Temperature Converter, Celsius, Fahrenheit, Kelvin

Three temperature scales explained

Three temperature scales handle almost every situation a normal person runs into. Celsius is the worldwide standard for everyday measurement. Its zero point is the freezing point of fresh water, and its hundred point is the boiling point of fresh water at sea level. That makes the scale easy to teach and easy to picture: a glass of ice water sits at roughly 0°C, a steaming pot sits at 100°C, and human body temperature is right around 37°C.

Fahrenheit was the standard temperature scale in much of the world for two hundred years and is still used in the United States, the Bahamas, the Cayman Islands, Palau, and a few US territories. Daniel Fahrenheit designed his scale so that 0°F was as cold as he could reasonably produce with a brine mixture and 100°F was close to human body temperature. Modern Fahrenheit puts water freezing at 32°F and water boiling at 212°F, with body temperature settled at 98.6°F.

Kelvin is the absolute temperature scale used in science. It starts at absolute zero, the theoretical point where particles have the lowest possible energy. A Kelvin degree is exactly the same size as a Celsius degree, so the only difference between the two scales is a constant shift of 273.15. Kelvin uses no degree symbol because it is treated as an absolute unit, not a relative scale.

Conversion formulas between scales

The formulas look complicated until you spot the pattern. Each pair of scales has a different zero point and a different degree size, so a conversion has to handle both. To convert Celsius to Fahrenheit, use °F = (°C x 9/5) + 32: you scale up by 9/5 (because a Celsius degree is 1.8 times bigger than a Fahrenheit degree) and then shift by 32 to align the zero points. To go from Fahrenheit to Celsius you reverse it with °C = (°F - 32) x 5/9: subtract 32, then scale by 5/9. The converter above works as a Celsius to Fahrenheit calculator and a Fahrenheit to Celsius calculator at the same time, so you never have to choose a direction.

For Kelvin the math is simpler because a Kelvin degree is the same size as a Celsius degree. To convert Celsius to Kelvin, add 273.15. To convert Kelvin to Celsius, subtract 273.15. To convert Fahrenheit to Kelvin, first convert to Celsius, then add 273.15. The Rankine scale is to Fahrenheit what Kelvin is to Celsius, so converting Fahrenheit to Rankine just means adding 459.67. Réaumur, used in old European cookbooks, scales water freezing at 0° and water boiling at 80°, so each Réaumur degree equals 1.25 Celsius degrees.

A quick mental shortcut for Celsius to Fahrenheit: double the Celsius value and add 30. So 20°C becomes about 70°F (the real answer is 68°F), and 30°C becomes about 90°F (the real answer is 86°F). The shortcut is off by a few degrees but useful when you do not have a calculator and only need a rough sense of the weather forecast you are reading.

Why most science uses Kelvin

Kelvin is the official SI base unit for thermodynamic temperature, and physics formulas almost always require it. Gas laws, blackbody radiation, thermal conductivity, and the Stefan Boltzmann law all involve multiplying or dividing by temperature directly. Negative or zero values would break the math. Using a scale that starts at the true bottom of the temperature range solves the problem.

Astronomy in particular relies on Kelvin. Stars are classified by surface temperature: red dwarfs sit around 3,000 K, our Sun is about 5,778 K, and the hottest blue stars reach 50,000 K. The cosmic microwave background, the faint glow left over from the early universe, is measured at 2.725 K. The temperatures inside particle accelerators and quantum computing experiments approach absolute zero so closely that scientists use millikelvins and microkelvins to describe them.

Engineers also use Kelvin and Rankine when designing engines, heat exchangers, and refrigeration systems. The performance of any device that converts heat into work depends on the difference between an absolute hot temperature and an absolute cold temperature. Putting those numbers in Celsius or Fahrenheit and treating them as relative scales gives you the wrong answers.

Absolute zero and the theory of temperature

Temperature is a measure of the average kinetic energy of particles in a substance. Hotter substances have particles moving faster, colder substances have particles moving slower. Absolute zero is the temperature at which classical motion stops entirely: 0 K, -273.15°C, -459.67°F. Quantum mechanics says particles never truly stop because of zero point energy, but the macroscopic motion that gives us our everyday sense of heat is gone.

