Micronesia and the Marshall Islands. The United Kingdom's Met Office began printing both scales in 1962 and dropped Fahrenheit from official reports in 1970, though tabloids still reach for it whenever a heatwave lets them print a bigger number. Canada replaced Fahrenheit in its weather reports in April 1975, yet Canadian kitchens keep running Fahrenheit ovens, largely because the appliances come from across the border. Australia's forecasts went Celsius-only on 1 September 1972, while continental Europe had settled on Celsius far earlier, along with the rest of the metric system. Science uses kelvin almost everywhere, which is exactly why this converter pivots through it: with a single shared zero at the bottom, each scale sits one clean shift-and-stretch away from the others. The Glasgow engineer William Rankine proposed an absolute scale with Fahrenheit-sized degrees in 1859, still seen in some American engineering, but Celsius, Fahrenheit and Kelvin cover nearly every thermometer you are likely to read.

Frequently asked questions

What is 20 degrees Celsius in Fahrenheit?

20 °C is exactly 68 °F. Multiply by 1.8 and add 32: 20 × 1.8 = 36, plus 32 gives 68. It sits at the comfortable end of room temperature, which is why thermostats around the world cluster between 18 and 21 °C.

At what temperature are Celsius and Fahrenheit equal?

At −40, and only there. Set C equal to F in the conversion F = 1.8C + 32 and the single solution is −40. The cold makes the distinction academic anyway — −40 means the same thing in Winnipeg as it does in Fairbanks.

Why does Kelvin not use a degree symbol?

Because since 1967 the kelvin has been an SI base unit in its own right, not a graduation mark on a scale. You write 300 K and say three hundred kelvins, the same way you would say three hundred meters. Celsius and Fahrenheit keep the degree sign because their zero points are arbitrary reference marks rather than a true zero.

Is there an easy way to convert Celsius to Fahrenheit in my head?

Double the Celsius figure and add 30. The trick is exactly right at 10 °C, where both the rule and the real formula give 50 °F, and it drifts by 0.2 °F for every degree away from that point, so a 25 °C afternoon reads as 80 °F instead of the true 77 °F. Close enough for a forecast, too loose for an oven.

Can anything be colder than absolute zero?

No. Absolute zero — 0 K, which is −273.15 °C or −459.67 °F — is where particles hold the minimum possible thermal energy, and no process can cool a system past it. If your input works out below 0 K, this converter shows an error instead of a number.