Pipe Diameter Calculator
To find pipe diameter from flow, use d = √(4Q ÷ (π × v)), where Q is flow in cubic feet per second and v is design velocity in ft/s. Size water supply lines for 5-8 ft/s. Quick rule of thumb: 1/2" carries ~2 GPM, 3/4" ~5 GPM, and 1" ~9 GPM at a safe velocity. Run the numbers in the free PlumbCalc Pro tool and skip the math.
→ Open the free Pipe Diameter Calculator — no signup, defaults set to the safe values.
The pipe diameter formula
Pipe diameter comes straight from the continuity equation, Q = A × v. Solve for the inside diameter:
d = √( 4Q ÷ (π × v) )
- d = inside diameter (feet)
- Q = volumetric flow rate (ft³/s)
- v = fluid velocity (ft/s)
To go from GPM to ft³/s, divide GPM by 448.8. Multiply the diameter answer by 12 for inches. The calculator does both conversions for you.
What velocity should I design for?
Keep cold water supply at 5-8 ft/s and hot water at 4-5 ft/s. Push past 8 ft/s and you invite erosion-corrosion in copper, water hammer, and pipe noise. Too slow wastes pipe and money. PlumbCalc Pro defaults to the safe side so you don't get a callback.
"Size for the velocity, not the guess — 8 ft/s is the wall you don't cross on potable copper."
Pipe diameter sizing chart (water supply)
Practical maximum flow by nominal size at safe design velocity. Use it as a fast field reference, then confirm with the tool.
| Nominal size | Typical ID (in) | Max flow @ 5 ft/s (GPM) | Max flow @ 8 ft/s (GPM) | Common use |
|---|---|---|---|---|
| 1/2" | ~0.545 | 3.6 | 5.8 | Single fixture branch |
| 3/4" | ~0.785 | 7.5 | 12.0 | 2-3 fixture branch |
| 1" | ~1.025 | 12.9 | 20.6 | Main to small home |
| 1-1/4" | ~1.265 | 19.6 | 31.4 | Main / riser |
| 1-1/2" | ~1.505 | 27.7 | 44.4 | Main / multi-bath |
| 2" | ~1.985 | 48.3 | 77.2 | Building main |
ID values shown for Type L copper; PEX and CPVC inside diameters run slightly smaller for the same nominal size, so lean to the larger pipe when close.
Does pipe material change the diameter?
Yes — sizing is based on inside diameter (ID), not the nominal label. Copper, PEX, and CPVC of the same nominal size have different IDs and different friction losses. PEX has a notably smaller bore, so a 3/4" PEX line moves less than 3/4" copper at the same pressure. Always size on ID and account for developed length and fittings. PlumbCalc Pro holds the material tables so you pick the pipe, not the math.
Why use PlumbCalc Pro
Built by 30+ years in the trades. The defaults are the safe ones, the units convert automatically, and every answer is jobsite-accurate. It's free, no login. Size your pipe now →
Frequently asked questions
How do I calculate pipe diameter from GPM?
Convert GPM to ft³/s (divide by 448.8), pick your velocity (5-8 ft/s for supply), then apply d = √(4Q ÷ (π × v)) and multiply by 12 for inches. Or enter GPM and velocity into the free calculator and read the size instantly.
What size pipe do I need for 10 GPM?
At a safe 8 ft/s, 10 GPM fits comfortably in 3/4" copper (good to ~12 GPM). For quieter, lower-velocity runs or PEX, step up to 1". When a flow sits near a size boundary, choose the larger pipe.
Is pipe diameter measured inside or outside?
Flow sizing uses inside diameter (ID). The nominal size (like "3/4 inch") is a label, not the true bore. Copper, PEX, and CPVC share nominal names but have different IDs, so always calculate on actual ID.
What is the maximum safe water velocity in a pipe?
Keep potable water at or below 8 ft/s (cold) and 4-5 ft/s (hot). Exceeding 8 ft/s accelerates erosion-corrosion, water hammer, and noise. PlumbCalc Pro caps velocity at safe defaults automatically.