Celsius to Fahrenheit Converter

Fahrenheit is Celsius × 1.8 + 32, and Celsius is (Fahrenheit − 32) ÷ 1.8

Celsius (°C) and Fahrenheit (°F) are two scales for the same temperature. The U.S. National Institute of Standards and Technology (NIST) defines the conversion as t/°C = (t/°F − 32)/1.8. Solve that for Fahrenheit and you get °F = °C × 1.8 + 32. So 25 °C is 25 × 1.8 + 32 = 77 °F, and 100 °F is (100 − 32) ÷ 1.8 = 37.777… °C, which rounds to 37.8 °C. Unlike length or weight, the two scales do not share a zero, so the conversion has an offset of 32 as well as a factor. Apply the steps in the wrong order and the answer lands far from the real value.

In the converter above, the Celsius and Fahrenheit fields follow each other. Whichever field you typed in last is the one the other is calculated from. If the other value had to be rounded, the two fields are joined by "≈"; if it is exact, by "=". The result sentence shows the unrounded value, and a decimal that never ends is cut short as in 37.777…, with the repeating digit written three times and then an ellipsis. Below the result you also get Kelvin (K), Rankine (°R) and the same number read as a temperature difference.

When you need to switch between Celsius and Fahrenheit

• Travel and weather: A U.S. forecast that says 90 °F, or a European one that says 30 °C, only means something once you read it on the scale you grew up with. 90 °F is 32.2 °C, and 30 °C is 86 °F, so both describe a hot day.
• Recipes from the other side of the Atlantic: An American recipe may give the oven setting as 350 °F or 400 °F, while your oven dial or another recipe may be marked in °C. 350 °F is about 176.7 °C and 400 °F is about 204.4 °C. In the other direction, 180 °C is 356 °F.
• Body temperature: In its port health guidance for reporting ill travelers on ships and aircraft, the CDC counts a measured temperature of 100.4 °F (38 °C) or greater as a fever. A thermometer or a health page set to the other scale needs a quick conversion to read it.
• Food safety: The USDA Food Safety and Inspection Service lists 145 °F (62.8 °C) with a rest of at least 3 minutes for steaks, chops and roasts of beef, pork, veal and lamb, and the FDA advises a refrigerator at 40 °F or below and a freezer at 0 °F. If your thermometer reads in °C, you need the converted numbers.
• Science and engineering: Physics and chemistry data may be given in kelvin (K), the SI unit of thermodynamic temperature. The Celsius degree has the same size as the kelvin, so you only add 273.15. U.S. engineering material that works in Fahrenheit degrees may use the Rankine scale (°R) instead.

How to use the converter

1. Type the temperature you know into the Celsius field or the Fahrenheit field. For a temperature below zero, put a minus sign in front, as in -10. A thousands comma such as 1,234.5 is accepted too.
2. Pick the decimal places (0, 1 or 2). The fields, the common values and the estimate error are rounded to that many places. The result sentence and the Kelvin and Rankine rows always show the exact value.
3. The dual-scale thermometer has Celsius on the left and Fahrenheit on the right. Two numbers at the same height are the same temperature. Drag the tube up or down, or select the thermometer and press the arrow keys (±1 degree) or Page Up and Page Down (±10 degrees). The temperature changes in the unit of the field you used last.
4. "Mental math" shows two ways to convert without a calculator, step by step. The error graph below it shows how far the quick estimate drifts from the exact value.
5. Click a value under "Common values" to put it in the field above. "Copy result" copies the result sentence, and "Copy link" copies an address that holds the current value.

How the conversion works

1. Celsius to Fahrenheit: multiply by 1.8 and add 32. The number 1.8 is the same as 9/5. 37 °C is 37 × 1.8 + 32 = 98.6 °F. If the Celsius value has a finite decimal, the Fahrenheit value always does too.
2. Fahrenheit to Celsius: subtract 32 first, then divide by 1.8. 350 °F is (350 − 32) ÷ 1.8 = 176.666… °C. Dividing by 1.8 is the same as multiplying by 5 and dividing by 9, so unless the result divides evenly by 9, one digit repeats forever at the end.
3. Why 1.8 and 32: The Fahrenheit scale puts the freezing point of water at 32° and the boiling point at 212°, with 180 divisions between them. Celsius splits the same span into 100, so one Celsius degree is 180 ÷ 100 = 1.8 Fahrenheit degrees, and the starting points are 32 apart.
4. Kelvin: K = °C + 273.15. The BIPM SI Brochure defines the Celsius temperature as t/°C = T/K − 273.15, and the kelvin is defined by fixing the Boltzmann constant at 1.380 649 × 10⁻²³ J/K. From Fahrenheit, kelvin is (°F + 459.67) ÷ 1.8.
5. Rankine: °R = °F + 459.67 = K × 1.8. Zero on the Rankine scale is the same absolute zero as 0 K, and one Rankine degree is the same size as one Fahrenheit degree.
6. Rounding: The converter first works out the exact value as a fraction, then rounds half up to the places you chose. Negative values are rounded by size, so −37.75 becomes −37.8. A long whole part is shown to 10 significant digits in a field, and a very small nonzero value gets extra places so that 3 significant digits are visible.

Common temperature tables

MedlinePlus, from the U.S. National Library of Medicine, says the average normal body temperature is generally accepted as 98.6 °F (37 °C), and that a normal reading can fall anywhere from 97 °F (36.1 °C) to 99 °F (37.2 °C). The 100.4 °F that CDC port health guidance (for reporting ill travelers on ships and aircraft) uses as its fever definition is exactly 38 °C.

The USDA calls the range from 40 °F to 140 °F the "Danger Zone" and states that bacteria grow most rapidly there, doubling in number in as little as 20 minutes. A refrigerator limit of 40 °F is exactly 4.444… °C, so a fridge set to 4 °C is a rounded stand-in. The freezer figure of 0 °F is −17.777… °C, which makes −18 °C its rounded counterpart.

Converting in your head

The quick estimate is to double the Celsius value and add 30 (going from Fahrenheit, subtract 30 and halve). Compared with the exact °C × 1.8 + 32, its error is 0.2 × °C − 2 (in °F). That is zero only at 10 °C (50 °F), and it grows by 2 °F for every 10 degrees away. For weather between 0 and 30 °C you are off by a few degrees at most, but at oven temperatures the gap becomes large.

Exact mental math splits the 1.8 into pieces. For Celsius to Fahrenheit, double the number, take off 10% of that, then add 32. For 25 °C that is 50, then 45, then 77 °F, the same as the formula. For Fahrenheit to Celsius, subtract 32, halve it, and add one ninth of the halved number. For 100 °F that is 68, then 34, then 34 + 3.777… = 37.777… °C. Halving and then adding a ninth is the same as multiplying by 5/9.

Flipping the digits gives a near match for some pairs. 16 °C is 60.8 °F, close to 61 °F, and 28 °C is 82.4 °F, close to 82 °F. Both are rounded, so treat them as a way to get a feel for the numbers, not as a conversion.

For a temperature difference, do not add 32

When you convert a change between two temperatures, such as "it warmed up by 10 degrees", you only multiply by 1.8. A 10 °C difference is an 18 °F difference, not 50 °F. The NIST table lists temperature intervals with a factor only, and 1 °F as an interval equals 5/9 = 0.555… K (and 0.555… °C). That is why "How many Celsius is 1 Fahrenheit?" has two answers. A temperature of 1 °F is −17.222… °C, while a difference of 1 °F is a difference of 0.555… °C.

Climate figures work the same way. The IPCC Special Report on Global Warming of 1.5 °C says global warming is likely to reach 1.5 °C above pre-industrial levels between 2030 and 2052 if it continues to increase at the current rate. That 1.5 °C is a difference, so in Fahrenheit it is a 2.7 °F difference. The "same number read as a temperature difference" row in the converter gives that value directly.

Why −40 is the same in Celsius and Fahrenheit

Set the two scales equal in °F = °C × 1.8 + 32 and you get x = 1.8x + 32. Solving gives x = −40, so −40 °C is −40 °F. Above that point the Fahrenheit number is larger than the Celsius number, and below it the Fahrenheit number is smaller. When the thermometer shows the weather range (down to −40 °C), the "Same on both" mark at the bottom of the dual-scale thermometer sits at this point.

