What size pipe do you need?

How do I size a water pipe?

Pick the smallest pipe that keeps the water speed under the limit at your flow. Speed in feet per second = GPM ÷ (2.448 × inside diameter²). 10 GPM through ¾ inch type L copper (0.785 inch inside) moves at 6.6 ft/s — fine for cold water, but too fast for hot, which needs 1 inch pipe (3.9 ft/s).

How fast should water flow in a pipe?

The Copper Development Association recommends at most 8 feet per second for cold water and 5 ft/s for hot water in copper tube. Faster water is noisy and wears the pipe, especially at elbows.

Why does the pipe material matter?

Pipes of the same nominal size have different inside diameters. ¾ inch type L copper is 0.785 inch inside, type M 0.811 inch and ¾ inch PEX only about 0.68 inch, so the same flow runs faster in PEX. The calculator uses the inside diameter of each.

How much flow does my house need?

Plumbing codes size the supply by fixture units — a count of the fixtures each pipe serves — in the IPC or UPC tables. Use this calculator to check the water speed for a flow you know, such as a pump rating or a bucket test.

Does a bigger pipe give more pressure?

Not more static pressure, but less is lost on the way: friction loss falls steeply with a larger diameter. For long runs, check the loss with the friction loss calculator too.

What size pipe do I need for 12 gallons per minute?

With Type L copper, 3/4 inch is the smallest size that stays under 8 ft/s for cold water, and 1 inch is the smallest that stays under 5 ft/s for hot water. In ¾ inch PEX the same 12 GPM would move at 10.6 ft/s.

Is the nominal size the same as the inside diameter?

No. The nominal size is a label. The inside diameter is smaller, and it is the number that sets the water speed and the flow area. That is why the calculator uses a table of real inside diameters for each pipe type.

How the speed is calculated

Flow is speed times the cross-section of the pipe. With the flow in US gallons per minute and the inside diameter d in inches, velocity (ft/s) = GPM ÷ (2.448 × d²). The constant 2.448 comes from converting the circle area π d² ÷ 4 from square inches to square feet and 7.48052 gallons per cubic foot, and minutes to seconds. Because d is squared, a small increase in diameter lowers the speed a lot: going from ¾ to 1 inch Type L copper cuts the speed by about 41% at the same flow.

A worked example

At 12 GPM in Type L copper, ¾ inch (0.785 inch inside) gives 12 ÷ (2.448 × 0.785²) = 7.95 ft/s, just under the 8 ft/s cold-water limit but well over the 5 ft/s hot-water limit. 1 inch (1.025 inch inside) gives 4.7 ft/s, under both limits. The calculator reports the smallest size that meets the limit you pick, and shows the speed in every size so you can see how much margin each one leaves.

Common mistakes

Using the nominal size in the formula gives the wrong speed. For ¾ inch PEX, whose inside diameter is only 0.681 inch, treating 0.75 inch as the diameter understates the speed by about 21%. Applying the copper limits to every material is another; the 8 and 5 ft/s figures are the Copper Development Association’s guidance for copper tube, and plastic pipe makers publish their own figures. Third, a flow that is correct for one fixture is not the flow for the whole house, since several fixtures can run at once. And a pipe that passes the speed test can still lose too much pressure over a long run; check that with the friction loss calculator.

Sources: the velocity formula follows from the flow-area and gallon definitions (1 US gallon = 231 in³, 1 ft³ = 7.48052 gal; NIST Handbook 44); velocity guidance from the Copper Development Association; inside diameters of copper tube are from ASTM B88 dimensions. Related: friction loss calculator, pipe volume calculator, GPM calculator and pipe velocity calculator.