What this calculator does
Turn any temperature into Celsius, Fahrenheit, Kelvin or Rankine, and see how the math works. Temperature is the one everyday measurement you can't convert by multiplying alone. Celsius and Fahrenheit start counting from different places, and their degrees are different sizes. So every conversion needs two moves: a multiply and a shift, called an offset. Doing them in the wrong order is the most common mistake people make by hand.
This converter covers the four scales people actually use. Celsius and Fahrenheit are for weather, cooking and medicine. Kelvin and Rankine start at the coldest possible temperature, and they're used in science and engineering. Enter a reading and pick two scales. You'll get the answer, the working, and the same temperature on every scale.
Where it's used
Why this math matters outside the classroom:
- Weather. A 30 °C summer day is 30 × 9 ÷ 5 + 32 = 86 °F.
- Boiling water. Water boils at 100 °C, which is 212 °F and 373.15 K at standard pressure.
- Engineering. Thermodynamics problems in US units use Rankine, so 70 °F becomes 529.67 °R.
- Cooking. An oven set to 425 °F is at about 218 °C.
- Cold climates. Travelers to cold climates read a forecast low of −20 °F as about −28.9 °C.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.EE.A.2.c. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the temperature. Negative numbers and decimals are fine, as long as the reading isn't below absolute zero, the coldest possible temperature.
- Choose the scale your reading is in and the scale you want.
- Read the answer and the step-by-step working. The table underneath shows the same temperature on all four scales.
What the inputs mean
- Temperature
- The temperature reading on the “From” scale. It can't be below absolute zero: 0 K, −273.15 °C, −459.67 °F or 0 °R.
- From
- The scale your number is measured on.
- To
- The scale you want the answer in.
Formula and method
°F = °C × 9/5 + 32 °C = (°F − 32) × 5/9 K = °C + 273.15 °R = K × 9/5 = °F + 459.67
In the formulas, °C is degrees Celsius, °F is degrees Fahrenheit, K is kelvins and °R is degrees Rankine. A Fahrenheit degree is 5/9 the size of a Celsius degree. Also, Fahrenheit reads 32 at the point where Celsius reads 0. So to go from Celsius to Fahrenheit, multiply first and then add 32. To go the other way, take away the 32 first and then multiply.
Kelvin uses Celsius-sized degrees but starts counting at absolute zero. That makes it just Celsius moved up by 273.15. Rankine does the same thing with Fahrenheit-sized degrees. When two scales aren't directly linked, the calculator goes through Celsius on the way. Fahrenheit ↔ Rankine and Kelvin ↔ Rankine use their own one-step formulas instead.
Worked example
Convert a body temperature of 98.6 °F to Celsius.
- Take away 32: 98.6 − 32 = 66.6.
- Multiply by 5/9: 66.6 × 5 ÷ 9 = 37.
Result: 98.6 °F is 37 °C. That's also 310.15 K and 558.27 °R.
Understanding the result
Two handy reference points: water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at normal sea-level air pressure. The two scales meet at −40, where −40 °C and −40 °F are the same temperature.
Take care with temperature changes. A change of 10 °C is a change of 18 °F, not 50 °F. That's because a change only needs the multiply step, not the shift. This converter treats your number as a reading on a thermometer, not as a change.
Assumptions and limitations
- Your number is a reading on a thermometer, not a change in temperature.
- Kelvin sits exactly 273.15 above Celsius, and Rankine exactly 459.67 above Fahrenheit, because that's how the scales are defined.
- Don't use this tool for a change in temperature (the gap between two readings). A change needs only the 9/5 factor.
- Old scales such as Réaumur, Delisle and Newton aren't included.
- Readings below absolute zero are turned away, because nothing can be colder than that.
Frequently asked questions
What's a quick mental shortcut for Celsius to Fahrenheit?
Double the Celsius number and add 30. It's close for everyday weather: 20 °C gives 70 by the shortcut, and 68 °F exactly. The shortcut drifts further off the further you get from about 10 °C. So use the exact formula for cooking or medical readings.
Why doesn't Kelvin use a degree sign?
The kelvin is one of the base units of the metric system (SI) in its own right. So a reading is written 300 K, not 300 °K. Celsius, Fahrenheit and Rankine keep the degree sign.
Why can't a temperature go below absolute zero?
Absolute zero is the point where something has the least heat energy it can possibly have. Kelvin and Rankine start counting there. So a negative kelvin or Rankine reading has no real meaning, and neither does anything below −273.15 °C or −459.67 °F.
Where is the Rankine scale used?
Mainly in some US engineering fields, such as heat engineering (thermodynamics) and aerospace. Those calculations need a scale that starts at absolute zero, but the rest of the data is in Fahrenheit-based units.
Is there a page for just Celsius and Fahrenheit?
Yes. Popular pairs have their own pages with a table of everyday temperatures: Celsius to Fahrenheit, Celsius to Kelvin and Fahrenheit to Kelvin. Each one converts both ways. Use this converter when Rankine is involved or you want all four scales at once.
How this calculator is checked
Method: Scale definitions: K = °C + 273.15; °F = °C × 9/5 + 32; °R = K × 9/5. Absolute zero = 0 K. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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