The answer in both scales
Below freezing means below 32 °F, or below 0 °C. Same temperature, two scales.
That pair is worth committing to memory, because it is the one anchor that makes the rest of the conversion intuitive. Zero and thirty-two are the same moment; every other correspondence follows from there.
Celsius was built around it — Anders Celsius fixed his scale using the freezing and boiling points of water, so freezing landing on zero was the design. Fahrenheit was not: Daniel Fahrenheit set his zero using a brine mixture, and water's freezing point ended up at 32 as a consequence rather than a choice.
Where the freezing point moves
Pure water at normal atmospheric pressure freezes at 0 °C. Very little water in the real world is pure water at normal pressure.
Dissolved substances lower it. Salt, sugar, alcohol and antifreeze all interfere with molecules arranging into a crystal lattice, so the water has to get colder before it can freeze — freezing-point depression. Sea water freezes around −2 °C. Road salt works to about −9 °C (15 °F) and tails off below that, which is precisely why gritting stops being effective in a hard freeze.
Pressure shifts it. Water is unusual here: higher pressure lowers its freezing point slightly, rather than raising it. The effect is small at everyday pressures.
Motion resists it. Moving water takes longer to freeze than still water, which is the smaller half of why a dripping tap protects a pipe. The larger half is pressure relief — see what temperature pipes freeze at.
Purity can let it fall far below. Very pure water in a smooth container has nothing to crystallise around and can supercool several degrees below zero while staying liquid, then freeze all at once when disturbed.
And where things freeze above it
The other direction catches more people out, and it is not really about water at all — it is about where the thermometer is.
Official air temperature is measured about 1.5 metres above open ground, shaded and ventilated. On a clear, still night every surface radiates heat directly to space, and with no cloud to return it the ground, the grass, the leaves and your car roof end up two to four degrees colder than the air above them.
So frost regularly appears when the reported temperature is 2, 3 or even 4 °C. The forecast is not wrong; the surface is simply colder than the air. This is why frost advisories are issued above freezing, covered in at what temperature frost forms.
The thresholds worth knowing near zero
| Temperature | What changes |
|---|---|
| 4 °C (39 °F) | Frost becomes possible on clear still nights |
| 2 °C (36 °F) | Frost becomes likely; frost advisory territory |
| 0 °C (32 °F) | Pure water freezes. Ice on standing water |
| −2 °C (28 °F) | Freeze warning; most tender annuals lost |
| −4 °C (25 °F) | Hard freeze; end of the growing season |
| −6 °C (21 °F) | Exposed pipes start to be at real risk |
| −9 °C (15 °F) | Road salt becomes much less effective |
| −18 °C (0 °F) | Domestic freezer; "sub-zero" in American usage |
| −40 °C (−40 °F) | The two scales read the same number |
Notice how many of the useful thresholds are not zero. Zero is where water changes state; almost every decision you actually make sits a few degrees either side of it.
Why "sub-zero" is ambiguous
Which zero depends on which country wrote the sentence.
In everyday American usage, sub-zero means below 0 °F, which is −18 °C — a genuinely dangerous cold. In Celsius-speaking countries it means below 0 °C, which is 32 °F, a frost. The two readings are 18 degrees apart, and nothing in the phrase tells you which is meant.
The full ordered scale from a cold night down to absolute zero is in the extreme cold scale, and any specific value converts in the converter.