What is the formula to convert Celsius to Fahrenheit?

Multiply the Celsius value by nine fifths and add thirty-two, so 20 degrees Celsius becomes 68 Fahrenheit. The multiplication accounts for a Fahrenheit degree being smaller, and the addition shifts the zero point, since water freezes at zero Celsius but at thirty-two Fahrenheit. Reversing it means subtracting thirty-two first and then multiplying by five ninths.

Why does Kelvin add 273.15 rather than 273?

Because absolute zero sits exactly 273.15 Celsius degrees below the freezing point of water under the current definition of the kelvin. The rounded figure of 273 is a classroom shortcut that introduces an error of 0.15 kelvin, which is negligible for a rough estimate but not acceptable in calibration, cryogenics or any published measurement.

Is there a temperature where Celsius and Fahrenheit are equal?

Yes, at minus forty. Setting the two scales equal and solving gives exactly minus forty degrees on both, which is why weather reports in northern Canada can quote minus forty without specifying a scale. It is also a useful check on any conversion you do by hand, since a correct method must reproduce that crossing point.

When do I need Kelvin instead of Celsius?

Whenever a formula multiplies or divides by temperature rather than adding to it. The ideal gas law, radiative heat transfer, thermal noise and Carnot efficiency all require absolute temperature. Using Celsius makes ratios meaningless and can produce negative values in places where only positive temperatures are physically possible, which usually shows up as an impossible final answer.

How do I convert a temperature difference rather than a reading?

Drop the offsets and use the ratio only. A difference of one Celsius degree equals one kelvin and equals 1.8 Fahrenheit degrees, so a ten-degree Celsius rise is an eighteen-degree Fahrenheit rise, not fifty. Adding the thirty-two is the most frequent error in heating, cooling and insulation calculations, where every figure is a difference.

What is Rankine used for?

It is the absolute scale that keeps Fahrenheit-sized degrees, with zero at absolute zero and water freezing at 491.67 degrees Rankine. It appears in United States thermodynamics, combustion and aerospace work so that engineers can use absolute temperature in formulas without leaving the customary unit system their other figures are in.

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Why use this calculator

Temperature is the one measurement where two incompatible scales are still in daily use across the English-speaking world, and where the conversion is not a simple multiplication. Because Fahrenheit and Celsius have different zero points as well as different degree sizes, you cannot scale one into the other the way you can with metres and feet, which is exactly why mental shortcuts drift badly at the extremes.

Getting it wrong has consequences: an oven set to 180 Fahrenheit instead of 180 Celsius will never cook anything, and a fever reported in the wrong scale is meaningless. This converter shows all four scales at once so the sanity check is immediate, adds Rankine for thermodynamics work, and flags any reading that falls below absolute zero rather than quietly converting an impossible number.

How to use it

Enter the reading you want to convert in the temperature box, taking it exactly as printed on the thermometer, the recipe, the weather report or the equipment datasheet, including the minus sign if it is below zero.
Then choose the scale that reading is already in from the dropdown; the symbol beside the number on the source tells you, with a plain degree sign usually meaning Celsius outside the United States and Fahrenheit inside it, and K or a capital K with no degree sign meaning kelvin.
All four converted values appear at once, with the scales you did not select highlighted.
The last row places the temperature against familiar landmarks such as freezing, body temperature and oven heat so you can tell at a glance whether the answer is plausible.

What each result means

Celsius: The reading on the scale used by almost every country and by all scientific work outside the United States.
Fahrenheit: The same temperature on the scale used for weather, cooking and body temperature in the United States.
Kelvin: The SI absolute temperature, counted from absolute zero, which is what physics and chemistry formulas require.
Rankine: The absolute scale built on Fahrenheit degrees, still used in some US thermodynamics and combustion engineering.
Where this sits: A plain-language landmark for the converted value so you can immediately tell whether the result is plausible.

The formula

Celsius to Fahrenheit

°F = °C × 9/5 + 32

Back the other way

°C = (°F − 32) × 5/9

Where the factor 9/5 accounts for a Fahrenheit degree being five ninths the size of a Celsius degree and the 32 accounts for the different zero points.

Kelvin is an absolute scale sharing the Celsius degree size

K = °C + 273.15 exactly

Rankine is the absolute companion to Fahrenheit

°R = °F + 459.67

Rankine converts to Celsius

°C = (°R − 491.67) × 5/9

Absolute zero is 0 K, −273.15 °C, −459.67 °F and 0 °R.

How this number is used in practice

Cooking is where the conversion bites hardest. Older British and Australian recipes give gas marks, American recipes give Fahrenheit, and modern ovens are usually marked in Celsius, so the same cake can be described as 180 degrees, 350 degrees or gas mark four. Meat safety targets are also quoted in both, with poultry needing a core of 74 degrees Celsius or 165 Fahrenheit.

Because ovens are rarely accurate to better than ten degrees, the practical advice is to convert precisely and then trust an oven thermometer rather than the dial.

In science and engineering the absolute scales exist because so many laws are proportional to temperature measured from absolute zero rather than from an arbitrary reference. The ideal gas law, radiative heat transfer through the Stefan-Boltzmann relation, and thermodynamic efficiency limits all require kelvin; feeding Celsius into any of them produces answers that are not merely inaccurate but nonsensical, including negative absolute temperatures.

Rankine survives in parts of US industry for the same reason, letting engineers keep Fahrenheit-sized degrees while still having a true zero.

