CalcStack — free, instant, no signup. The interactive calculator loads automatically.

Pipe Size Calculator

Count the fixtures, size the line.

Plumbing lines are not sized for every tap running at once — they are sized for probable simultaneous demand, which is what fixture units encode. This calculator totals your fixture units, converts them to peak GPM, and picks the smallest supply line that keeps water velocity under the 8 ft/s noise-and-wear ceiling.

Why people use this

Remodelers adding a bathroom use this to answer "can I tee off the existing line?" with fixture-unit math instead of hope. Plumbers use it as a fast first pass before the official UPC/IPC design.

How it works

1. Count each fixture type the line serves — toilets, sinks, showers, appliances, hose bibs.
2. Read total fixture units and probable peak demand in GPM.
3. Get the recommended pipe diameter at ≤8 ft/s velocity.
4. Long runs, low street pressure, or flushometer fixtures need a licensed plumber's design — code governs.

Common questions

What size water line does a typical house need?

A standard 2–2.5 bath home totals roughly 15–20 fixture units, demanding about 7–10 GPM at peak — a ¾-inch main handles it comfortably. One-inch service lines are common where street pressure is low or runs are long.

Why not just run 1-inch pipe everywhere?

Oversized supply lines waste money and, worse, let hot water sit and cool in the pipe — longer waits at the tap and more wasted water. Right-sizing balances pressure, cost, and hot-water delivery time.

Browse all 500+ free calculators →