Common mistakes

• Subtracting 32 last: Working out 100 °F as 100 ÷ 1.8 − 32 gives 23.6, which is more than 14 degrees away from the correct 37.8 °C. From Fahrenheit to Celsius, subtract 32 first and divide afterwards.
• Adding 32 to a difference: A rise of 5 °C is not a rise of 41 °F. The difference is 5 × 1.8 = 9 °F.
• Using the quick estimate for ovens or fevers: Estimating 350 °F as (350 − 30) ÷ 2 gives 160 °C, while the exact value is 176.7 °C, a gap of 16.7 °C. The same shortcut turns 100.4 °F into 35.2 °C and hides a 38 °C fever. For ovens and body temperature, use the formula or the exact mental method.
• Treating a rounded recipe value as exact: Some recipes turn 350 °F into 180 °C, but the exact value is 176.7 °C, which is 3.3 °C lower. If the dish is sensitive to temperature, go back to the original Fahrenheit figure and convert it again.
• Writing a degree sign with kelvin: The symbol is K, not °K. Celsius, Fahrenheit and Rankine do take the degree sign.

Limits

• The converter applies the defined formulas to the number you type. It does not account for thermometer error, for the difference between body sites or thermometer types, or for the gap between an oven dial and the real temperature inside.
• The boiling point of water depends on air pressure. The "water boils" mark (100 °C, 212 °F) shown on the thermometer and in the table is the approximate value at 1 atm near sea level.
• Values below absolute zero (−273.15 °C, −459.67 °F) are rejected, and input runs from −1 billion to 1 billion in the unit you enter.
• Fields are rounded to the decimal places you pick. For a record or a document, write the exact value from the result sentence together with the original unit.
• Health decisions should follow advice from a medical professional, not this calculation. The body temperature table only changes the unit.
• The calculation runs in your own browser and nothing you type is sent anywhere. As soon as you change a value, the page address (?v=…) is updated with it, so reloading or sharing the address opens the same value.

Basis: the relationships among Celsius, Fahrenheit, kelvin and Rankine come from the definitions in NIST SP 811 Appendix B.9 and the BIPM SI Brochure (9th edition), which do not change from year to year. The food safety temperatures (USDA, FDA), the fever definition (CDC), the body temperature norms (MedlinePlus), the climate report (IPCC) and the scale history (Britannica) were checked against the original pages on October 20, 2026.

Sources

• 01NIST SP 811 Appendix B.9 — Factors for units listed by kind of quantityt/°C = (t/°F − 32)/1.8, T/K = t/°C + 273.15, T/K = (t/°F + 459.67)/1.8, T/K = (T/°R)/1.8, temperature interval 1 °F = 5.555 556 E−01 K — checked 2026-10-20
• 02NIST SP 811 Chapter 4 — 4.2.1.1 Celsius temperatureThe degree Celsius (°C) is equal in magnitude to the kelvin
• 03BIPM — The International System of Units (SI Brochure, 9th edition)t/°C = T/K − 273.15
• 04BIPM — SI base unit: kelvin (K)Defined by fixing the Boltzmann constant at 1.380 649 × 10⁻²³ J/K
• 05Britannica — Fahrenheit temperature scaleFreezing point of water 32°, boiling point 212°, 180 divisions between them
• 06Britannica — Celsius temperature scaleAnders Celsius, 1742; at first 0° was the boiling point and 100° the temperature at which snow melts
• 07USDA FSIS — Safe Minimum Internal Temperature ChartBeef, pork, veal and lamb steaks, chops and roasts 145 °F (62.8 °C) plus a rest of at least 3 minutes, ground meat 160 °F (71.1 °C), poultry 165 °F (73.9 °C)
• 08USDA FSIS — "Danger Zone" (40 °F – 140 °F)The range in which bacteria grow most rapidly
• 09FDA — Refrigerator Thermometers: Cold Facts about Food SafetyRefrigerator at 40 °F or below, freezer at 0 °F
• 10CDC — Definitions of Signs and Symptoms of Reportable IllnessGuidance for reporting ill travelers on ships and aircraft. Fever: measured temperature of 100.4 °F [38 °C] or greater
• 11MedlinePlus — Body temperature normsAverage normal 98.6 °F (37 °C), range 97 °F (36.1 °C) to 99 °F (37.2 °C)
• 12IPCC — Global Warming of 1.5 °C, Summary for PolicymakersA.1 About 1.0 °C of warming above pre-industrial levels, global warming likely to reach 1.5 °C between 2030 and 2052 if it continues to increase at the current rate

Frequently asked questions

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