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Pipe Size Calculator
Use this free pipe size calculator to find the right pipe diameter for a water flow. Enter the flow rate and the velocity you want, and it works out the required inside diameter from the flow equation, shows the nearest standard pipe sizes with the actual velocity in each, and estimates the friction and pressure loss over your pipe run with the Hazen-Williams equation.
Aim for 2 to 8 ft/s for home water supply. About 5 ft/s is a good starting point; keep hot water lines near 4 to 5 ft/s.
Material sets the Hazen-Williams roughness factor used for the friction estimate below. Smoother pipe loses less pressure.
Friction loss (optional)
Friction loss is calculated for the recommended pipe size over the length you enter, using Hazen-Williams.
| Nominal size | Actual ID | Velocity |
|---|
Friction loss in 100 ft of 1 in PVC: 5.5 ft of head (2.4 psi).
How it works
A pipe carries water by giving it a hole to flow through, and the size of that hole decides how fast the water moves. The flow equation says it plainly: flow rate equals cross-sectional area times velocity, or Q = A × v. If you know the flow you need and the velocity you want, the area follows by division, and the diameter follows from the area.
Velocity is the number you choose, and it matters more than it looks. The usual guidance for home water supply pipes is 2 to 8 feet per second. Above 8 feet per second the flow turns noisy, fittings and elbows wear out faster, and water hammer, the bang you hear when a valve shuts quickly, gets much worse.
A = Q ÷ v
Diameter = √(4 × A ÷ π)
Velocity in a pipe = 0.4085 × GPM ÷ (inside diameter in inches)²
Once the required diameter is known, the calculator compares it with standard pipe sizes. A 1 inch Schedule 40 pipe measures 1.049 inches inside, because the wall takes up the rest, and a thicker-walled Schedule 80 pipe of the same nominal size is only 0.957 inches inside. Flow only cares about the inside diameter, so the table in the result uses actual Schedule 40 inside diameters: ½ inch is 0.622, ¾ inch is 0.824, 1 inch is 1.049, 1¼ inch is 1.380, 1½ inch is 1.610, 2 inch is 2.067, 3 inch is 3.068, and 4 inch is 4.026 inches.
The friction estimate uses the Hazen-Williams equation, the standard way to estimate pressure loss in water pipes. The loss grows with flow rate to the power of 1.852, so doubling the flow nearly quadruples the loss, and it shrinks with diameter to the power of 4.87, so going up one pipe size can cut the loss by more than half. The calculator reports the loss both as feet of head and as psi, since 1 foot of head equals 0.4335 psi.
Worked example
You need to supply 10 gallons per minute to a garden tap and you want the water to move at 5 feet per second through PVC pipe, with a 100 foot run.
Step 1: Convert the flow to cubic feet per second — 10 ÷ 448.8 = 0.0223 ft³/s.
Step 2: Divide by the velocity to get the area — 0.0223 ÷ 5 = 0.00446 ft², which is 0.642 in².
Step 3: Work out the diameter — √(4 × 0.642 ÷ π) = √0.817 = 0.90 inches inside diameter.
Step 4: Pick the next standard size up — the smallest Schedule 40 size with an inside diameter of at least 0.90 inches is a 1 inch pipe, with an actual inside diameter of 1.049 inches.
Step 5: Check the real velocity in that pipe — 0.4085 × 10 ÷ 1.049² = 3.7 ft/s, comfortably inside the 2 to 8 ft/s range.
Step 6: Friction loss over 100 feet of 1 inch PVC — Hazen-Williams gives about 5.5 feet of head, or about 2.4 psi.
The result is a required inside diameter of 0.90 inches, a recommended 1 inch pipe, a real velocity of 3.7 ft/s, and a friction loss of about 2.4 psi over the 100 foot run.
FAQ
What water velocity should I design a pipe for?
For most home water supply pipes, design for 2 to 8 feet per second, and aim for about 5 feet per second as a starting point. Below 2 feet per second the pipe is bigger and more expensive than it needs to be, and water can sit and stagnate. Above 8 feet per second the flow gets noisy, fittings wear faster, and water hammer becomes a real risk when a valve closes quickly. Hot water lines and copper pipe are best kept toward the lower end, around 4 to 5 feet per second, because fast hot water erodes copper from the inside.
What is the difference between pipe size and pipe inside diameter?
The size printed on a pipe, like 1 inch, is its nominal size, not its actual inside diameter. The inside diameter is smaller because the pipe wall takes up space, and it changes with the wall thickness, or schedule. A 1 inch Schedule 40 pipe has an inside diameter of 1.049 inches, while a 1 inch Schedule 80 pipe, with thicker walls, is only 0.957 inches inside. Flow depends on the inside diameter, so this calculator works with the actual inside diameter of each standard size.
Why does my pipe lose pressure over a long run?
Water rubbing against the inside of the pipe creates friction, and that friction eats pressure over distance. The loss grows quickly with flow rate, roughly with the square of the flow, and shrinks quickly with pipe size, roughly with the fifth power of the diameter. Doubling the flow through the same pipe nearly quadruples the pressure loss, while going up one pipe size can cut the loss by more than half. That is why a long run to a distant fixture often needs a bigger pipe than a short run carrying the same flow.
Does pipe material change the size I need?
Material does not change the diameter the flow equation gives you, because the diameter comes from the flow rate and the velocity you want. Material does change the friction loss: smooth plastic pipe like PVC and PEX loses less pressure than rougher metal pipe, so a long PVC run can be a little smaller than a long steel run for the same pressure drop. This calculator uses the Hazen-Williams roughness factor for your material, 150 for PVC, 140 for copper and PEX, 130 for cast iron, and 120 for galvanized steel.
How do I convert GPM to pipe diameter by hand?
First turn the flow into cubic feet per second by dividing GPM by 449. Then divide by the velocity in feet per second to get the area in square feet. Then turn that area into a diameter: diameter in inches equals the square root of 4 times the area in square inches divided by pi. For 10 GPM at 5 feet per second, the flow is 0.0223 cubic feet per second, the area is 0.00446 square feet or 0.642 square inches, and the diameter is the square root of 4 times 0.642 divided by pi, which is 0.90 inches. Pick the next standard size up, which is a 1 inch pipe.