How do I convert Celsius to Fahrenheit?

Multiply by 9/5 and add 32. So 20°C is (20 × 1.8) + 32 = 68°F.

Is there a quick mental shortcut?

Double the Celsius figure and add 30. For 20°C that gives 70 against a true 68 — close enough for deciding on a jacket, and increasingly wrong at extremes.

Why is −40 the same in both scales?

Because the difference in degree size and the difference in zero point exactly cancel there. It is the single crossing point of the two scales.

Why does Kelvin not use the degree symbol?

Because it is an absolute unit rather than a scale of degrees. It was officially dropped in 1967, so correct usage is 300 K.

What is absolute zero?

−273.15°C or −459.67°F — the temperature at which molecular motion reaches its minimum. It is the zero point of both Kelvin and Rankine.

How do I convert a temperature change rather than a temperature?

Use only the ratio: multiply a Celsius change by 1.8 for Fahrenheit. Never add the 32, since a difference has no zero point to shift.

Why is body temperature 98.6°F such an odd number?

Because it is a conversion of 37°C, which was itself a rounded nineteenth-century average. The false precision comes from converting a round number in the other scale.

Does water always boil at 100°C?

Only at standard pressure. At altitude it boils lower — around 93°C at 2,000 metres — which is why high-altitude cooking takes longer.

Temperature Calculator

Convert Celsius, Fahrenheit, Kelvin, Rankine, and more.

About the Temperature Calculator

Convert between Celsius, Fahrenheit and Kelvin. The two everyday scales cross at minus 40, which is the only point where they read the same.

How to use it

Enter the temperature.
Choose the scale you are converting from.
Read the equivalents.

The formula

Four scales, three conversions, and one detail that explains all of them: Celsius and Fahrenheit have both different degree sizes and different zero points.

°F = °C × 9/5 + 32 · °C = (°F − 32) × 5/9
K = °C + 273.15 · °R = °F + 459.67

The 9/5 exists because there are 180 Fahrenheit degrees between freezing and boiling and only 100 Celsius ones, so each Celsius degree is 1.8 times larger. The 32 exists because the scales put zero in different places. You need both operations, in that order, which is why the reverse conversion subtracts before multiplying.

Kelvin and Rankine are absolute scales — their zero is absolute zero, the point where molecular motion stops. They need no negative numbers, which is why physics and engineering prefer them.

Worked examples

Celsius	Fahrenheit	Kelvin	Rankine	Reference point
−40°	−40°	233.15	419.67	the scales cross
0°	32°	273.15	491.67	water freezes
20°	68°	293.15	527.67	room temperature
37°	98.6°	310.15	558.27	body temperature
100°	212°	373.15	671.67	water boils

The first row is a genuine curiosity worth remembering: −40° is the same temperature on both everyday scales. It is the one point where the different degree sizes and different zeros exactly cancel, which is why manufacturers of antifreeze and cold-weather gear can print a single number and have it be correct everywhere.

Common mistakes

Multiplying before subtracting when converting to Celsius. Subtract 32 first, then multiply by 5/9. Reversing the order gives an answer wrong by nearly 18 degrees.
Writing 'degrees Kelvin'. Kelvin is not a degree scale — it is simply 300 K, not 300°K. Rankine, oddly, does take the degree symbol.
Converting a temperature difference like a temperature. A change of 10°C is a change of 18°F, not 50°F. For differences you apply only the ratio, never the offset.
Rounding 273.15 to 273. Fine for weather, not for laboratory work. The 0.15 is part of the definition and matters wherever precision does.

Terms explained

Celsius: Zero at water's freezing point, 100 at its boiling point at standard pressure. The everyday scale almost everywhere.
Fahrenheit: Freezing at 32 and boiling at 212, giving 180 degrees between them. Standard in the United States.
Kelvin: The SI absolute scale. Same degree size as Celsius, with zero at absolute zero.
Rankine: The absolute scale using Fahrenheit-sized degrees. Found in some American engineering work.
Absolute zero: −273.15°C, the coldest possible temperature. Approached in laboratories, never reached.
Standard pressure: One atmosphere, at which the familiar freezing and boiling points are defined. Water boils lower at altitude.

Common questions

How do I convert Celsius to Fahrenheit?: Multiply by 9/5 and add 32. So 20°C is (20 × 1.8) + 32 = 68°F.
Is there a quick mental shortcut?: Double the Celsius figure and add 30. For 20°C that gives 70 against a true 68 — close enough for deciding on a jacket, and increasingly wrong at extremes.
Why is −40 the same in both scales?: Because the difference in degree size and the difference in zero point exactly cancel there. It is the single crossing point of the two scales.
Why does Kelvin not use the degree symbol?: Because it is an absolute unit rather than a scale of degrees. It was officially dropped in 1967, so correct usage is 300 K.
What is absolute zero?: −273.15°C or −459.67°F — the temperature at which molecular motion reaches its minimum. It is the zero point of both Kelvin and Rankine.
How do I convert a temperature change rather than a temperature?: Use only the ratio: multiply a Celsius change by 1.8 for Fahrenheit. Never add the 32, since a difference has no zero point to shift.
Why is body temperature 98.6°F such an odd number?: Because it is a conversion of 37°C, which was itself a rounded nineteenth-century average. The false precision comes from converting a round number in the other scale.
Does water always boil at 100°C?: Only at standard pressure. At altitude it boils lower — around 93°C at 2,000 metres — which is why high-altitude cooking takes longer.

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