Why does PEX flow less than copper of the same size?

PEX has a thicker wall, so its inside diameter is meaningfully smaller than Type L copper at the same nominal size — 0.475 in versus 0.545 in for 1/2 in. Flow area scales with the diameter squared, so even though PEX is smoother, the smaller bore usually wins and PEX carries less flow at the same pressure drop.

Is there a flow difference between PEX-A and PEX-B?

Hydraulically they are nearly identical: both have the same ASTM F876 inside diameter and the same smooth-bore Hazen-Williams C of 150. PEX-A's expansion fittings keep the full bore at joints, while PEX-B insert fittings restrict it slightly, so PEX-A has a small real-world edge at the connections rather than in the straight run.

What Hazen-Williams C values does this use?

It uses C=150 for both PEX types (very smooth plastic) and C=130 for Type L copper, which reflects mild interior aging. These are standard plumbing design values; new copper can be treated as C=140-150 if you want a best-case figure.

What velocity limit should I enter?

A common guideline is 5 ft/s for cold water and 8 ft/s for hot, because higher velocities in copper accelerate erosion-corrosion and cause noise. PEX tolerates higher velocity but the same comfort and noise limits usually apply.

Should I upsize PEX by one step?

Often yes for long trunk lines or pressure-sensitive fixtures. Running 3/4 in PEX where you would have used 1/2 in copper recovers the lost bore and keeps velocity and pressure drop in check. For short branch runs to a single fixture the difference is usually negligible.

What is the PEX vs Copper Flow Rate Comparison Calculator?

Uses Hazen-Williams friction with PEX-A and PEX-B (C=150) versus Type L copper (C=130) and published inside diameters to compare maximum flow rate and pressure drop per 100 ft for the same nominal pipe size. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

PEX vs Copper Flow Rate Comparison Calculator

Name: PEX vs Copper Flow Rate Comparison Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

PEX and copper of the same nominal size are not hydraulically equal. PEX has a smaller inside diameter but a smoother wall; copper has a larger bore but more friction as it ages. This calculator puts PEX-A, PEX-B, and Type L copper side by side so you can see which actually moves more water at your design conditions.

How it works

Two standard fluid relationships drive the comparison. Maximum flow at a velocity limit comes from continuity, and pressure drop comes from the Hazen-Williams friction equation:

Max flow:   Q(gpm) = V(ft/s) × d(in)² / 0.4085
Head loss:  h(ft/100ft) = 0.2083 × (100/C)^1.852 × Q^1.852 / d^4.8655
Pressure:   Δp(psi) = h × 0.4335

d is the published inside diameter (PEX 0.475/0.671/0.862 in; copper 0.545/0.785/1.025 in for 1/2, 3/4, 1 in), and C is the Hazen-Williams roughness coefficient (150 for PEX, 130 for copper).

Example and notes

At 3/4 in nominal and a 5 ft/s limit, Type L copper (0.785 in bore) carries notably more GPM than PEX (0.671 in bore), and at any shared design flow the PEX shows higher velocity and a larger pressure drop per 100 ft. That is why many plumbers upsize PEX one nominal step on long trunk lines. The numbers here are for straight pipe only — add fitting and valve equivalent lengths, and remember PEX-B insert fittings shave a little more off the bore at each joint.

Inside diameter comparison at common sizes

The inside diameter difference is the root cause of PEX’s lower flow capacity at the same nominal size:

Nominal size	PEX-A / PEX-B OD (in)	PEX ID (in)	Type L copper OD (in)	Type L copper ID (in)
1/2 in	0.625	0.475	0.625	0.545
3/4 in	0.875	0.671	0.875	0.785
1 in	1.125	0.862	1.125	1.025

Both pipe types use the same outside diameter at each nominal size — they are designed to accept the same fittings. But PEX has a thicker wall, leaving a meaningfully smaller bore. Flow area scales with the square of the diameter, so the 0.070-inch difference at 1/2-in nominal reduces PEX’s flow area by about 23% compared to Type L copper.

PEX-A vs PEX-B: the fitting distinction

Hydraulically, PEX-A and PEX-B are nearly identical in straight-run flow — same inside diameter, same smoothness coefficient. The practical difference appears at fittings:

PEX-B uses insert fittings that press inside the pipe and are held with a crimp ring or clamp. The fitting’s barbed body occupies part of the bore, further reducing the effective flow area at each joint.
PEX-A uses expansion fittings that stretch the pipe and then allow it to contract around a fitting with the same outside diameter as the pipe itself. The fitting sits at the ID without reducing the bore, preserving full-bore flow through the connection.

In a typical home installation with many short runs and many fittings, PEX-A’s expansion connections provide a measurable advantage in overall system pressure drop, even though the straight-run calculations for PEX-A and PEX-B are identical.

When to upsize PEX

The rule of thumb “upsize PEX by one nominal size on trunk lines” is a reasonable starting point, but the actual decision depends on run length, number of fittings, inlet pressure, and what fixtures are served. Use this calculator to check:

Does your velocity stay under 5 ft/s for cold and 8 ft/s for hot at your design flow?
Is the pressure drop per 100 ft acceptable given your available inlet pressure and the total run length?

For short branch runs to a single fixture (for example, 6 ft of 1/2-in PEX from a manifold to a sink), the pressure drop difference between PEX and copper is negligible. For a 60-ft trunk line serving a full bathroom group, the compounding pressure drop at the smaller PEX bore becomes meaningful and upsizing to 3/4 in where copper design would use 1/2 in is often worthwhile.