Celsius to Fahrenheit Converter

For a centigrade to Fahrenheit conversion, multiply the Celsius temperature by 1.8 and add 32. So 20 °C is 68 °F, 25 °C is 77 °F and 37 °C is 98.6 °F. Going the other way, subtract 32 and divide by 1.8. The converter above handles °C, °F, kelvin and Rankine in any direction. It can also convert a temperature change, where the +32 must be left out, and it shows how far the popular "double and add 30" shortcut is from the exact answer.

Celsius to Fahrenheit formula

°F = °C × 1.8 + 32 (1.8 is the same as 9/5). °C = (°F − 32) ÷ 1.8. NIST publishes these as exact relations in its SI temperature guide and in Special Publication 811. Two things happen in the formula. First, a Celsius degree is larger than a Fahrenheit degree: NIST notes that an interval of one Celsius degree corresponds to 1.8 Fahrenheit degrees. Second, the zero points differ, because water freezes at 0 °C but 32 °F. Multiply to fix the degree size, then add 32 to line up the zeros.

1. Start with the Celsius reading, for example 23 °C.
2. Multiply by 1.8: 23 × 1.8 = 41.4.
3. Add 32: 41.4 + 32 = 73.4 °F.

For Fahrenheit to Celsius, reverse the order: subtract first, then divide. 86 °F − 32 = 54, and 54 ÷ 1.8 = 30 °C. Doing the steps in the wrong order is the most common error. If you divide 86 by 1.8 before subtracting, you get a number that has nothing to do with the right answer.

Kelvin and Rankine

Kelvin is the SI unit of thermodynamic temperature. The BIPM SI Brochure defines Celsius temperature through t/°C = T/K − 273.15, and states that the degree Celsius equals the kelvin in size. So K = °C + 273.15 and a change of 1 °C is a change of 1 K. Kelvin is written without a degree sign: 300 K, not 300 °K. The SI Brochure records that the 13th CGPM (1967/68) renamed the "degree Kelvin" to simply "kelvin", and in 1980 the CIPM stopped permitting the °K symbol.

Rankine is the absolute scale built on Fahrenheit-sized degrees. NIST SP 811 gives T/K = (T/°R) ÷ 1.8, which means °R = K × 1.8, or °R = °F + 459.67. Both absolute scales start at absolute zero: 0 K = 0 °R = −273.15 °C = −459.67 °F. Absolute scales matter whenever a formula multiplies or divides by temperature, as the gas laws do; plugging in °C or °F there gives wrong answers.

Converting a temperature difference

A forecast that says "5 degrees warmer tomorrow" or a spec that allows "±2 °C" describes an interval, not a reading. For intervals, multiply by 1.8 and do not add 32. A 5 °C rise is a 9 °F rise; a tolerance of ±2 °C is ±3.6 °F. If you use the full formula on a difference, you get a nonsense answer: a 5 °C change would become "41 °F". Choose "A difference or change" in the converter to get this right automatically. For kelvin and Celsius the interval is the same number, since the degrees are the same size.

Quick mental math: double and add 30 (approximate)

A popular shortcut for weather is to double the Celsius number and add 30. It is only an approximation. It uses 2 instead of 1.8 and 30 instead of 32, so the two errors cancel at exactly 10 °C (50 °F) and grow as you move away. At 0 °C it gives 30 °F, 2 °F too cold. At 25 °C it gives 80 °F versus 77 °F. At 35 °C, a heat-wave day, it gives 100 °F versus 95 °F. The error is 0.2 °F for every degree Celsius away from 10 °C. The chart in the converter shows the two lines drifting apart. Use the shortcut to get a feel for a forecast; use the formula for cooking, medicine, lab work or anything written down.

A more accurate trick: double, subtract 10%, then add 32. For 25 °C: 50 − 5 = 45, and 45 + 32 = 77 °F. That is the exact formula done in your head, because doubling and removing a tenth is the same as multiplying by 1.8.

