Bilge Pump Sizing: How Many GPH Do You Really Need?
A pump labelled 1,100 GPH is rated at zero head and 13.6 V. Lift the water 3–4 ft through 15 ft of hose on a 12.2 V battery and you'll often see half that. Size with that loss in mind.
A practical sizing approach
- Under 20 ft: one 500–800 GPH automatic pump.
- 20–30 ft: 1,100–1,500 GPH primary, plus a manual override switch at the helm.
- 30 ft+: 2,000 GPH primary plus a second high-water pump mounted higher, with an alarm.
- Rule Mate 1100 Automatic Bilge Pump 12V $135.31
- Rule 1100 GPH Standard 12V Bilge Pump $67.35
- Rule 1500 GPH Non Automatic Bilge Pump $130.00
- Rule 2000 GPH Non Automatic Bilge Pump 12V $162.82
- Rule Mate 2000 GPH Bilge Pump 12V $312.76
Float switches
If your bilge collects hair, leaves or oily debris, a mechanical float with a removable base is easier to clean and replace.
- Rule Rule-A-Matic Float Switch w/ Detachable Base $49.42
- Rule Rule-A-Matic Plus Mercury Free Float Switch $63.38
- Rule Super Switch Float Switch w/ Removable Base 5YR Warranty $73.56
Wire it like your boat depends on it
Bilge circuits are critical loads, so size the wire for a 3% drop (see our wire gauge guide). Use a dedicated fuse at the battery and adhesive-lined heat-shrink butt connectors above the high-water line. Smooth-wall discharge hose and a vented loop prevent back-siphoning.
Shop all bilge pumps.
FAQ
How big a bilge pump does my boat need?
As a rule of thumb, 20–25 ft boats carry at least one 1,100 GPH automatic pump. Over 30 ft, run a 2,000 GPH primary plus a high-water backup. Always derate the label by 40–60% for head height and wiring losses.
Automatic pump or float switch?
Built-in automatic pumps (like the Rule-Mate) are simpler. A separate float switch lets you replace the pump or switch independently and is more reliable in debris-prone bilges.
Should the bilge pump be on the battery switch?
The automatic circuit should be wired direct to the battery through its own fuse, so it works even with the main switch off.