Pipe Size Comparison at This Flow
| Nominal size | Velocity | Loss / 100 ft |
|---|---|---|
| 3/8 in. | — | — |
| 1/2 in. | — | — |
| 3/4 in. | — | — |
| 1 in. | — | — |
| 1-1/4 in. | — | — |
| 1-1/2 in. | — | — |
| 2 in. | — | — |
Highlighted row is the recommended size. Velocity and friction loss update with your flow and material.
GPM Capacity Guide (copper, 8 ft/s)
| Size | Approx. max GPM | Typical use |
|---|---|---|
| 3/8 in. | ~3.5 | Single low-flow fixture |
| 1/2 in. | ~5.5 | One or two fixtures |
| 3/4 in. | ~12 | Branch to several fixtures |
| 1 in. | ~20 | Small house main |
| 1-1/4 in. | ~31 | Larger main / trunk |
| 1-1/2 in. | ~44 | Whole-house / multi-bath |
| 2 in. | ~77 | Large or commercial main |
Result Summary
| Recommended size | — |
| Design flow | — |
| Velocity | — |
| Pressure drop over run | — |
| Material | — |
What Size Pipe Do I Need?
The right pipe size balances two limits: water must not move too fast (high velocity causes noise and erosion) and must not lose too much pressure over the run. Start with your design flow in gallons per minute, or let the calculator convert fixture units to flow using a demand curve. Pick the material, enter the run length and fittings, and the tool tests each standard size against an 8 ft/s velocity limit for cold lines (5 ft/s for hot) and your target friction loss. The smallest size that passes both is the recommended pipe.
Pipe Size vs Flow Rate
Flow capacity rises sharply with diameter because area grows with the square of the radius, so a 3/4 in. line carries roughly three times the flow of a 1/2 in. line before hitting the same velocity. That is why a single fixture is happy on 1/2 in. pipe but a whole-house main needs 3/4 in. or 1 in. The capacity guide above lists approximate maximum flow for each size at the cold-water velocity limit. Undersize the pipe and you get water-hammer noise and pressure loss; oversize it and you pay more for material and let water sit longer in the lines.
Water Pressure Drop Explained
As water flows, friction against the pipe wall steadily bleeds off pressure. The Hazen-Williams equation estimates this loss from flow, diameter, and a roughness coefficient that depends on material: smooth PEX and copper lose less than rougher galvanized steel. Every elbow and tee adds resistance equal to a length of straight pipe, which is why this calculator lets you add fittings as equivalent length. Keep total loss within the pressure your supply can spare after the meter, backflow device, and elevation rise.
Frequently Asked Questions
GPM or fixture units? Use fixture units when sizing a whole system from a fixture list; use GPM when you already know the demand for one run.
Why does material change the size? PEX has a smaller inner diameter than copper at the same nominal size, and steel is rougher, so both can push you up a size versus copper or PVC.
What velocity is safe? Keep cold water under about 8 ft/s and hot under about 5 ft/s to limit erosion and noise.
Does length matter? Yes. Longer runs and more fittings increase total pressure drop, sometimes forcing a larger pipe even at the same flow.