Temperature Converter (°C, °F, K, °R)

Understanding the temperature converter — concept and principle

Temperature is reported on four common scales: Celsius (°C) in everyday use in most of the world, Fahrenheit (°F) in the United States, kelvin (K) in science as the SI base unit, and Rankine (°R) in some US engineering work. This converter takes a value on any scale and shows it on all four, which helps when reading foreign recipes, weather reports, lab data or thermodynamic tables.

The same warmth reads 25, 77, 298.15 or 536.67 depending on the ruler.

How it is calculated

The calculator first converts your input to Celsius, then derives the others:

From °F: °C = (°F − 32) × 5/9.
From K: °C = K − 273.15.
From °R: °C = °R × 5/9 − 273.15.
Then °F = °C × 9/5 + 32, K = °C + 273.15, and °R = (°C + 273.15) × 9/5 = °F + 459.67.

The principle

Each scale is defined by a zero point and a degree size. Celsius and kelvin share the same degree size; kelvin simply starts at absolute zero, the lowest possible temperature, which is −273.15 °C. Fahrenheit degrees are smaller: 180 °F span the same interval as 100 °C, hence the ratio 9/5, and its zero is shifted so that 0 °C equals 32 °F. Rankine is to Fahrenheit what kelvin is to Celsius: Fahrenheit-sized degrees counted from absolute zero, so 0 °R = 0 K = −459.67 °F. Since 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K, and the Celsius scale is defined from it.

Worked example

Input: 25 in °C, so the Celsius value is 25.
Fahrenheit: 25 × 9/5 + 32 = 45 + 32 = 77 °F.
Kelvin: 25 + 273.15 = 298.15 K.
Rankine: 298.15 × 9/5 = 536.67 °R (check: 77 + 459.67 = 536.67).

Each point is the calculator re-run with only “Temperature” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Temperature	Celsius
6.25	6.25
12.5	12.5
18.8	18.75
25	25
31.2	31.25
37.5	37.5
50	50

How to read the result

Useful reference points: water freezes at 0 °C (32 °F, 273.15 K) and boils at about 100 °C (212 °F) at standard atmospheric pressure; normal body temperature is about 37 °C (98.6 °F); 25 °C is a common "room temperature" in laboratory data. −40 is the one value where Celsius and Fahrenheit agree. Kelvin and Rankine are never negative, so a negative result on those scales means an input error.

Common mistakes

Converting a temperature difference with the offset. A change of 10 °C is a change of 18 °F, not 50 °F, and exactly 10 K.
Writing "degrees kelvin"; the unit is simply kelvin, symbol K, without a degree sign.
Using 273 instead of 273.15 when precision matters.
Applying ratios to Celsius or Fahrenheit values; 20 °C is not "twice as hot" as 10 °C. Ratios only make sense on absolute scales.

Keep in mind: Conversions are exact by definition, but the precision of the result is only as good as the measured input. Rounding the displayed value repeatedly can introduce small errors.

Frequently asked questions

What is the quick mental rule for °C to °F?

Double it and add 30 for a rough estimate; 25 °C gives about 80 °F (exact: 77 °F). It works best for everyday weather temperatures.

Why does science use kelvin?

Gas laws, radiation and thermodynamic efficiency depend on absolute temperature, so formulas such as PV = nRT need a scale that starts at absolute zero.

Where is Rankine used?

Mainly in older US thermodynamics and engineering tables, for example in some steam and refrigeration calculations done in imperial units.

Sources: BIPM, The International System of Units (SI Brochure), 9th ed., 2019 · NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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