What pipe size do I need for 10 GPM?

For 10 GPM with a target velocity of 5-7 fps, a 1-inch pipe is typically sufficient. However, for runs over 100 feet or if you want to minimize pressure loss, 1.25-inch pipe is better. Always check the actual velocity to ensure it's under 8 fps to avoid noise and erosion.

What is the maximum water velocity in pipes?

Plumbing codes generally recommend keeping water velocity below 8 feet per second (fps) to prevent erosion, water hammer, and noise. For quiet residential systems, 5 fps is a better target. High-velocity flow (>10 fps) can erode copper pipes over time.

How do I calculate GPM from pipe size?

The formula is: GPM = (Velocity in fps × Area in sq inches × 60) / 231. For example, a 1-inch pipe (ID ~1.05") at 5 fps: Area = π × (0.525)² = 0.865 sq in. GPM = (5 × 0.865 × 60) / 231 ≈ 11.2 GPM.

What is the difference between nominal and actual pipe size?

Nominal pipe size (e.g., 1-inch) is the label, but the actual inside diameter (ID) varies by material. A 1-inch copper Type L has an ID of ~1.025", while 1-inch Schedule 40 PVC has an ID of ~1.05". Always use the actual ID for flow calculations.

How much pressure do I lose per 100 feet of pipe?

Pressure loss depends on pipe size, flow rate, and material roughness. A rough estimate using the Hazen-Williams equation: For 3/4" copper at 5 GPM, expect ~5-8 psi loss per 100 feet. For 1" pipe at the same flow, loss drops to ~1.5 psi per 100 feet.

💧 Pipe Sizing Calculator

Calculate Water Pipe Size by GPM & Velocity

Flow Rate (GPM)

Gallons per minute

Target Velocity (fps)

3-8 fps recommended

Pipe Material

Pipe Run Length (ft)

For pressure loss estimate

💧 Pipe Sizing Fundamentals

Proper pipe sizing balances adequate flow capacity with controlled velocity to prevent noise, erosion, and excessive pressure loss.

Key Formula

GPM = (Velocity × Area × 60) / 231

Where Area = π × (Diameter / 2)² (in square inches)

Velocity Guidelines

Under 3 fps: May have sediment buildup in horizontal runs
3-5 fps: Ideal for quiet residential systems
5-8 fps: Standard range (code compliant)
8-10 fps: Noisy, higher pressure loss
Over 10 fps: Risk of erosion and water hammer

Sample Sizing Chart (Copper Type L, 6 fps target)

Pipe Size	Inside Diameter	GPM @ 6 fps
1/2"	0.545"	3.4
3/4"	0.785"	7.0
1"	1.025"	11.9
1-1/4"	1.265"	18.0
1-1/2"	1.505"	25.5
2"	1.985"	44.0

💡 From Carlos Rivera, Master Plumber (17 years):

"The biggest mistake DIYers make is undersizing the main supply line. They calculate the peak fixture demand (say, 12 GPM for shower + washing machine + sink) and size the pipe at 12 GPM, forgetting about friction loss. By the time the water gets to the second floor, you've lost 20 psi! Always size up one increment if your run is over 50 feet, and NEVER go smaller than 3/4" for a main house line, even if the math says you can."

⚠️ Common Mistakes

1. Ignoring Fittings: Every 90° elbow adds friction equivalent to ~3 feet of straight pipe. A run with 6 elbows effectively adds 18 feet to your length for pressure loss calculations.

2. Using Nominal Size for Calculations: A "1-inch pipe" is NOT 1 inch inside! Always use the actual ID. Copper Type M has a different ID than Type L.

3. Forgetting Peak Demand: Don't size for average use. Size for when the shower, dishwasher, and outdoor hose are all running at once (simultaneity factor).

Reviewed by Carlos Rivera

Licensed Master Plumber, 17 years experience in residential and commercial plumbing design.

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