Will My Pipes Freeze Tonight?

Enter your ZIP code and tell us where the pipe runs. We check the hourly National Weather Service forecast and model how quickly the water in that pipe cools and starts to freeze.

How pipes actually freeze

A water pipe does not care what the thermometer on your porch says. It responds to the air right around it — the inside of a wall cavity, a crawl space, a garage, an attic. Those spaces sit somewhere between your indoor temperature and the outdoor air, and how close they get to outdoor conditions decides whether the pipe freezes. That is why this calculator asks where the pipe runs before anything else.

Once the surrounding air is below 32 °F, the water starts losing heat. First it cools down to the freezing point; how fast depends on how much water there is (a ¾-inch pipe holds about twice the water of a ½-inch one), what the pipe is made of, and how much insulation slows the flow of heat. Then the water has to give up its latent heat of fusion — about 144 BTU per pound — before it turns to ice. That second stage takes far longer than the first, which is why a pipe can sit at exactly 32 °F for hours before it is in real trouble.

Why pipes burst

Water expands about 9% when it freezes, but the ice itself rarely splits the pipe where it forms. Research at the University of Illinois showed that an ice plug traps the water between it and a closed faucet. As more ice grows, the pressure in that trapped section climbs until the pipe fails — often at a spot that never froze at all. That is why a slow drip works: an open faucet gives the pressure somewhere to go.

Which locations are most at risk

Attics are the worst: they are deliberately kept close to outdoor temperature, and on clear nights the roof radiates heat to the sky and can make the attic colder than the outside air.Vented crawl spaces and attached garages are buffered by the ground and the house, but cold wind pushes them toward outdoor temperature. Pipes in uninsulated exterior walls — common in older homes and under kitchen sinks on outside walls — freeze more often than people expect, especially on the windward side. Pipes in heated rooms are safe as long as the heat stays on.

Reading your result

Low means the model does not expect the water to get within a few degrees of freezing in the next 48 hours. Medium means the water gets close to or reaches 32 °F and a little ice may form — worth acting on, especially for pipes you cannot see. High means enough ice is expected to block the pipe, the point where pressure builds and bursts become likely. The time shown is when the water first drops to about 35 °F, so you have a margin to act before ice forms.

The calculation is deliberately cautious: the forecast is shifted 2 °F colder, the water is assumed to be standing still, and we take no credit for supercooling (water can briefly stay liquid below 32 °F, which would delay freezing). Thin pipes and bare copper react fastest; insulation and larger pipes buy time but do not change the final outcome on a long, bitterly cold night.

Cheap fixes that work

Keep the thermostat steady through the night instead of setting it back. Open sink cabinet doors on exterior walls. Close garage doors and crawl space vents during a cold snap. Let a faucet drip. For pipes that freeze every winter, foam sleeves plus a self-regulating heat cable — or rerouting the pipe inside the insulated envelope — is the permanent fix. And know where your main shutoff valve is before you need it.

Frequently asked questions

At what outdoor temperature do pipes freeze?

There is no single number, because the pipe sees the temperature of the space it runs through, not the outdoor air. A common rule of thumb from insurance research is that pipes in unheated spaces become vulnerable when it gets down to about 20 °F outside. Attic pipes can freeze in the upper 20s, while a pipe in a heated room usually needs the heat to fail first.

Does letting a faucet drip really prevent frozen pipes?

Yes, it helps in two ways. Moving water brings in warmer water from the main, and an open faucet relieves the pressure that builds between an ice blockage and the closed faucet — that pressure, not the ice itself, is what usually splits the pipe. A pencil-thin stream is enough.

Will foam pipe insulation stop my pipes from freezing?

Insulation slows heat loss, so it buys time — often several hours — but it does not add heat. On a short cold snap that can be enough. On a long night well below freezing, an insulated pipe in a cold space will still freeze, just later. Pair insulation with heat (room air, heat cable) or flow (dripping).

Are PEX pipes freeze-proof?

No. PEX is more flexible than copper or CPVC and often survives a freeze, but brass fittings, valves and the joints between PEX and other materials can still crack. Treat PEX as freeze-resistant, not freeze-proof.

How can I tell if a pipe is already frozen?

Common signs are a faucet that only trickles or gives nothing on a cold morning, frost on an exposed pipe, or a bulge in the pipe. Keep the faucet open, warm the pipe gently with a hair dryer, heating pad or space heater kept at a safe distance, and never use an open flame. If you cannot reach the frozen section or a pipe has split, shut off the main water and call a plumber.

How accurate is this calculator?

It is an estimate built to err on the cautious side. The forecast is shifted 2 °F colder, the water is assumed to be standing still, and no credit is taken for supercooling. Real homes vary — drafts, sun on a wall, or a nearby furnace duct can make your pipe warmer or colder than the model. Use it as an early warning, not a guarantee.

Do you store my ZIP code?

No. The ZIP code is converted to coordinates in your browser using a static lookup file. Only rounded coordinates are sent to the National Weather Service (through our forecast relay) to fetch the forecast, and the calculation runs entirely in your browser.