Reaching absolute zero is impossible in practice. The third law of thermodynamics, set out by Walther Nernst in 1906, shows that an infinite number of steps would be required to remove the last bit of energy from a system. The closest anyone has come is around 38 picokelvin (38 trillionths of a kelvin), produced in 2021 by German physicists studying ultracold rubidium atoms in microgravity. Even outer space is much warmer than that, sitting at the 2.7 K background left from the big bang.

The story of how scientists worked out the relationship between temperature and energy spans three centuries. Galileo built an early thermoscope around 1593. Anders Celsius proposed his scale in 1742. Lord Kelvin (William Thomson) defined the absolute scale in 1848 by extrapolating gas behavior backward to find the point where pressure would vanish. Ludwig Boltzmann tied temperature directly to particle motion through statistical mechanics in the late 19th century, giving us the equation S = k log W and the foundation of modern thermodynamics.

Temperature in cooking around the world

Recipes are where most people first run into the difference between Celsius and Fahrenheit. A British or Australian cookbook calls for a moderate oven at 180°C. An American cookbook calls for 350°F. Both describe the same temperature, give or take a few degrees. UK and European ovens commonly use gas marks instead of degrees: gas mark 4 is 180°C, gas mark 6 is 200°C, gas mark 8 is 230°C.

Meat doneness uses Fahrenheit in the US and Celsius almost everywhere else, but the underlying temperatures are the same. Rare beef is 52 to 55°C (125 to 130°F). Medium is 60 to 63°C (140 to 145°F). Well done is above 71°C (160°F). Chicken should reach 74°C (165°F) for food safety. Pork is now considered safe at 63°C (145°F) followed by a three minute rest.

Sugar work uses its own set of cooking stages with specific Fahrenheit ranges: soft ball at 235 to 240°F (113 to 116°C), firm ball at 245 to 250°F (118 to 121°C), hard crack at 300 to 310°F (149 to 154°C). Coffee brewing wants water between 90 and 96°C (194 to 205°F). Tea varies by leaf: green tea wants 70 to 80°C (158 to 176°F), black tea wants near boiling at 95°C (203°F). Knowing how to switch quickly between scales is the difference between a recipe that works the first time and one you redo.

How to convert Celsius to Fahrenheit

Celsius to Fahrenheit is the single most searched temperature conversion, so it pays to know it well. The formula is °F = (°C x 9/5) + 32. You multiply the Celsius value by 9/5, which is the same as 1.8, because a Celsius degree is 1.8 times larger than a Fahrenheit degree, then add 32 to line up the freezing points. To convert Celsius to Fahrenheit in your head, double the number and add 30 for a quick estimate. Whether you type into the Celsius box here or use it as a Celsius to Fahrenheit calculator, the math is identical and the result appears instantly.

Common Celsius to Fahrenheit conversions

A handful of Celsius to Fahrenheit values come up constantly, so it helps to know them. 20 degrees C to F is 68°F, a comfortable room temperature. 30 degrees C to F is 86°F, a warm summer day. 32 Celsius to Fahrenheit is 89.6°F, and 40 degrees C to F is 104°F, the kind of heat that triggers warnings. 90 C to F is 194°F, near coffee brewing range, and 100°C is 212°F, the boiling point of water. For oven users, 200 degrees Celsius to Fahrenheit is 392°F, a hot-roasting setting. Each of these uses the same formula, °F = (°C x 9/5) + 32, and the converter above returns the exact figure the instant you type the Celsius value.

How to convert F to Celsius

Going from Fahrenheit to Celsius reverses the same two steps. The formula is °C = (°F - 32) x 5/9. You subtract 32 first to align the zero points, then multiply by 5/9 (about 0.556) because a Fahrenheit degree is smaller than a Celsius degree. To convert F to Celsius quickly in your head, subtract 30 and halve the result for a rough figure. This is the conversion travelers reach for most when reading a US weather forecast or setting an American thermostat, and the converter above handles it the moment you type a value into the Fahrenheit box.

