How to use the Temperature Converter
- Enter the temperature reading you have.
- Select the scale that reading is on.
- Read all four scales in the results — no second dropdown needed.
- Use Kelvin for any physics or gas-law work; use Rankine only alongside imperial engineering data.
- Remember that a temperature difference converts differently from a temperature — see the methodology below.
How the calculation works
Temperature is not a simple ratio conversion because the scales have different zero points as well as different degree sizes. Celsius and Kelvin share a degree size but are offset by 273.15. Fahrenheit and Rankine share a smaller degree — exactly 5/9 of a Celsius degree — and are offset by 459.67. Converting therefore requires both a multiplication and an addition, which is why doubling 20 °C does not give you double the heat.
Kelvin and Rankine are absolute scales starting at absolute zero, the point where thermal motion reaches its quantum minimum. They carry no degree symbol in the Kelvin case and are the only scales valid inside physics equations such as the ideal gas law, Stefan-Boltzmann radiation or Carnot efficiency. Substituting Celsius into any of those produces nonsense, often negative, results.
Temperature differences behave differently from temperatures. A rise of 10 °C is a rise of 18 °F and also a rise of 10 K, because the offsets cancel when you subtract. Confusing an interval with a point is the single most common error in HVAC and thermal specification work: a heat exchanger rated for a 40-degree delta means something very different depending on which scale the specification assumed.
°F = °C × 9/5 + 32; K = °C + 273.15; °R = (°C + 273.15) × 9/5; °C = (°F − 32) × 5/9Source: BIPM SI Brochure 9th edition; ITS-90 International Temperature Scale; NIST Special Publication 811.
Worked example
An oven recipe from a US site says 425 °F and your oven dial is in Celsius.
- Enter 425 and select Fahrenheit.
- Subtract 32: 425 − 32 = 393.
- Multiply by 5/9: 393 × 0.5556 = 218.3 °C.
- Round to the nearest oven step: 220 °C, or gas mark 7.
425 °F is 218.3 °C — set the dial to 220 °C and reduce by about 20 °C if the oven is fan-assisted.
Frequently asked questions
At what temperature do Celsius and Fahrenheit meet?+
At −40. Minus forty degrees Celsius is exactly minus forty degrees Fahrenheit, which is the only point where the two scales agree.
Why does Kelvin have no degree sign?+
Because it is an absolute unit, not a scale of degrees relative to an arbitrary zero. You write 300 K, never 300 °K.
What is absolute zero in each scale?+
0 K, −273.15 °C, −459.67 °F and 0 °R. It is a theoretical limit that cannot be reached, only approached.
How do I convert a temperature difference?+
Use only the ratio: a difference of 1 °C equals 1 K and 1.8 °F. Do not apply the 32 or 273.15 offsets to a difference.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.