Nominal size is a label, not a measurement
This is the whole reason the page exists. A half-inch pipe is called half-inch because of a naming convention that stopped describing anything physical a long time ago. What actually decides how much water it holds, and how well it flows, is the inside diameter, and the inside diameter depends on the material and on the wall thickness of that particular series.
| Nominal | Type K copper | Type L copper | Type M copper | PEX (SDR-9) | CPVC (CTS) | PVC sch 40 | Steel sch 40 |
|---|---|---|---|---|---|---|---|
| 3/8" | 0.402 | 0.430 | 0.450 | 0.350 | — | 0.493 | 0.493 |
| 1/2" | 0.527 | 0.545 | 0.569 | 0.475 | 0.489 | 0.602 | 0.622 |
| 3/4" | 0.745 | 0.785 | 0.811 | 0.671 | 0.715 | 0.804 | 0.824 |
| 1" | 0.995 | 1.025 | 1.055 | 0.862 | 0.921 | 1.029 | 1.049 |
| 1-1/4" | 1.245 | 1.265 | 1.291 | 1.054 | — | 1.360 | 1.380 |
| 1-1/2" | 1.481 | 1.505 | 1.527 | 1.244 | — | 1.590 | 1.610 |
| 2" | 1.959 | 1.985 | 2.009 | 1.629 | — | 2.047 | 2.067 |
All figures in inches. Read across the half-inch row: PEX at 0.475 and steel at 0.622 are both sold as half-inch, and the steel holds seventy percent more water per foot. Because area goes with the square of the diameter, small differences in the bore make large differences in volume and larger ones still in flow.
These are published nominal figures. Real pipe carries a manufacturing tolerance, plastics grow slightly when hot, and old galvanised steel can lose a startling fraction of its bore to scale. If the number matters, measure the pipe you have and use the override field. The actual ID governs, not the table.
The arithmetic
Gallons = π × (ID ÷ 2)2 × length in inches ÷ 231
There are 231 cubic inches in a US gallon, which is the only awkward constant in the whole thing. Working in inches throughout avoids the unit slip that catches people who mix a diameter in inches with a length in feet. If you prefer cubic feet, there are 1,728 cubic inches in one, so 1,728 divided by 231 gives 7.4805 gallons per cubic foot.
Water weighs about 8.345 lb per gallon near 60 °F. That figure drifts a little with temperature but not enough to matter for anything you would use this page for. Where it does matter is on long horizontal runs of large pipe: two hundred feet of 2-inch type L copper holds about 32 gallons, which is 270 lb hanging on the hangers, and that is before you add the weight of the pipe itself.
Why anyone needs the flush volume
Line volume comes up more often than pipe volume does. After a repair, after a period of stagnation, after a chlorine disinfection, or when you are drawing a sample that has to represent the source rather than the pipe, the question is how much water has to move before what comes out of the tap is what is in the main.
One line volume is not enough. Flow in a pipe is not a clean plug — the water at the wall moves slower than the water down the middle, so the old water tails off gradually rather than ending at a stroke. Three line volumes is the figure people generally work to, and more where the line has sat a long time or the pipe is large relative to the flow you can pull through it. The calculator lets you set that multiplier because the right number depends on why you are flushing.
The flow rate to enter is the one you can actually achieve at the open end, not what the pipe could theoretically carry. A modern lavatory faucet is throttled to around 1.5 GPM by its aerator, a kitchen faucet nearer 2.2, a hose bib on a decent supply somewhere between 5 and 10. Pulling a line volume through a bathroom tap takes a great deal longer than people estimate.
What this figure is not
Volume tells you what a full pipe holds. It says nothing about whether the pipe is the right size, which is a question about flow, pressure, fixture demand and the code your jurisdiction adopted. Minimum pipe sizes for supply are set by that code and vary; so does the list of materials approved for a given use, and materials that are ordinary in one jurisdiction are not permitted in another. Plumbing is code-regulated and the requirements are not the same everywhere. Drain slope, venting, fixture unit loading, minimum pipe sizes and which materials are approved for which use are set by the plumbing code your local authority has adopted, along with whatever local amendments came with it. Most permanent work needs a permit and a licensed plumber. What is described here is common practice, not a rule you can hold up to an inspector.
The other thing volume will not tell you is anything about a drain. Waste lines are sized to run partly full by design, so the bore capacity of a 3-inch drain has almost nothing to do with what it carries. That is a slope and venting question, not a volume one.
Questions people ask
How many gallons are in 100 feet of 1/2 inch pipe?
It depends entirely on the material, which is the point. At 100 feet, type L copper with a 0.545 inch bore holds about 1.21 gallons, type M at 0.569 holds about 1.32, type K at 0.527 about 1.13, PEX at 0.475 about 0.92, CPVC at 0.489 about 0.98, schedule 40 PVC at 0.602 about 1.48, and schedule 40 steel at 0.622 about 1.58. The spread between PEX and steel is a factor of about 1.7 for pipe sold under the same name. If you see a single figure quoted for half-inch pipe with no material attached, it is an average of things that are not the same.
Does the pipe volume change the water heater sizing or the wait for hot water?
It has nothing to do with heater sizing and everything to do with the wait. The delay before hot water arrives is the volume of pipe between the heater and the fixture divided by the flow rate at that fixture, plus the time it takes to heat the pipe wall itself. Fifty feet of 3/4 inch type L copper holds about 1.26 gallons; at a 1.5 GPM aerated faucet that is the better part of a minute of running before the hot water even reaches you, and copper takes a further slug of heat out of the leading edge to warm the tube. This is why smaller-bore branch runs to distant fixtures, or a recirculation loop, deal with the complaint that a bigger heater never fixes.
Should I use the inside or outside diameter?
Inside, always, for volume and for flow. The outside diameter is what determines fittings and what you measure with calipers on the outside of the tube, and for copper and CTS plastics it is the nominal size plus one eighth of an inch. Half-inch copper measures 0.625 across the outside regardless of whether it is K, L or M; the type only changes the wall thickness, and therefore the bore. That is the trap: two pieces of pipe that look identical from the outside and take the same fittings can differ by ten percent in the volume they hold.
Why is my old galvanised pipe carrying less water than the table says?
Because the bore is not the bore any more. Galvanised steel scales up internally over decades, and the deposit is not thin. It is common to open an old half-inch galvanised line and find an opening closer to a quarter inch, with the loss concentrated at horizontal runs and at fittings. Volume falls with the square of the diameter, so half the bore is a quarter of the volume, but flow falls faster still because friction climbs steeply as the passage narrows and roughens. If you are calculating anything about an old galvanised system, cut a piece out and look at it rather than trusting a published ID.