Temperature Converter

What this converter answers

It shows one temperature, or one change in temperature, on all four scales in use: degrees Celsius (°C), degrees Fahrenheit (°F), kelvins (K) and degrees Rankine (°R). Every step is shown with exact values, so you can check the arithmetic by hand. Temperatures (readings such as an oven setting) and temperature changes (such as a 10-degree rise) convert differently, and the converter keeps the two apart.

How to use it

• Type in any box. Enter your number in the box for the scale you have. The other three boxes fill in as you type, and Converting from moves to the box you typed in. The ± button next to each box switches the sign, for phone keypads that have no minus key.
• Convert: choose A temperature for a reading (an oven setting, today’s high, a thermometer) or A change for a difference between two readings (a rise of 10 °C, a kiln’s ramp rate per hour).
• Decimal places: results show 2 decimal places, with trailing zeros dropped, so 98.60 reads 98.6. Choose 0 to 6 places, or up to 12 significant figures.
• Table step: the spacing of the nearby-values table under the result. Auto uses 25 °F (10 °C) steps for oven settings, 1 °F (0.5 °C) steps around body temperature and a round step elsewhere.

Results update as you type. The Try buttons load the cases worked through on this page: body temperature, a thermostat setting, a rise of 10 °C and a kiln ramp. Save for comparison keeps up to three results; with two temperatures saved, the change column shows how far apart they are on every scale.

How to convert Celsius to Fahrenheit

Multiply the Celsius temperature by 9/5 (that is, 1.8) and add 32.

• $T_C$ is the temperature in degrees Celsius.
• $T_F$ is the same temperature in degrees Fahrenheit.

For a body temperature of 37 °C: 37 × 9/5 = 66.6, and 66.6 + 32 = 98.6 °F. The same temperature is 37 + 273.15 = 310.15 K and 98.6 + 459.67 = 558.27 °R.

How to convert Fahrenheit to Celsius

Subtract 32 first, then multiply by 5/9. The 32 is there because water freezes at 32 °F but at 0 °C, and the 5/9 because a Fahrenheit degree is 5/9 the size of a Celsius degree.

Worked example: an oven set to 350 °F

1. Subtract 32: 350 − 32 = 318.
2. Multiply by 5/9: 318 × 5/9 = 1,590/9 = 176.666… °C, which rounds to 176.67 °C.
3. Add 273.15 for kelvins: 176.666… + 273.15 = 449.81666… K, or 449.82 K.
4. Add 459.67 to the Fahrenheit number for Rankine: 350 + 459.67 = 809.67 °R, exactly.

Oven charts round further: NIST’s guide to old recipe terms describes a moderate oven as 350 to 375 °F, about 180 to 190 °C. A thermostat set to 68 °F is (68 − 32) × 5/9 = 36 × 5/9 = 20 °C exactly.

Converting to and from kelvin and Rankine

Kelvin and Rankine both begin at absolute zero, the coldest possible temperature, so no temperature on either scale is negative. A kelvin is the same size as a degree Celsius, and a degree Rankine is the same size as a degree Fahrenheit.

Absolute zero is 0 K = −273.15 °C = −459.67 °F = 0 °R. Room temperature, about 20 °C, is 293.15 K; 68 °F is 527.67 °R. To go back: $T_C = T_K - 273.15$, $T_F = T_R - 459.67$ and $T_K = T_R \times 5/9$.

Converting a temperature change

A change converts with the size of the degree only. Leave out the 32, the 273.15 and the 459.67:

A rise of 10 °C is a rise of 10 × 9/5 = 18 °F, not 50 °F. 50 °F is what the temperature 10 °C reads in Fahrenheit, which is a different question. A tool that only converts readings adds the 32 to whatever you type, so choose A change here; the result then says rise or drop, and a tile shows the wrong answer the 32 would have given.

Rates of change convert the same way, because the time unit stays the same on both sides. A kiln ramp of 150 °F per hour is 150 × 5/9 = 83.33 °C per hour. For readings versus differences in other units, see the unit conversion pitfalls guide under Related guides.

Common temperatures in °C and °F

Values are rounded to 2 decimal places; the calculator shows the exact digits.

Oven settings

Weather in Fahrenheit

Celsius to Fahrenheit

For body temperature, MedlinePlus gives an average of 98.6 °F (37 °C) and a normal range of about 97 to 99 °F (36.1 to 37.2 °C); over 100.4 °F (38 °C) most often means a fever.

What temperature is the same in Celsius and Fahrenheit?

−40: −40 °C is −40 °F. Set both readings to the same number $x$ in the Celsius formula: $x = x \times 9/5 + 32$, so $-\tfrac{4}{5}x = 32$ and $x = -40$. Above −40 the Fahrenheit number is the larger of the two; below it, the smaller.

A quick mental conversion, and how accurate it is

Double the Celsius number and add 30 to estimate Fahrenheit. The estimate is exact only at 10 °C (50 °F); it is 2 °F too high at 20 °C and 18 °F too high at 100 °C. The error (estimate minus exact value) is 0.2 × °C − 2 degrees Fahrenheit, so it grows the further you are from 10 °C.

The reverse, subtracting 30 from the Fahrenheit number and halving, is exact at 50 °F and drifts the other way: at 350 °F it gives (350 − 30) ÷ 2 = 160 °C, 16.67 °C below the exact 176.67 °C. The Quick estimate tile works the shortcut for your own temperature and shows how far off it is.

Reading the result

• The headline is the everyday partner of the scale you typed in: Celsius for Fahrenheit or kelvin, Fahrenheit for Celsius or Rankine. Below it, the conversion as one line, and whether the number shown is exact or rounded, with the exact digits when it is rounded. Copy value copies the headline; Copy all four copies every scale.
• The tiles give the other two scales, each marked exact or with its exact digits, and the quick estimate. For a change, the third tile shows what the change would wrongly become if treated as a temperature.
• How this was calculated applies the formulas above to your number, and adds the formula for a change so you can see the difference.
• Reference temperatures (temperatures only) places your temperature among absolute zero, −40, water freezing and boiling, room temperature, body temperature and fever, and a moderate oven.
• The nearby table converts values either side of yours in the table step, for a whole oven dial or a range of forecasts. Download it as a CSV file.

Assumptions and limitations

• The scales are fixed by definition, so the conversions are exact: every number you type is taken as exact, and results are rounded only for display.
• Water’s freezing and boiling points, room temperature and body temperature are reference values from NIST and MedlinePlus. Body temperature varies by person and time of day; the converter is not a medical tool.
• The box you type in accepts up to 1,000,000,000,000,000 either way. Temperatures can’t go below absolute zero; changes can be any sign.
• Only the four scales in use today are included. The converter doesn’t do other quantities that contain a temperature, such as heat capacity per degree.

Common mistakes

• Adding 32 to a change. A rise of 10 °C is a rise of 18 °F. 10 × 9/5 + 32 = 50 °F answers a different question: what the temperature 10 °C reads.
• Multiplying before subtracting. It’s (°F − 32) × 5/9. Typing 350 − 32 × 5/9 into a calculator multiplies only the 32 and gives 332.22 instead of 176.67.
• Rounding the factor. 5/9 is 0.555…, not 0.55: 318 × 0.55 = 174.9 °C for 350 °F, 1.77 °C too low. Even 0.5556 gives 176.68 °C instead of 176.67 °C.
• Using 273 instead of 273.15. Every kelvin result comes out 0.15 K off.
• Typing a Celsius number in the Kelvin box. 37 K is −236.15 °C. When a kelvin or Rankine temperature is colder than −100 °C, the converter points this out under the box.