PEX Manifold Sizing Calculator — UPC 604 / ASTM F876

Size a PEX manifold feed or home-run trunk from WSFU load. Converts fixture units to gpm, selects the smallest PEX tube whose actual inside diameter keeps velocity at or below the 8 ft/s UPC 604 limit, and reports Hazen-Williams friction loss.

By TradeCalc, Plumber Calculators — Code-Referenced — UPC 604, ASTM F876, ASTM F877, ASPE Domestic Water Heating Design Manual

Related Calculators

How to Size PEX Manifold Piping (UPC 604 / ASTM F876)

Why PEX Uses Actual Inside Diameter

Unlike copper (Type L) or CPVC, PEX tubing is dimensioned by its outside diameter per ASTM F876 (SDR-9 dimension ratio). Because the wall thickness scales with OD, a nominal 1/2″ PEX tube has an actual inside diameter of only 0.475 in — noticeably smaller than 1/2″ Type-L copper (0.545 in ID). If you size PEX the same as copper, you will undersize it and see high velocity, noise, and friction loss. Always use the actual ID when checking velocity and pressure drop.

PEX Actual Inside Diameters (ASTM F876 SDR-9)

Nominal	OD (in)	Actual ID (in)
3/8″	0.500	0.350
1/2″	0.625	0.475
3/4″	0.875	0.677
1″	1.125	0.862
1-1/4″	1.375	1.092
1-1/2″	1.625	1.305
2″	2.125	1.737

WSFU → gpm Conversion

The manifold trunk feeds every fixture on its branch, so it must carry the peak coincident demand, not the sum of all individual flows. The Hunter-based residential demand envelope converts total WSFU to gpm:

gpm = 0.80 × WSFU^0.86 (residential / private use)

gpm = 0.92 × WSFU^0.86 (commercial / public use, ≈ +15%)

The 8 ft/s Velocity Limit (UPC 604)

UPC 604 limits water velocity in thermoplastic piping to 8 ft/s. Higher velocity causes erosion at fittings, accelerates corrosion of any upstream copper or brass, and creates audible flow noise. The required inside diameter to stay under the limit comes from the continuity equation v = 0.4085 × Q / d², solved for d:

d_required = √(0.4085 × gpm / 8)

Then select the smallest PEX size whose actual ID ≥ d_required. That size guarantees velocity stays at or below 8 ft/s.

Worked Example

Scenario: A residential bathroom group and kitchen sink on one PEX manifold total 8 WSFU. The manifold is fed by a 30 ft run from the main. Size the feed tube.

Peak demand: gpm = 0.80 × 8^0.86 = 0.80 × 5.98 = 4.78 gpm
Required ID: d = √(0.4085 × 4.78 / 8) = √0.2441 = 0.494 in
Compare to PEX IDs: 3/8″=0.350 (✗), 1/2″=0.475 (✗, just under), 3/4″=0.677 (✓)
Selected size = 3/4 in PEX (actual ID 0.677 in)
Velocity check: v = 0.4085 × 4.78 / 0.677² = 1.953 / 0.458 = 4.26 ft/s ✓ (well under 8)
Friction (Hazen-Williams, C=150, L=30 ft): hf = 4.52 × 30 × 4.78^1.85 / (150^1.85 × 0.677^4.87) = 1.55 ft = 0.67 psi

A 1/2″ PEX tube (ID 0.475) would have given v = 8.66 ft/s — over the UPC 604 limit — confirming the 3/4″ selection is correct.

Practical Tips

PEX is not copper. A 1/2″ Type-L copper tube (ID 0.545) carries the same load as a 3/4″ PEX tube (ID 0.677). When switching material, always re-check with the actual ID.
Home-run branches are sized per fixture. A single lavatory (≈ 1.5 WSFU) is fine on a 3/8″ PEX home-run line; a shower (≈ 2 WSFU) usually takes 1/2″. This calculator sizes the manifold feed/trunk, not individual fixture branches.
PEX has very low friction. The smooth thermoplastic wall gives C ≈ 150 in Hazen-Williams, so pressure drop over a 30 ft run is usually small — velocity, not friction, typically governs the size.
Commercial loads are ~15% higher. Public-use fixtures carry more WSFU, so the same physical bathroom in a commercial building needs a larger trunk. The calculator applies a 1.15 multiplier for commercial.

Code References

UPC 604, ASTM F876, ASTM F877, ASPE Domestic Water Heating Design Manual

Frequently Asked Questions

Why is my PEX tube bigger than the copper it replaces?

PEX is dimensioned by outside diameter (ASTM F876 SDR-9), so its actual inside diameter is smaller than nominal-size copper. A 1/2" PEX tube has an ID of 0.475 in versus 0.545 in for 1/2" Type-L copper. To carry the same flow at the same velocity you typically need one trade size larger in PEX (1/2" copper ≈ 3/4" PEX). Always size PEX using the actual ID, not the nominal size.

What is the 8 ft/s velocity limit in PEX?

UPC 604 recommends keeping water velocity at or below 8 ft/s in thermoplastic piping (PEX, CPVC, PVC). Higher velocity accelerates erosion at fittings, increases flow noise, and can cause water-hammer damage. The limit is enforced by selecting a tube whose actual inside diameter keeps v = 0.4085 × gpm / d² at or under 8 ft/s.

Should I use 3/8" or 1/2" PEX for individual fixture home-runs?

Single low-flow fixtures like a lavatory (≈ 1.5 WSFU, ≈ 1 gpm) are fine on 3/8" PEX (velocity ≈ 3.4 ft/s). A shower or kitchen sink (≈ 2 WSFU, ≈ 2 gpm) typically takes 1/2" PEX to stay under the 8 ft/s limit. This calculator sizes the manifold feed/trunk line that supplies all branches — individual home-run lines are sized separately per fixture.

Does this calculator work for commercial PEX manifolds?

Yes — select "Commercial (public use, +15%)" and the WSFU-to-gpm conversion applies a 1.15 multiplier, reflecting the higher peak demand of public-use fixtures. For very large commercial systems, UPC Appendix M (the IAPMO Water Demand Calculator) gives a more precise probabilistic estimate; this calculator uses the simpler Hunter envelope and may over-size slightly, which is conservative.