Pipe Size Calculator

How erosion-corrosion destroys copper from the inside

Copper does not resist corrosion because it is inert. It resists because it builds a thin, tightly adhering layer of copper oxide on its inner surface within days of being filled, and that film is what the water actually touches thereafter.

Fast-moving water applies shear stress to that film. Below a threshold the film regenerates faster than it is removed and the pipe is stable indefinitely. Above it, bare copper is continuously exposed, dissolves slightly, and re-oxidises — a cycle that removes metal every second the water flows.

Temperature accelerates every part of this, which is why the hot water limit is lower. So does turbulence, which is why failures appear at elbows, tees and just downstream of valves rather than in straight runs.

The signature is unmistakable once seen: a pinhole leak with a horseshoe-shaped erosion pattern on the inside of the pipe, pointing in the direction of flow. It typically appears five to fifteen years after installation, long enough that nobody connects it to the original pipe sizing.

The fix is not a better fitting. It is a larger pipe, because the only variable that matters is velocity.

Sizing by fixture units

Plumbing codes do not size supply pipe from flow rate directly. They use fixture units, and understanding why explains where the flow figure for this calculator should come from.

Each fixture is assigned a fixture unit value reflecting both its flow and how often it is used — a water closet, a shower and a hose bib all draw differently and are used with different frequency. Summing fixture units gives a measure of demand potential rather than simultaneous flow.

That sum is then converted to expected demand through a probability curve, because fixtures are not all open at once. Ten fixtures do not demand ten times one fixture; the curve flattens sharply as the count rises, which is why a large building needs proportionally less pipe than a small one.

For a single-family house the practical outcome is that peak demand on the main lands somewhere between 8 and 15 GPM regardless of fixture count. Sizing to the arithmetic sum of every fixture produces an absurdly large service.

If you are working from a code table, take the demand figure it gives you and enter that here rather than adding fixtures yourself.

Pressure loss is a separate question from velocity

Velocity determines whether the pipe survives and whether it is quiet. Pressure at the far fixture is a different calculation, and a pipe can pass one and fail the other.

Total pressure loss has three parts: friction along the pipe, losses through fittings and valves, and static loss from elevation at roughly 0.43 psi per foot of rise. A second-storey bathroom starts nine feet up, which is four psi gone before any water moves.

The friction figure here is for straight pipe. Real runs include elbows, tees and valves, and each has an equivalent length — the length of straight pipe that would lose the same amount. A gate valve is nearly free; a globe valve can be equivalent to 40 feet of pipe in the same size.

The working rule is to keep total loss such that the least favoured fixture still sees adequate residual pressure — commonly taken as 15 to 20 psi at the fixture for normal operation, more for some appliances.

Long runs sometimes need upsizing for pressure even when velocity is comfortable. That is a legitimate reason to go up a size and this calculator will not tell you about it.