Temperature: the exact defined values
0 °C = 273.15 K exactly, and °F = °C × 9/5 + 32 exactly — these are definitions, not approximations.
How the base unit is defined
Since 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380 649 × 10⁻²³ J/K (SI Brochure, 9th edition).
Exact values table
"Exact" means the value is fixed by definition and carries no measurement uncertainty.
| Quantity | Exact value | Notes |
|---|---|---|
| 0 °C | 273.15 K | The Celsius scale is defined as t/°C = T/K − 273.15. Exact. |
| Absolute zero | 0 K = −273.15 °C = −459.67 °F | Lowest possible temperature; exact on all three scales. |
| °C → °F | °F = °C × 9/5 + 32 | Exact ratio, no rounding involved. Open converter → |
| °F → °C | °C = (°F − 32) × 5/9 | Exact inverse of the above. Open converter → |
| Scales cross at | −40 °C = −40 °F | The single point where Celsius and Fahrenheit read the same. |
Rounding traps that actually change your result
A temperature and a temperature difference convert differently
To convert a reading, use °F = °C × 9/5 + 32. To convert a difference or an interval, drop the offset: a rise of 10 °C is a rise of 18 °F, not 50 °F. This is the single most common temperature conversion error.
Kelvin takes no degree sign
Write 300 K, not 300 °K. The degree symbol was dropped from the kelvin in 1967.
Where these values come from
- BIPM SI Brochure, 9th edition
Kelvin definition via the Boltzmann constant and the exact 273.15 offset for Celsius.
- NIST Special Publication 811
Temperature conversion relationships and correct usage of degree symbols.
Frequently asked questions
Is 0 °C exactly 273.15 K?
Yes. The Celsius scale is defined by the exact relation t/°C = T/K − 273.15, so the offset carries no uncertainty.
At what temperature do Celsius and Fahrenheit meet?
At −40 degrees. −40 °C = −40 °F, which follows directly from solving x = x × 9/5 + 32.
Convert temperature now
These converters use the exact factors listed above.