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.

PEX vs Copper Flow Rate Comparison Calculator

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.