Bilge Pump Sizing Calculator (GPH, Head Loss & Flooding Rate)

How it works

flooding rate (gal/min) ≈ 20 × d² × √h (d = hole diameter in inches, h = depth in feet) · delivered flow ≈ rated × (1 − equivalent head ÷ 12.5 ft) × 0.85 · equivalent head = lift + 0.25 ft per ft of corrugated hose beyond 3 ft · run time = ½ × Ah ÷ amps

Bilge pumps are rated at open flow with no hose and a fresh battery, and installed on a boat they lift water several feet through a narrow corrugated hose on a sagging voltage, so they typically deliver half to two-thirds of the label — the reason guidelines quote generous total capacities that scale with boat length and hull depth. The sobering comparison is with how fast water comes in: flow through a hole grows with the square of its diameter and the square root of its depth, and a one-inch fitting failure two feet down admits nearly thirty gallons a minute, about 1,700 gallons an hour, more than most small boats' pumps deliver against a real head. Pumps exist to handle nuisance water and buy time; plugs, alarms and a second pump are what handle a breach.

Worked example

A 24 ft powerboat should carry about 2,000 GPH of rated capacity. A single 2,000 GPH pump lifting 4 ft through 8 ft of corrugated hose (≈ 5.3 ft equivalent head) actually moves about 990 GPH. A 1-inch hole 2 ft below the waterline floods at 28 gal/min — 1,700 GPH — so the pump falls 700 GPH short and the bilge gains 12 gallons a minute; at 8 A it runs about 6 hours on half of a 100 Ah battery.

Frequently asked questions