Sizing a bilge pump: GPH, lift and float switches

Post · 3 October 2026 · Figures reviewed 6 October 2026

Every boat takes on water: rain, spray, a leaking gland or a hose that gives way. A bilge pump sends it back over the side, and a float switch does it when no one is watching. The number on the pump is where sizing starts, not where it ends.

What does GPH mean?

GPH is US gallons per hour; one US gallon is 3.785 litres, so an 1100 GPH pump is rated at about 4,160 litres an hour. That rating is measured in ideal conditions: water flowing straight out with no lift, through a short outlet, at full test voltage.

What cuts the flow

• Lift, or head: the height from the pump to where the water leaves the boat. Every metre costs flow.
• The hose: its length, every bend, and any check valve.
• Voltage: a long or thin cable, or a tired battery, slows the motor.

Rule’s own published figures for its 360 GPH pump show the effect: 360 GPH with no lift, 265 GPH at 3.35 ft (about 1 m) and 190 GPH at 6.7 ft (about 2 m).

How big a pump?

There is no single rule. Size for the worst leak the boat could suffer, then allow for the losses above. Many boats carry two: a smaller pump at the lowest point for everyday water, and a larger one mounted a little higher as a back-up, plus a manual pump that needs no battery at all.

How a float switch works

A float switch has a hinged float that rises with the water and closes the circuit, so the pump runs until the level falls again. Mount it beside the pump at the lowest point, clear of debris that could jam the float, and wire it with a fuse as the pump maker’s diagram shows, so the pump can run with the boat unattended. A panel switch with manual, off and automatic settings lets the crew run the pump by hand as well.

In MM’s range

The Rule and Jabsco lines are listed on the Pumps & Plumbing page; both names are the marks of their respective owners. Follow the pump maker’s installation instructions; ask us to match a pump to a boat.