Body, kitchen and weather reference points

These temperatures come from NIST, the NIH's MedlinePlus, the CDC and the National Weather Service. The source column shows which one. Converted values are rounded to one decimal.

Body temperature: MedlinePlus gives 98.6 °F (37 °C) as the generally accepted average and a normal range of 97 °F to 99 °F, and notes that a temperature over 100.4 °F (38 °C) most often means a fever. Readings vary with the time of day and where they are taken, so follow the instructions for your thermometer. Food: the CDC's safe internal temperatures are 145 °F for whole cuts of beef, pork and lamb (with a 3-minute rest) and fish, 160 °F for ground meats, and 165 °F for poultry and leftovers. The Celsius equivalents are not round numbers, which is why a recipe written in one scale and cooked with a thermometer in the other needs the conversion.

Worked examples

A child's thermometer reads 38.5 °C

38.5 × 1.8 = 69.3, plus 32 = 101.3 °F. That is above the 100.4 °F (38 °C) level MedlinePlus associates with a fever, so the unit change does not change the reading's meaning, only the number.

A European forecast says −5 °C

−5 × 1.8 = −9, plus 32 = 23 °F. Multiply the negative number first and keep the sign; −9 + 32 is 23, not 41.

A recipe calls for 190 °C

190 × 1.8 = 342, plus 32 = 374 °F. The nearest common US setting is 375 °F, which is 190.6 °C, about half a degree higher than asked.

A lab sample is stored at 4 °C

4 × 1.8 + 32 = 39.2 °F and 4 + 273.15 = 277.15 K. In Rankine that is 498.87 °R.

Converting in a spreadsheet or code

In a spreadsheet with the Celsius value in cell A1, use =A1*1.8+32; for the reverse, =(A1-32)/1.8. Keep the brackets in the reverse formula, otherwise the division happens first. In code the same expressions work. Store the unrounded result and round only for display, so that a value converted to Fahrenheit and back returns exactly what you started with.

How to read the scales

Everyday Celsius readings are easy to anchor: 0 is freezing, 10 is a cool day, 20 is a comfortable room, 30 is hot and 40 is extreme heat. On the Fahrenheit side, those same points are 32, 50, 68, 86 and 104. Negative Celsius numbers are common in winter; negative Fahrenheit numbers mean it is colder than −17.8 °C. When a thermometer shows both scales, the Fahrenheit side has 1.8 marks for each Celsius mark.

Centigrade to Fahrenheit conversion table: −40 to 100 °C

Every whole degree from −40 °C to 100 °C, rounded to one decimal place.

Higher temperatures: 110 to 300 °C

Oven temperatures: Fahrenheit to Celsius

US recipes usually give oven settings in 25 °F steps. The exact Celsius values are awkward decimals, so round to the nearest mark your oven dial actually has.

Fahrenheit to Celsius chart

Common mistakes

• Adding 32 before multiplying. (20 + 32) × 1.8 = 93.6 is wrong; 20 × 1.8 + 32 = 68 is right.
• Using 9/5 one way and 9/5 the other way. Celsius to Fahrenheit multiplies by 9/5; Fahrenheit to Celsius multiplies by 5/9.
• Adding 32 to a change. Differences only scale by 1.8.
• Writing °K. The kelvin takes no degree sign.
• Trusting the shortcut for exact work. "Double and add 30" is off by 18 °F at boiling.

More unit tools: mm to inches, mL to ounces and the molarity unit converter.

Frequently asked questions

Sources & method

• NIST — SI Units: Temperature (exact °C, °F and K conversion formulas)
• NIST SP 811, Appendix B.8 — degree Fahrenheit, degree Rankine and degree Celsius factors
• BIPM, The International System of Units (SI Brochure, 9th ed.), §2.3.1 — t/°C = T/K − 273.15
• MedlinePlus (NIH) — Body temperature norms
• CDC — Preventing Food Poisoning: safe internal temperatures, fridge and freezer
• National Weather Service — Wind chill chart and definition

Results are estimates for general information. Found an error? It helps everyone — see our methodology.