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Formula
°C = (°F − 32) × 5/9 · °F = °C × 9/5 + 32 · K = °C + 273.15
- °F
- degrees Fahrenheit (water freezes at 32 °F and boils at 212 °F at sea level)
- °C
- degrees Celsius (0 °C and 100 °C for the same two points)
- K
- kelvin, the SI unit, which starts at absolute zero (0 K = −273.15 °C)
Three temperature scales, two kinds of difference
Converting between temperature units is not like converting inches to centimeters. Length scales all start at zero length, so a single multiplier does the job. Temperature scales put their zero in different places, so a conversion needs a multiplier and an offset:
- Fahrenheit sets water’s freezing point at 32 °F and its boiling point at 212 °F, 180 degrees apart. It is the everyday scale in the United States.
- Celsius sets the same two points at 0 °C and 100 °C, 100 degrees apart. Most of the world and all of science use it.
- Kelvin uses Celsius-sized steps but starts at absolute zero, so 0 K = −273.15 °C. It is the SI base unit for temperature and the one physics and chemistry formulas expect.
The temperature converter above handles all three in any direction. Pick the scale you have and the scale you want, type the reading, and the working appears line by line, so you can see exactly where the 32 and the 273.15 come in.
Temperature converter formulas
| Conversion | Formula | Check with a known point |
|---|---|---|
| Fahrenheit to Celsius | °C = (°F − 32) × 5/9 | 212 °F → 100 °C |
| Celsius to Fahrenheit | °F = °C × 9/5 + 32 | 0 °C → 32 °F |
| Celsius to kelvin | K = °C + 273.15 | −273.15 °C → 0 K |
| Kelvin to Celsius | °C = K − 273.15 | 373.15 K → 100 °C |
| Fahrenheit to kelvin | K = (°F − 32) × 5/9 + 273.15 | 32 °F → 273.15 K |
| Kelvin to Fahrenheit | °F = (K − 273.15) × 9/5 + 32 | 0 K → −459.67 °F |
The order of operations matters. For Fahrenheit to Celsius, subtract 32 before multiplying. Doing it the other way round gives the wrong answer for every value except −40.
Converting Fahrenheit to Celsius by hand
Take a spring afternoon of 64 °F:
- Subtract the offset: 64 − 32 = 32.
- Scale the degree size: 32 × 5/9 = 17.78.
- Result: 64 °F is about 17.8 °C.
For kelvin, add 273.15 to the Celsius figure: 17.78 + 273.15 = 290.93 K.
Temperature converter chart: Fahrenheit, Celsius and kelvin
Values below come from the converter’s own formulas, rounded to one or two decimals.
| Fahrenheit | Celsius | Kelvin | What it feels like |
|---|---|---|---|
| −10 °F | −23.3 °C | 249.82 K | Bitter winter cold |
| 0 °F | −17.8 °C | 255.37 K | A very cold January day in the northern US |
| 20 °F | −6.7 °C | 266.48 K | Hard frost |
| 32 °F | 0 °C | 273.15 K | Water freezes |
| 40 °F | 4.4 °C | 277.59 K | Refrigerator setting |
| 50 °F | 10 °C | 283.15 K | Cool fall morning |
| 68 °F | 20 °C | 293.15 K | Standard room temperature |
| 75 °F | 23.9 °C | 297.04 K | Warm indoor setting |
| 90 °F | 32.2 °C | 305.37 K | Hot summer afternoon |
| 110 °F | 43.3 °C | 316.48 K | Desert heat |
| 212 °F | 100 °C | 373.15 K | Water boils at sea level |
| 350 °F | 176.7 °C | 449.82 K | Typical baking temperature |
Converting temperature differences
A forecast that says “15 degrees warmer than yesterday” is a difference, not a reading, and differences convert without the offset. Multiply a Celsius or kelvin difference by 1.8 to get Fahrenheit degrees, or divide a Fahrenheit difference by 1.8 for Celsius. A 15 °F rise is 15 ÷ 1.8 = 8.3 °C warmer, not −9.4 °C, which is what you get if you wrongly run 15 °F through the reading formula. A change of 1 K is exactly a change of 1 °C.
Where kelvin is used
- Gas laws and chemistry. Formulas such as PV = nRT only work with absolute temperatures, because doubling the kelvin value really does double the molecules’ average kinetic energy. The ideal gas law calculator expects kelvin for that reason.
- Lighting. Bulb color is rated in kelvin: about 2,700 K for warm white and 5,000–6,500 K for daylight-style light.
