Bilge pump sizing

Marine Werks buying guide · Updated November 2026

A bilge pump keeps rain, spray and small leaks out of the boat. Sizing one well comes down to a simple fact: the number on the box is a best case, and your installation decides what you actually get.

1. What the GPH rating means

Pumps are rated in gallons per hour (GPH) with no hose, no lift and full voltage. Once the pump has to push water up a hose to a thru-hull above the waterline, through bends and on a battery that's sagging, real output often drops by a third or more. Size with that loss in mind.

2. How much pump

These are common starting points, not a rule. Bigger boats, offshore use and boats left in the water benefit from more capacity.

3. Automatic, float switch or both

• Pump plus float switch. The switch turns the pump on when water rises. Reliable and easy to test.
• Automatic pumps sense water on their own and need no separate switch.
• Backup pump and alarm. A second pump mounted a few inches higher, wired to a high-water alarm, tells you when the primary can't keep up or has failed.

4. Install it so it performs

• Use smooth-wall bilge hose in the size of the pump's outlet, and keep the run short with no dips where water can sit.
• Avoid check valves unless the manufacturer calls for one; they cut flow and can stick.
• Use the wire size and fuse or breaker the manufacturer specifies. Undersized wire costs real GPH.
• Wire the automatic circuit so it works with the main battery switch off.
• Clean the strainer and test the float at the start of every season.