Temperature Converter
°C, °F, K, and °R — the one conversion with offsets, done right.
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The engineering
Temperature is the only common conversion with offsets, which is exactly why people botch it. Labs and datasheets run °C, US HVAC and weather run °F, radiometry and physics run kelvin, and US thermodynamics (steam tables, gas-turbine cycle decks) still runs Rankine — absolute like kelvin, sized like Fahrenheit.
The classic mistakes are all offset crimes: converting a temperature *difference* with the full formula (a 10 °C rise is 18 °F, not 50), forgetting that °C and °F cross at −40, and feeding Celsius into an absolute-temperature law — radiated power goes as T⁴ in kelvin, and 25 °C entered as '25 K' is a five-order-of-magnitude error that a spreadsheet will happily commit.
Where this math comes from
Daniel Gabriel Fahrenheit built the first reliably reproducible mercury thermometers and published his scale in 1724, anchored on an ice-salt brine zero and body heat. Anders Celsius proposed his centigrade scale in 1742 — upside down, with 100 at freezing and 0 at boiling; it was flipped to the modern sense shortly after his death (Linnaeus among the claimants, 1745).
William Thomson (Lord Kelvin) cut the thermometer out of the definition entirely in 1848 with an absolute scale grounded in Carnot's thermodynamics, and William Rankine supplied the Fahrenheit-sized absolute twin in 1859 for the engineers. The kelvin was redefined in 2019 via a fixed Boltzmann constant — temperature is now bookkeeping on energy, and every scale on this card is a linear map of the same physics.
- 1724Daniel Gabriel FahrenheitReproducible mercury thermometers and the °F scale.
- 1742Anders CelsiusCentigrade scale proposed (inverted; flipped by 1745).
- 1848William Thomson (Kelvin)Absolute thermodynamic temperature scale.
- 1859William RankineFahrenheit-sized absolute scale for engineering.
- 201926th CGPMKelvin redefined via the Boltzmann constant.
See the full timeline of the math behind every calculator →
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