- Astronomy and cryogenics. The cosmic microwave background is about 2.7 K, and liquid nitrogen boils at about 77 K.
Common mistakes
- Adding 32 before multiplying. °F = °C × 9/5 + 32, not (°C + 32) × 9/5.
- Using the reading formula for a change. See the section above: differences skip the 32.
- Writing degree kelvin. The unit is simply “kelvin” (K), with no degree sign.
- Rounding 273.15 to 273. Fine for weather, but it introduces a 0.15 K error that matters in lab work.
If you mostly go from Celsius to Fahrenheit and want a mental shortcut, cooking temperatures and body temperatures, the Celsius to Fahrenheit converter is built around that one direction.
Worked examples
Body temperature in Celsius
The traditional normal body temperature of 98.6 °F in degrees Celsius.
Inputs: Value = 98.6; From = Fahrenheit (°F); To = Celsius (°C)
°C = (°F − 32) × 5/9 (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37 °C
Answer: 98.6 °F = 37 °C
Kelvin to Fahrenheit
A lab reading of 300 K expressed in degrees Fahrenheit.
Inputs: Value = 300; From = Kelvin (K); To = Fahrenheit (°F)
°C = K − 273.15 300 − 273.15 = 26.85 °C °F = °C × 9/5 + 32 26.85 × 9/5 + 32 = 48.33 + 32 = 80.33 °F
Answer: 300 K = 80.33 °F
Coldest air temperature on record
The −128.6 °F reading at Vostok Station, Antarctica (1983), in degrees Celsius.
Inputs: Value = -128.6; From = Fahrenheit (°F); To = Celsius (°C)
°C = (°F − 32) × 5/9 (−128.6 − 32) × 5/9 = (−160.6) × 5/9 = −89.222222 °C
Answer: −128.6 °F = −89.222222 °C
Liquid helium, near absolute zero (edge case)
Liquid helium boils at about 4.2 K, only a few degrees above absolute zero.
Inputs: Value = 4.2; From = Kelvin (K); To = Celsius (°C)
°C = K − 273.15 4.2 − 273.15 = −268.95 °C
Answer: 4.2 K = −268.95 °C
Assumptions and limitations
- Conversions use the exact defining relationships, °F = °C × 9/5 + 32 and K = °C + 273.15. Nothing is rounded until the result, which shows up to 6 decimal places.
- The converter converts temperature readings, not temperature differences. A change of 10 °C is a change of 18 °F, not 50 °F; see the section on differences below.
- Values below absolute zero (0 K, −273.15 °C, −459.67 °F) are rejected because no temperature can be lower.
- The Rankine and Réaumur scales are not included in this version. Rankine equals kelvin × 1.8 if you need it.
Frequently asked questions
How do you convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. For 59 °F, 59 − 32 = 27 and 27 × 5/9 = 15 °C. Dividing by 1.8 instead of multiplying by 5/9 gives the same answer.
What is the formula to convert Celsius to kelvin?
Add 273.15. Room temperature of 20 °C is 293.15 K. To go from kelvin back to Celsius, subtract 273.15. Kelvin is written without a degree sign.
How do you convert Fahrenheit to kelvin?
Convert to Celsius first, then add 273.15. For 50 °F, (50 − 32) × 5/9 = 10 °C, and 10 + 273.15 = 283.15 K. The converter shows both steps.
At what temperature are Fahrenheit and Celsius equal?
At −40. Setting °F = °C in the formula gives x = 1.8x + 32, so x = −40. It is the only point where the two thermometers agree.
Why can't kelvin be negative?
The kelvin scale starts at absolute zero, the point where particles have the minimum possible energy. There is nothing colder, so 0 K is the bottom of the scale and all kelvin values are zero or positive.
Why does a 10 degree change in Celsius become 18 degrees in Fahrenheit?
A Fahrenheit degree is smaller. There are 180 Fahrenheit degrees between freezing and boiling but only 100 Celsius degrees, so each Celsius degree is 1.8 Fahrenheit degrees. Differences scale by 1.8 without adding 32.
Is it Celsius or centigrade?
Celsius. The scale was called centigrade until 1948, when it was renamed after the astronomer Anders Celsius. The values are the same.
Sources
- SI Units – Temperature — National Institute of Standards and Technology (NIST). Checked October 12, 2026.
- The International System of Units (SI Brochure) — Bureau International des Poids et Mesures (BIPM). Checked October 12, 2026.
- NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically — National Institute of Standards and Technology (NIST). Checked October 12, 2026.