Weather and health are the everyday cases. A body temperature of 39 degrees Celsius is a serious fever at 102.2 Fahrenheit, while a fridge above 5 degrees Celsius is outside the food-safe range in most national guidance. Travellers benefit from the two mental anchors that actually work: the scales cross at minus 40, and doubling Celsius and adding 30 gets you within a couple of degrees of Fahrenheit across the ordinary weather range.

Worked examples

Room temperature reported as 72°F in the United States
22.22°C, 295.37 K and 531.67°R. A comfortable indoor reading.
Water boiling at sea level, 100°C
212.00°F, 373.15 K and 671.67°R.
Absolute zero entered as 0 K
-273.15°C and -459.67°F, the coldest temperature that can exist.
A moderate oven set to 180°C for baking
356.00°F, which is why American recipes round it to 350°F.

When people use it

Converting oven temperatures between an American recipe and a Celsius-marked oven
Reading an overseas weather forecast in the scale you actually think in
Turning a Celsius body temperature into Fahrenheit for a clinical reference range
Preparing Kelvin inputs for gas law, thermal radiation or efficiency calculations
Checking equipment datasheets that quote operating ranges in a scale you do not use

Tips

Minus 40 is the same number on both Celsius and Fahrenheit, which makes it the easiest reference point to remember.
For quick mental weather conversion, double the Celsius figure and add 30; it is within two degrees across the normal range.
Kelvin takes no degree symbol. Writing degrees Kelvin is incorrect, and the conversion offset is 273.15, not 273.
Never feed Celsius into a gas law or a radiation formula. Those require absolute temperature or the answer is meaningless.
A temperature difference converts differently from a temperature reading: a change of 10 degrees Celsius is a change of 18 Fahrenheit, with no offset applied.
Boiling point falls with altitude, so 100 degrees Celsius is a sea-level figure and cooking times lengthen in the mountains.

What it assumes, and where it stops being accurate

Freezing and boiling landmarks assume pure water at standard atmospheric pressure of 101.325 kilopascals.
Results are rounded to two decimals, which is far finer than domestic thermometers and ovens can actually resolve.
Readings below absolute zero are converted arithmetically but flagged, because no physical system can reach them.
The converter handles absolute temperature readings, not temperature differences, where the offset terms must be dropped.
Gas mark, oven fan-forced offsets and thermostat calibration error are not modelled; fan ovens typically run 20 degrees Celsius hotter than marked.
Rankine is included for legacy US engineering work and is not used outside that context.

Terms used on this page

Absolute zero: The lowest possible temperature, 0 K, equal to -273.15 °C and -459.67 °F, where thermal motion is minimal.
Kelvin: The SI base unit of thermodynamic temperature, sharing the Celsius degree size but counted from absolute zero.
Rankine: An absolute temperature scale using Fahrenheit-sized degrees, with zero at absolute zero.
Gas mark: A British oven scale where mark 4 is roughly 180 °C; each mark is about 14 °C apart.
Standard pressure: 101.325 kilopascals at sea level, the condition under which water is defined to boil at 100 °C.

Questions people ask

What is the formula to convert Celsius to Fahrenheit?: Multiply the Celsius value by nine fifths and add thirty-two, so 20 degrees Celsius becomes 68 Fahrenheit. The multiplication accounts for a Fahrenheit degree being smaller, and the addition shifts the zero point, since water freezes at zero Celsius but at thirty-two Fahrenheit. Reversing it means subtracting thirty-two first and then multiplying by five ninths.
Why does Kelvin add 273.15 rather than 273?: Because absolute zero sits exactly 273.15 Celsius degrees below the freezing point of water under the current definition of the kelvin. The rounded figure of 273 is a classroom shortcut that introduces an error of 0.15 kelvin, which is negligible for a rough estimate but not acceptable in calibration, cryogenics or any published measurement.
Is there a temperature where Celsius and Fahrenheit are equal?: Yes, at minus forty. Setting the two scales equal and solving gives exactly minus forty degrees on both, which is why weather reports in northern Canada can quote minus forty without specifying a scale. It is also a useful check on any conversion you do by hand, since a correct method must reproduce that crossing point.
When do I need Kelvin instead of Celsius?: Whenever a formula multiplies or divides by temperature rather than adding to it. The ideal gas law, radiative heat transfer, thermal noise and Carnot efficiency all require absolute temperature. Using Celsius makes ratios meaningless and can produce negative values in places where only positive temperatures are physically possible, which usually shows up as an impossible final answer.
How do I convert a temperature difference rather than a reading?: Drop the offsets and use the ratio only. A difference of one Celsius degree equals one kelvin and equals 1.8 Fahrenheit degrees, so a ten-degree Celsius rise is an eighteen-degree Fahrenheit rise, not fifty. Adding the thirty-two is the most frequent error in heating, cooling and insulation calculations, where every figure is a difference.
What is Rankine used for?: It is the absolute scale that keeps Fahrenheit-sized degrees, with zero at absolute zero and water freezing at 491.67 degrees Rankine. It appears in United States thermodynamics, combustion and aerospace work so that engineers can use absolute temperature in formulas without leaving the customary unit system their other figures are in.

Use this with

Temperature Converter answers one part of the question. These pick up where it stops: Oven Temperature Converter, Length Converter, Weight Converter and Roasting Time Calculator.

Where these figures come from

Works anywhereLast reviewed 2026-09·How we build and check calculators

This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.

NIST: SI base unit: kelvin. The kelvin, absolute zero and the thermodynamic temperature scale
BIPM: The International System of Units (SI Brochure). The defining document for the SI base units and their definitions
US National Weather Service. Operational temperature reporting, wind chill and heat index references

Temperature Converter produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.

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