Pipe Sizing Calculator
To size a water pipe, work backward from flow and velocity: pick your peak demand in GPM, cap velocity at 5–8 ft/s, then solve for inside diameter with d = √(4Q / (π·v)). The field shortcut is GPM = 2.448 × d² × v (d in inches, v in ft/s). A free PlumbCalc Pro pipe sizing calculator does this instantly and defaults to the safe velocity. Rule of thumb: 1/2" carries ~4 GPM, 3/4" ~7 GPM, 1" ~13 GPM at a quiet 5 ft/s.
The pipe sizing formula
Every water-supply sizing job comes down to one relationship: flow equals area times velocity.
- Solve for diameter: d = √(4Q / (π·v)), with Q in ft³/s and v in ft/s.
- Plumber's shortcut: GPM = 2.448 × d² × v, where d is the pipe inside diameter in inches.
- Rearranged for flow check: v = GPM / (2.448 × d²).
Always size on the inside diameter (ID), not the nominal size. A "3/4-inch" copper Type L line has a 0.785" ID; PEX and CPVC of the same nominal size run smaller, so they hit peak velocity sooner.
What velocity should I design for?
Velocity is the safety limit that keeps pipe quiet and long-lived. Standard practice:
- Cold water: 8 ft/s maximum.
- Hot water: 5 ft/s maximum — heat plus speed drives erosion-corrosion.
- Quiet/premium design: keep everything at or below 5 ft/s to kill water hammer and pipe noise.
The statistic that matters: sustained velocity above 8 ft/s can more than double the erosion-corrosion rate inside copper, pitting elbows and pinholing lines years early. That is why our defaults are the safe ones.
"Size for the velocity, not the fixture count alone — a line that flows fast enough to whistle is a line that will leak before its time."
Pipe sizing chart (max GPM by size)
Approximate carrying capacity for common water lines, based on inside diameter. Use the 5 ft/s column for hot and for quiet homes; the 8 ft/s column is the cold-water ceiling.
| Nominal size | Typical ID (in) | Max GPM @ 5 ft/s (quiet) | Max GPM @ 8 ft/s (peak) | Typical use |
|---|---|---|---|---|
| 1/2" | 0.545 | 3.6 | 5.8 | Single fixture branch |
| 3/4" | 0.785 | 7.5 | 12 | Small-home main / 2–3 fixtures |
| 1" | 1.025 | 13 | 21 | 2-bath home main |
| 1-1/4" | 1.265 | 20 | 31 | Large home / light commercial |
| 1-1/2" | 1.481 | 27 | 43 | Multi-bath / branch main |
| 2" | 1.959 | 47 | 75 | Building main / irrigation |
Values use copper Type L ID. For PEX and CPVC, drop to the next size up when demand is near the top of a row, since their smaller bore raises velocity.
How to size a run in four steps
- Total the demand. Add fixture GPM or use water supply fixture units (WSFU) converted to GPM.
- Set the velocity cap. 5 ft/s hot, 8 ft/s cold.
- Match the chart. Pick the smallest size whose column meets your GPM.
- Check pressure loss. Confirm friction loss over the run length still leaves ~40–50 PSI at the fixture.
Run your numbers in the free PlumbCalc Pro calculator — enter GPM and length, and it returns the pipe size, velocity, and friction loss with the safe defaults already dialed in. No sign-up, no callbacks, built by 30+ years in the trades.
Frequently asked questions
What size water line does a typical house need?
Most single-family homes run a 3/4" or 1" main. A 3/4" line handles roughly 7–12 GPM — enough for a small 2-bath home — while a 1" main (13–21 GPM) covers larger homes with simultaneous fixture use. The street-to-meter service is often 1" to protect against pressure drop.
What is the maximum flow through a 3/4-inch pipe?
About 7.5 GPM at a quiet 5 ft/s and up to ~12 GPM at the 8 ft/s cold-water ceiling, based on a 0.785" copper ID. Push it past 12 GPM and you get noise, water hammer, and accelerated erosion.
Do PEX and copper size the same?
No. Same nominal size, different inside diameter. 3/4" PEX has a smaller bore than 3/4" copper, so it reaches max velocity at a lower GPM. When demand is near the top of a size row, step up one size for PEX or CPVC.
How do I convert velocity to GPM?
Use GPM = 2.448 × d² × v, where d is the inside diameter in inches and v is velocity in ft/s. For a 1" line (ID 1.025") at 5 ft/s: 2.448 × 1.05 × 5 ≈ 13 GPM. The PlumbCalc Pro tool does the math for you.