Plumbing estimating has its own unit of measurement that doesn't map directly to square footage or linear wall length: the fixture unit, a standardized value assigned to each fixture type that determines pipe sizing for both supply and drain lines.
Fixture count
Fixture count comes directly from the plan's plumbing schedule — sinks, toilets, tubs/showers, washing machine connections, water heaters — each contributing a specific fixture-unit value that determines the minimum pipe diameter needed to serve it without undersizing.
Supply pipe runs
Supply pipe runs are estimated similarly to electrical wire runs: trace an approximate path from each fixture back to the main supply line or manifold, with a routing multiplier to account for going around framing and through walls rather than a straight line.
Drain, waste, and vent piping
DWV piping requires attention to slope requirements (typically 1/4 inch per foot for most drain lines) and vent stack routing to the roof — this is where plumbing takeoffs differ most from other trades, since pipe slope and vent placement are governed by code requirements that materially affect routing, not just distance.
Commonly missed items
Shutoff valves at each fixture, cleanouts at required intervals along drain runs, and water hammer arrestors on appliance connections — small-cost items individually, but numerous enough to add up on a full rough-in.