Bilge Pump Sizing Calculator

The number on the box is a lab-ideal figure — real-world output through actual hose and head height typically runs a third to half below the rated GPH. This turns your boat's length into a delivered-GPH target, then translates that into what the combined box ratings should actually add up to, plus a sensible main/backup/midship split. Full reasoning in the bilge pump buying guide.

How this is figured

The boat-length brackets are the same rough industry guideline covered in the buying guide — a floor, not a ceiling, meant for total combined pump capacity rather than any single pump. Checking "long hose run" pushes the target to the top of the bracket instead of the bottom, since more vertical lift and hose friction eats into real output faster than a short, straight run does. The box-rating figure multiplies that target by roughly 1.5x, which is a reasonable middle ground for the commonly-cited one-third-to-one-half real-world derate — buy to the box-rating number, not the delivered-GPH number, or you'll end up under-pumped the one time it actually matters.

Two pumps — main and backup — is the default layout here, and it's the right call for most boats. A lower-mounted main pump handles routine water and cycles the most, while a higher-mounted backup on its own separate switch carries the real emergency capacity, so a jammed float switch or dead motor on the main doesn't take out your only pump. Check the third-pump box if the boat is over 30 feet or has a compartmented or stepped hull — that's when a midship pump earns its keep covering the blind spot the bow and stern units can't reach, not before. None of this replaces a high water alarm as the layer above all three; see the buying guide for why that's a separate system, not just a bigger pump.