WATER SUPPLY SIZE · REVIEWED MAY 2026 · BY BRENT

WATER SUPPLY PIPE

size = f(Σ WSFU)

Full bathrooms?

Half baths (powder rooms)?

Kitchen sinks

Dishwashers

Clothes washers

Outdoor hose bibs

RESULT

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FILL IN ABOVE

Residential service entry, 40–60 psi static. PEX is bumped one nominal size vs copper because of thinner ID. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.

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About this calculator

Water supply line branching from a service entry to bath, kitchen, and laundry fixtures, each with a fixture unit value

This water supply pipe sizing calculator gives the minimum supply pipe diameter for a residential service line, based on the fixtures it serves. Enter how many full baths, kitchens, washing machines, and hose bibs the line feeds; the calculator sums Water Supply Fixture Units (WSFU) per IPC Table E202.1, then picks the smallest pipe that can carry that demand at typical residential pressure (40–60 psi static). Copper Type L and CPVC use the nominal size returned. PEX has a smaller inside diameter for the same nominal size, so the PEX answer is bumped up one size for equivalent flow. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.

How to use this calculator

Enter the count of each fixture type the supply line serves. Full bathrooms (toilet + lav + tub/shower) = 7 WSFU each. Half baths (toilet + lav) = 3.5 WSFU. Kitchen sinks = 1.5, dishwashers = 1.4, clothes washers = 4, hose bibs = 2.5.

The calculator sums WSFUs and returns the minimum copper/CPVC pipe size plus the equivalent PEX size (PEX is bumped one nominal size because its thinner ID gives less flow capacity than copper at the same nominal). Also returns estimated peak demand in GPM. Use for sizing the building service entry, manifolds, and trunk lines — branch lines feeding individual fixtures use smaller pipe per IPC.

Worked example

For a typical 2-bath home with kitchen, dishwasher, washing machine, and 2 hose bibs:

WSFU: 2 × 7 + 1.5 + 1.4 + 4 + 2 × 2.5 = 14 + 1.5 + 1.4 + 4 + 5 = 25.9 WSFU.

Looking up: 25.9 falls in 14-32 range → ¾" copper Type L (or CPVC). PEX equivalent: 1".

Peak demand: 5 + √25.9 × 1.8 = 5 + 9.16 = 14.2 GPM.

For a larger 3-bath home with 2 hose bibs, second kitchen sink, and second washer (in-law suite):

WSFU: 3 × 7 + 2 × 1.5 + 1.4 + 2 × 4 + 2 × 2.5 = 21 + 3 + 1.4 + 8 + 5 = 38.4 WSFU.

38.4 → 1" copper. PEX: 1¼".

For a small 1-bath cabin: WSFU = 7 + 1.5 + 4 + 2.5 = 15. → ¾" copper. Same as the 2-bath because demand is bursty, not continuous.

Common mistakes & waste factors

Using nominal PEX size as if it were copper. PEX has a thicker wall and smaller ID than copper at the same nominal size. ½" PEX flows like ⅜" copper — always bump up one size when substituting PEX for copper.

Forgetting hose bibs. Outdoor hose bibs add WSFU just like indoor fixtures. A common 2-bib home adds 5 WSFU just from hoses — significant on small services.

Sizing for static pressure instead of demand. Static pressure (the no-flow reading at the meter) and dynamic pressure (with fixtures running) are different. Pipe size is determined by flow demand at the worst-case fixture combination.

Undersizing the meter. A ¾" service pipe needs at least a ¾" meter (5/8" or 3/4" service meter). Mismatched meter size is a common code violation that limits flow.

Rules of thumb

WSFU table: full bath 7, half bath 3.5, kitchen sink 1.5, dishwasher 1.4, washer 4, hose bib 2.5.

Copper/CPVC sizes by total WSFU: ½" up to 2 WSFU, ¾" up to 14 WSFU, 1" up to 32, 1¼" up to 50, 1½" up to 90.

PEX: bump one size from the copper recommendation (½" copper = ¾" PEX).

Residential static pressure: 40-80 psi typical. Below 40 psi requires a booster pump; above 80 psi requires a pressure-reducing valve (PRV).

Peak demand approximation: 5 + √WSFU × 1.8 GPM (for WSFU ≥ 6). Below 6 WSFU: 1.5 × WSFU GPM.

IRC requires service line ≥ ¾" for any home with multiple bathrooms.

Common questions

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What is a Water Supply Fixture Unit (WSFU)?

A WSFU is a dimensionless number assigned to each fixture in IPC Table E202.1 that captures both its peak flow rate and its likely simultaneous use with other fixtures. A toilet is 2.5, a tub is 4, a kitchen sink is 1.5. Summing WSFUs lets you size a single supply line that serves many fixtures without assuming all of them open at once — Hunter's probabilistic curve does the rest.

Why is the PEX size bigger than the copper size?

PEX has a thicker wall than copper for the same nominal size, so the inside diameter is smaller and friction loss is higher per foot. To deliver the same flow rate, PEX usually needs to be one nominal size up. A ¾" copper line is roughly equivalent to a 1" PEX line at typical residential pressure. A PEX cinch clamp tool is the cheapest entry to PEX work; expansion-style fittings need a different (and pricier) PEX expansion tool.

Does this work for well water systems?

Use this for the line downstream of the pressure tank, which sees the same 40–60 psi residential range. The line from the pump to the pressure tank sizes differently — driven by pump GPM and lift, not WSFU. For long underground service runs (over 100 ft), bump up one size to offset friction loss regardless of what this calculator returns.