Common Fahrenheit to Celsius conversions

The reverse direction matters just as often, especially for travelers reading US weather or thermostats. 20 degrees Fahrenheit to Celsius is -6.7°C, a cold winter morning. 50°F is 10°C, 68°F is 20°C, and 75°F is 23.9°C, a pleasant indoor setting. 98.6°F is 37°C, normal body temperature, and 100°F is 37.8°C, the edge of a low fever. To convert F to Celsius, subtract 32 and multiply by 5/9. The tool handles this automatically, so typing a value into the Fahrenheit box gives you the Celsius and Kelvin equivalents at once.

Oven temperature conversions for cooking

Oven settings are one of the most searched temperature conversions. A slow oven of 150°C is 300°F, a moderate oven of 180°C is 350°F, and a hot oven of 220°C is 425°F. UK recipes that list gas marks line up neatly: gas mark 1 is 140°C (275°F), gas mark 4 is 180°C (350°F), and gas mark 7 is 220°C (425°F). When you switch a recipe between metric and US measurements, convert the oven temperature first, then adjust bake time only if your oven runs hot or cold. The cooking reference table above lists the full range of baking, roasting, and pizza-oven settings.

Understanding weather temperatures abroad

Weather forecasts are where conversion confusion hits travelers hardest, because most countries report in Celsius while the US reports in Fahrenheit. A forecast of 0°C means freezing (32°F), 10°C is a cool jacket day (50°F), 25°C is mild and pleasant (77°F), and 35°C is a hot day (95°F). Going the other way, a US forecast of 90°F is a warm 32°C, and 20°F is a cold -6.7°C. The double-and-add-30 mental shortcut for Celsius to Fahrenheit is close enough to decide what to wear, but the converter gives the precise figure when packing or planning matters.

200 degrees Celsius to Fahrenheit and other oven values

Oven temperature questions drive a large share of temperature searches. 200 degrees Celsius to Fahrenheit is 392°F, a strong roasting heat used for vegetables, potatoes, and crisp-skinned chicken. Other common oven points follow the same °F = (°C x 9/5) + 32 formula: 160°C is 320°F, 180°C is 356°F, 220°C is 428°F, and 230°C is 446°F. US recipes usually round these to the nearest familiar dial setting, so 180°C is written as 350°F and 220°C as 425°F. When you switch a recipe between metric and US units, convert the oven temperature first and keep the bake time the same unless your oven is known to run hot or cold.

20 degrees Fahrenheit to Celsius and cold weather values

Cold US forecasts confuse visitors used to Celsius. 20 degrees Fahrenheit to Celsius is -6.7°C, a hard winter cold that calls for a heavy coat. Other cold-weather points include 0°F at -17.8°C, 10°F at -12.2°C, and 32°F at exactly 0°C, the freezing point of water. Going warmer, 50°F is 10°C and 75°F is 23.9°C. To convert F to Celsius for any of these, use °C = (°F - 32) x 5/9, or just type the Fahrenheit value into the converter and read the Celsius and Kelvin equivalents at once.

Why -40 degrees is the same in Celsius and Fahrenheit

There is exactly one temperature where the Celsius and Fahrenheit scales meet: -40 degrees. At that point -40°C equals -40°F. It happens because the two scales have different zero points and different degree sizes, and their lines cross only once. You can prove it by solving °F = (°C x 9/5) + 32 with °F set equal to °C, which gives -40. It is a handy fact for science problems and a quick sanity check that a Celsius to Fahrenheit calculator is working correctly.

How to use this temperature converter

The tool is built to be the fastest way to convert temperature without picking a direction. Click any of the Celsius, Fahrenheit, or Kelvin boxes, type a number, and the other two scales update on every keystroke. Negative values, decimals, and large numbers all work, so it handles everything from freezer settings to star temperatures. The preset chips jump straight to common points such as freezing, body temperature, and boiling. Nothing is stored, nothing is sent anywhere, and there is no signup, so it works as a quick Celsius to Fahrenheit calculator any time you need one.