Why does PEX sometimes have a smaller inner diameter than copper of the same nominal size?

Nominal pipe size is a trade label, not an exact measurement. PEX-A inner diameters are typically slightly smaller than Type L copper at 1/2" and 3/4", which can raise velocity and pressure drop at higher flow rates. The difference narrows at larger sizes.

Which pipe material has less friction loss?

Copper (Type L) has a Hazen-Williams C of about 130–140 while PEX has C ≈ 130–150 depending on brand. In practice the difference in friction coefficient is small; inner diameter matters more. At sizes where PEX ID is slightly smaller, copper may show marginally lower pressure drop.

What is the maximum recommended velocity for each material?

For copper, keep velocity below 8 ft/s to prevent erosion corrosion. For PEX, most manufacturers allow up to 10–12 ft/s, though keeping it under 8 ft/s reduces noise and wear. This tool flags velocities above 8 ft/s as a warning.

Does this account for fittings?

No — this calculator covers straight-pipe friction loss only. Add 10–20% to the pressure drop estimate for fittings such as elbows, tees, and valves. PEX expansion fittings typically have lower restriction than standard PEX crimp fittings, though both add more friction than full-bore copper solder fittings.

Which PEX type does this use?

Inner diameters are based on PEX-A (expansion/ASTM F876), the most common standard. PEX-B and PEX-C share similar dimensions at most sizes, so results are representative for all PEX tubing.

PEX vs Copper Flow Rate Comparison

Compare flow rates and pressure drop between PEX and copper pipes of the same nominal size to choose the right material for your plumbing project.

Pipe Parameters

Nominal Pipe Size

Pipe Run Length (ft)

Flow Rate (GPM)

Inner Diameters Used

Set your parameters and click Compare Pipes to see results.

PEX-A

Inner diameter: —
Velocity: —
Head loss: —
Pressure drop: —

Copper (Type L)

Inner diameter: —
Velocity: —
Head loss: —
Pressure drop: —

Comparison Summary

PEX pressure drop

—

Copper pressure drop

—

Pressure Drop Comparison

PEX-A —

Copper (Type L) —

Summary

PEX and copper pipes of the same nominal size have different inner diameters and surface roughness, which affects flow rate and pressure drop. This tool applies the Hazen-Williams equation to compare both materials side by side — enter pipe size, length, and flow rate to see friction loss, velocity, and which pipe performs better for your application.

How it works

Select the nominal pipe size (1/2", 3/4", 1", etc.).
Enter the total pipe run length in feet.
Set the desired flow rate in gallons per minute (GPM).
The tool looks up the actual inner diameters for PEX-A and Type L copper at that nominal size.
Friction head loss is calculated with the US-customary Hazen-Williams form (h_f = 10.44 × L × Q^1.852 / (C^1.852 × D^4.87)), using GPM, inches, and feet.
Results show pressure drop (PSI), velocity (ft/s), and a side-by-side summary so you can pick the better fit.

Use cases

Decide between PEX and copper when re-piping a bathroom or kitchen.
Verify that a PEX retrofit will deliver the same flow as the original copper supply line.
Estimate pressure drop for a long run from main to fixtures.
Compare materials when sizing a water supply line for a new fixture.
Check pipe velocity to stay within the 8 ft/s erosion limit for copper.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu