PEX Sizing Calculator

Inputs

Expected flow (gpm)

Use demand flow for the segment you are sizing.

Pipe run length (ft)

Measure along the planned routing.

Fittings equivalent length (ft)

Adds elbows, tees, valves as extra length.

Elevation change (ft)

Positive means water must rise.

Max velocity (fps)

Common targets: 4–5 fps for quiet operation.

Max allowed drop (psi)

Budget pressure loss for this segment.

Hazen-Williams C value

PEX often uses ~150 as a planning value.

Safety factor (×)

Multiplies flow to add design margin.

Water temperature (°F)

Shown for context; friction uses a standard model.

Reset

Scenario	Flow (gpm)	Run (ft)	Fittings (ft)	Max Velocity (fps)	Max Drop (psi)	Typical Outcome
Small branch	3.0	40	10	5.0	6.0	Often 1/2 in
Bathroom group	5.5	70	20	5.0	8.0	Often 3/4 in
Main trunk	10.0	120	30	4.5	10.0	Often 1 in

Outcomes vary with layout, fittings, and local design criteria.

Velocity: Convert flow to cubic feet per second, then v = Q / A, where A = πD²/4.
Required diameter by velocity: D = √(4Q / (πv)).
Hazen-Williams headloss: h_f (ft/100ft) = 4.52·Q^1.85 / (C^1.85·d^4.87).
Pressure conversion: ΔP (psi) ≈ 0.433·h (ft).

This calculator uses approximate inside diameters and a standard friction model for planning.