Temperature Converter
Convert between Celsius, Fahrenheit, Kelvin, Rankine, and Réaumur all at once. Just enter a value and pick its unit to see the full conversion table.
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Input
Result
In Celsius (℃)
25℃
| Unit | Symbol | Converted value |
|---|---|---|
| Celsius | ℃ | 25 |
| Fahrenheit | ℉ | 77 |
| Kelvin | K | 298.15 |
| Rankine | °R | 536.67 |
| Réaumur | °Ré | 20 |
How it works
- Five units are supported: Celsius (°C), Fahrenheit (°F), Kelvin (K), Rankine (°R), and Réaumur (°Ré).
- Celsius (°C) is used as the base unit. Your input is first converted to Celsius and then to every other unit, which keeps all results consistent.
- Temperature scales differ not only in the size of each degree but also in where their zero point sits, so this tool uses a dedicated formula for each unit rather than the simple ratio used for length or weight.
- To use it, just enter a number and pick its unit. The Celsius value is shown prominently, and the table below lists the result in every unit at the same time.
- Results are rounded to about six decimal places, switching to exponential notation when the magnitude is very large or very small. Keep the rounding in mind when you need exact values.
- As a handy reference, water freezes at 0°C (32°F, 273.15 K) and boils at 100°C (212°F).
Oven settings and recipes
Recipes written for an American audience give oven temperatures in Fahrenheit, while most of the world sets ovens in Celsius. The exact conversion of 350°F is (350 − 32) × 5/9 = 176.7°C, which is why European cookbooks round it to 180°C. Most ovens only step in 5 or 10 degree increments and drift by more than that anyway, so the usual pairings work fine in practice.
- 325°F = 162.8°C (usually set to 160 or 165°C)
- 350°F = 176.7°C (usually set to 180°C)
- 400°F = 204.4°C (usually set to 200°C)
- 425°F = 218.3°C (usually set to 220°C)
- 450°F = 232.2°C (usually set to 230°C)
Body temperature and fever
Normal body temperature is commonly given as 37°C, which is exactly 98.6°F. 38°C is 100.4°F, 39°C is 102.2°F, and 40°C is 104°F. Near body temperature, every 0.1°C equals 0.18°F, so a rise of one full Fahrenheit degree is only about 0.56°C. Keep that in mind when comparing readings from thermometers set to different scales.
Mental shortcuts and how far off they are
The popular shortcut "double it and add 30" gives a Fahrenheit value whose error equals 0.2 × °C − 2. It is exact at 10°C, two degrees high at 20°C (70 vs 68) and four degrees high at 30°C (90 vs 86). That is fine for deciding whether you need a jacket, but it breaks down at higher temperatures: it turns 37°C into 104°F instead of 98.6°F, and 180°C into 390°F instead of 356°F.
If you want an exact mental method, use "double it, subtract 10 percent, add 32". For 25°C: 25 × 2 = 50, minus 5 is 45, plus 32 is 77°F, which is the exact answer because doubling and removing 10 percent is the same as multiplying by 1.8. Going the other way, "subtract 32, halve it, add 10 percent" is within about 1 percent: 212°F gives 99°C rather than 100°C.
One handy landmark: −40 is the same on both scales, so −40°C and −40°F describe the same cold.
Temperature differences are not temperatures
A change in temperature converts differently from a temperature reading. If the forecast says it will be 5°C warmer tomorrow, that is a rise of 5 × 1.8 = 9°F, not 41°F. Entering 5°C in this converter correctly shows 41°F for a reading of 5°C, but that figure is wrong for a difference. Kelvin degrees are the same size as Celsius degrees, so a difference of 1°C is also a difference of 1 K.
Ratios only make sense on absolute scales. Going from 20°C to 40°C does not double the heat; in kelvin it is 293.15 K to 313.15 K, an increase of about 6.8 percent. Gas law and radiation calculations therefore need kelvin or Rankine, the absolute scale built on Fahrenheit-sized degrees that is still used in some US engineering work.
Finally, the converter accepts any number, including values below absolute zero (−273.15°C, −459.67°F, 0 K). If a result in kelvin or Rankine comes out negative, the input or the selected unit is wrong.
Frequently asked questions
- What is the formula to convert Celsius to Fahrenheit?
- Multiply the Celsius value by 9/5 and add 32 (°F = °C × 9/5 + 32). Because temperature scales differ in both their spacing and their zero point, the tool applies a dedicated formula per unit rather than a simple ratio.
- How do Kelvin, Rankine, and Réaumur differ from Celsius?
- Kelvin starts at absolute zero, so 0°C = 273.15 K; Rankine is the absolute scale tied to Fahrenheit degrees; and Réaumur puts water's freezing at 0 and boiling at 80. All five scales convert simultaneously through a Celsius base.
- At what temperatures does water freeze and boil in each unit?
- Water freezes at 0°C (32°F, 273.15 K) and boils at 100°C (212°F). Results are rounded to about six decimal places, switching to exponential notation for extreme values.
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