Reckoning / Measure / Temperature
Measure · 4 units
Temperature
Temperature scales are not simple multiples of each other: they have different zeros. The kelvin starts at absolute zero, the Celsius degree is the same size with its zero at 273.15 K, and the Fahrenheit degree is 5/9 as large.
Convert temperature
Then and now
Anders Celsius’s scale of 1742 ran backwards, with 0 at boiling and 100 at freezing; it was reversed soon after his death. Since 2019 the kelvin is fixed through the Boltzmann constant rather than the triple point of water.
All temperature units
“Exact” means the unit is defined as an exact multiple of the SI unit; the decimal is printed in full when it ends. Rows marked historical or obsolete are kept for reading old documents.
| Unit | Defined as | In SI | Status | Source |
|---|---|---|---|---|
| SI | ||||
| kelvin K | SI base unit: fixed by the Boltzmann constant k = 1.380 649 × 10⁻²³ J/K | T/K = 1 · x + 0 | exact | BIPM SI Brochure |
| degree Celsius °C | T/K = t/°C + 273.15 | T/K = 1 · x + 273.15 | exact | BIPM SI Brochure |
| US | ||||
| degree Fahrenheit °F | t/°F = 1.8 · t/°C + 32 | T/K = 0.555 555 555 556… · x + 255.372 222 222… | exact | NIST SP 811 |
| US engineering | ||||
| degree Rankine °R | T/°R = 1.8 · T/K | T/K = 0.555 555 555 556… · x + 0 | exact historical | NIST SP 811 |