The Freezing Point of Water in A. C, B. F, and C. K

The freezing point of pure water is 0 °C, 32 °F, or 273.15 K. These values express the same temperature on the Celsius, Fahrenheit, and Kelvin scales. The search phrase “freezing point of water a. c b. f c. k” refers to these three values.

Why the numbers differ

The scales use different zero points and temperature intervals. On the Celsius scale, water freezes at 0 °C. On the Fahrenheit scale, it freezes at 32 °F. Kelvin starts at absolute zero, so water freezes at 273.15 K. The kelvin is the SI base unit of thermodynamic temperature, defined by fixing the Boltzmann constant (BIPM SI Brochure). Because Kelvin starts at absolute zero, water’s freezing point is 273.15 K, not 0 K (NIST Guide to the SI, Chapter 4).

Celsius and Kelvin use the same interval size: a change of 1 °C equals a change of 1 K. Their readings differ by 273.15, so add 273.15 to a Celsius temperature to convert it to Kelvin. Fahrenheit uses a different interval size, so conversion also requires multiplication.

One conversion example

At water’s freezing point, convert 0 °C to Fahrenheit like this:

(0 × 9/5) + 32 = 32 °F

To convert the same temperature to Kelvin, add 273.15:

0 + 273.15 = 273.15 K

Both conversions express the same temperature. They do not describe different freezing points.

Key Takeaways

• Pure water freezes at 0 °C, 32 °F, or 273.15 K under ordinary pressure conditions.
• The three values are equivalent temperatures on different scales.
• Celsius and Kelvin have the same interval size; add 273.15 to a Celsius temperature to convert it to kelvin.
• Dissolved substances can lower water’s freezing point, so mixtures such as seawater may freeze below 0 °C.