Temperature Converter
Convert between Celsius, Fahrenheit, Kelvin and Rankine, with the offset handled correctly.
How to use this calculator
- 1Enter the value you want to convert.
- 2Pick the unit you are converting from and the one you want.
- 3The table below shows the same value in every other supported unit.
How the calculation works
result = (value × factor(from) + offset(from) − offset(to)) / factor(to)- factor
- How many kelvin one unit represents
- offset
- Additive shift, needed where scales do not share a zero point
Every value is converted to kelvin first, then out to the target unit. This means n units need n factors rather than n² conversion pairs.
Temperature scales do not share a zero point, so conversion needs both a multiplier and an offset. This is why you cannot convert a temperature the way you convert a length.
C to F: (°C × 9/5) + 32. F to C: (°F − 32) × 5/9. Both fall out of the factor-and-offset model used here.
A temperature *difference* converts differently from a temperature *value*: a change of 1 °C equals a change of 1.8 °F, with no offset applied.
Worked example
25 celsius to fahrenheit
- 1.Convert to the base unit: 25 × 1 + 273.15 = 298.15 kelvin.
- 2.Convert out again: (298.15 − 255.37222) ÷ 0.5555556 = 77.
- 3.These scales have different zero points, which is why an offset is needed as well as a multiplier.
Result: 77 °F
What temperature scales actually measure
Temperature is a measure of how much thermal energy is packed into the random motion of a substance's molecules — the faster they jostle around, the hotter it feels. Unlike length or weight, temperature scales are not simple multiples of each other, because each one picks its own zero point. That is the whole reason converting temperature needs both a multiplier and an offset, while converting length or weight needs only a multiplier.
Kelvin is the odd one out among the everyday scales: it starts at absolute zero, the coldest temperature theoretically possible, where molecular motion stops almost entirely. Celsius and Fahrenheit both use zero points chosen for practical convenience rather than any physical limit, which is why negative Celsius and negative Fahrenheit temperatures are completely normal.
Four scales, four design choices
Each scale in this converter was built around a different reference need — human comfort, laboratory precision, or compatibility with an existing system of units.
- Celsius — set 0° at water's freezing point and 100° at its boiling point, giving a scale that lines up neatly with a substance nearly everyone has direct experience with. Used for almost all everyday and scientific purposes worldwide.
- Fahrenheit — an older scale, still standard for everyday use in the United States, originally anchored to a brine solution's freezing point and roughly the human body's temperature. It divides the range between water freezing and boiling into 180 degrees rather than 100, which gives finer-grained whole-number readings for everyday weather.
- Kelvin — shares Celsius-sized degrees but starts at absolute zero, so it has no negative values for physically possible temperatures. This makes it the standard for scientific work, where ratios of temperature (twice as hot, half as hot) need to mean something physically real.
- Rankine — the Fahrenheit-scaled equivalent of Kelvin — also zeroed at absolute zero, but using Fahrenheit-sized degrees. Mostly seen in older US engineering and thermodynamics references.
Why the US still uses Fahrenheit
Fahrenheit was the standard scale across the English-speaking world before most countries switched to Celsius during their broader adoption of the metric system in the 20th century. The United States never made that switch for everyday use, so weather reports, oven dials and thermostats in the US still run in Fahrenheit even though US science, medicine and engineering generally work in Celsius or Kelvin.
One argument in Fahrenheit's favour for weather is granularity: because its degrees are smaller than Celsius degrees, whole-number Fahrenheit readings describe outdoor temperature changes a person can actually feel, without needing decimal points the way Celsius sometimes does for the same precision.
Reading a conversion correctly
A few habits prevent the most common temperature-conversion errors.
- Never treat 0 as "nothing" — unlike length or weight, 0 °C and 0 °F are just points on arbitrary scales, not an absence of temperature — only 0 K represents a true absence of thermal energy.
- Differences convert differently from absolute values — a change of 10 °C equals a change of 18 °F, not 50 °F — when converting a temperature *swing* rather than a single reading, drop the offset and use the ratio alone.
- The scales cross at −40 — −40 °C and −40 °F are the same temperature, which is a useful sanity check: if a quick mental conversion lands far from that crossover in the expected direction, it is probably right.
What this assumes, and where it stops
Assumptions
- Values are absolute temperatures, not temperature differences.
Limitations
- Converting a temperature difference requires the ratio only, without the offset. This converter handles absolute temperatures.
- Kelvin and Rankine have no degree symbol and cannot be negative — a negative result indicates an input below absolute zero, which is unphysical.
Common questions
How do I convert Celsius to Fahrenheit in my head?
Double it and add 30 for a rough answer — 20 °C becomes about 70 °F (the exact value is 68). For precision, multiply by 1.8 and add 32. The two scales cross at −40, where both read the same number.
Why does Kelvin not use degrees?
Because it is an absolute scale starting at absolute zero, not a scale defined by two arbitrary reference points. Since 2019 the kelvin has been defined by fixing the Boltzmann constant. You write "300 K", not "300 °K".
Sources
- The kelvin: definition — BIPM
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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