Temperature Converter: Celsius, Fahrenheit, Kelvin
Convert temperatures between Celsius (°C), Fahrenheit (°F), Kelvin (K) and Rankine (°R), and convert temperature differences correctly too.
Things to keep in mind:
- Inputs below absolute zero do not raise an error; the result is calculated but the "At or above absolute zero?" line shows 0. Such values are physically meaningless.
- The temperature-difference line only applies to rises and falls; use the main result to convert a thermometer reading.
- Historic scales such as Réaumur or Delisle are not included.
Calculator
Result
Enter the values and press Calculate to see the result here.
About this tool
This tool converts a temperature between the Celsius, Fahrenheit, Kelvin and Rankine scales and shows the value on all four scales at once.
Formulas (the value is converted to kelvin first, then to the target scale):
- K = °C + 273.15
- °F = °C × 9/5 + 32 and °C = (°F − 32) × 5/9
- K = (°F + 459.67) × 5/9
- °R = K × 9/5 = °F + 459.67
Temperature scales differ not only by a factor but also by an offset (a shifted zero point), so the conversion is linear but not proportional. 20 °C is not "half as hot" as 40 °C; ratios are only meaningful on absolute scales (K, °R).
Temperature differences: when converting a rise or fall ("the temperature went up by 10 degrees") the offset is not used; only the degree sizes are compared: 1 °C = 1 K = 1.8 °F = 1.8 °R. The tool shows this value on a separate line.
Example: 100 °C = 100 × 9/5 + 32 = 212 °F = 373.15 K = 671.67 °R. The same 100 read as a temperature difference equals a difference of 180 °F.
The scale definitions follow the SI (kelvin is the base unit, °C = K − 273.15) and the NIST SP 811 conversion rules. Values below absolute zero (0 K = −273.15 °C = −459.67 °F) are not physically possible, so the tool marks them with a validity flag.
How to use
- Enter the temperature you want to convert (negative values are allowed).
- Select the unit of the value under From unit.
- Select the unit you want the result in under To unit.
- Select Calculate.
Notes
The main result is rounded half-up (HALF_UP) to 6 decimals, the scale equivalents to 4 decimals. The 5/9 factor can produce repeating decimals, so the last digit may be rounded. The Celsius scale is tied to the kelvin with an offset of 273.15 (SI definition).
Frequently asked questions
Multiply the Celsius value by 9/5 (1.8) and add 32: °F = °C × 1.8 + 32. For example 25 °C = 25 × 1.8 + 32 = 77 °F.
Subtract 32 from the Fahrenheit value and multiply by 5/9: °C = (°F − 32) × 5/9. For example 70 °F ≈ 21.1111 °C.
The degree size is the same; only the zero point differs. K = °C + 273.15. Kelvin is an absolute scale with no negative values and is written without "degree" (300 K).
At −40 degrees: −40 °C = −40 °F. Enter −40 °F in the tool to check.
In a difference the offset (32 or 273.15) cancels out. A rise of 10 °C therefore equals a rise of 18 °F, not 50 °F. The tool shows this on the As a temperature difference line.
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