Bilge Pump Sizing Calculator

A bilge pump's job is to clear water out of the hull faster than it can accumulate, whether that's routine seepage or a genuine flooding emergency, and sizing it starts with knowing how much water the hull could actually hold. This calculator estimates interior bilge volume from length, beam, and bilge depth using a coefficient that discounts for keel, deadrise, and machinery space rather than treating the hull as a solid rectangular box, then converts that volume to gallons. It targets clearing that volume within a fixed benchmark time -- a simplifying rule of thumb this calculator uses for routine sizing, not a citation to ABYC or any other specific published standard -- and divides gallons by that time to get the minimum required pump capacity in gallons per hour (GPH), which is then matched to the nearest standard commercially available pump size. Very large or unusually proportioned hulls can exceed what a single pump handles; in that case, multiple pumps or a dedicated high-capacity damage-control pump are the real-world solution rather than a single oversized unit.

Separately, the calculator estimates worst-case flooding risk from your below-waterline through-hull fittings: more fittings, a larger fitting diameter, and more water pressure head (the vertical distance from the fitting to the waterline) all increase how fast water could pour in if a fitting failed -- this flooding-rate estimate is itself an approximation with a wide published spread, so use it as a ballpark figure rather than a precise prediction. The larger of that flooding-based estimate or twice the routine minimum becomes the recommended emergency pump size. Bilge pump capacity ratings from manufacturers are typically measured under ideal lab conditions with no lift or hose restriction, so real-world output at your actual installed head height and hose run will be lower than the rated number -- oversizing on paper leaves margin for that real-world falloff.