Water pipe sizing calculator.
Add up your fixtures or type in a GPM. The calculator totals the water supply fixture units, converts to peak demand with Hunter’s curve, and tells you the smallest copper, PEX or CPVC pipe that holds your chosen velocity.
| Fixture | WSFU each | Count | Subtotal WSFU |
|---|
| Material | Recommended size | Velocity at size | How it’s figured |
|---|
How the math works
Three steps, all visible so you can audit them: fixture units in, peak GPM out, pipe size by velocity.
- Fixture units (WSFU) — each fixture carries a standard weighting: water closet 2.2 (flush tank), lavatory 1.0, kitchen sink 1.5, shower 2.0, bathtub 2.0, clothes washer 2.0, dishwasher 1.5, hose bibb 2.5. Counts × weighting, summed, gives total WSFU.
- WSFU → GPM — total WSFU is converted to peak probable demand using a Hunter’s-curve lookup (5 WSFU ≈ 6 GPM, 20 ≈ 14, 50 ≈ 29, 100 ≈ 44, 200 ≈ 70), interpolated between points. The curve is non-linear because not every fixture runs at once.
- GPM → pipe size — pipe capacity in GPM is 2.448 × d² × velocity, where d is the inside diameter in inches and velocity is in ft/s. The required diameter is √(GPM ÷ (2.448 × velocity)); the tool picks the smallest stocked size whose inside diameter meets it.
- Material matters — Type L copper has the largest inside diameters; PEX and CPVC (CTS) are a bit smaller for the same nominal size, so a run that fits in 1″ copper can need 1-1/4″ PEX. The result shows the size and the actual velocity each material lands at.
This is the easy 20%. The other 80% is on your drawings.
Sizing one run from a fixture count is the simple part. What it can’t do is read a full plumbing set: count every fixture off the riser diagram, follow the developed length through 60 fittings, check residual pressure at the worst-case fixture, and price the copper, PEX, valves and hangers by the foot. That’s the part of a plumbing bid that leaks money — and it’s exactly what Pilars AI takeoff does from your actual plans: it classifies every line and fixture on every sheet and returns a full quantity takeoff. $100 per trade, per plan.
Questions estimators actually ask
How does the calculator turn fixtures into a pipe size?
It sums the WSFU for every fixture you add, converts that total to a peak demand in GPM with a Hunter’s-curve lookup, then finds the smallest pipe whose capacity (2.448 × inside-diameter² × velocity) meets the demand at your chosen maximum velocity.
What is a water supply fixture unit (WSFU)?
A WSFU is a relative weighting of how much water a fixture draws and how often. A flush-tank water closet is ≈2.2 WSFU, a lavatory 1.0, a kitchen sink 1.5, a shower or bathtub 2.0, a hose bibb 2.5. Totaling them estimates peak demand instead of summing every fixture’s full flow at once.
What velocity should I size to?
Most designers cap cold-water velocity around 6–8 ft/s and hot water nearer 5 ft/s to limit noise and erosion. This tool defaults to 6 ft/s and lets you choose 4, 6 or 8. Lower velocity gives a larger, quieter pipe; higher velocity a smaller, cheaper one.
Does it handle copper, PEX and CPVC?
Yes. It uses Type L copper inside diameters plus the smaller CTS inside diameters of PEX and CPVC, so the same demand can need a larger nominal size in PEX than in copper. The output shows the recommended size and resulting velocity for each material.
Is this a code-compliant pipe sizing design?
No. It’s a fast sizing guide built on the common IPC/UPC fixture-unit method and a velocity check. It doesn’t account for static and residual pressure, developed length, fittings or elevation. Always verify with your local plumbing code and a licensed engineer.
Let Pilars take off your whole set.
Upload your plans. Pilars reads the riser diagram, counts every fixture, measures every line by type, and returns the full plumbing takeoff. $100 per trade.