Boat systems
Livewell pump sizing calculator
Quick answer
A well measuring 36 by 16 by 14 inches holds about 35 gallons, and turning it over every 15 minutes needs roughly 140 GPH, which a stocked 360 GPH pump covers easily. Most factory pumps are oversized, which is why a timer matters more than a bigger pump.
Two halves to this, and they are different kinds of number. Volume is published arithmetic: length by width by depth in inches, divided by 231 cubic inches per gallon. Turnover rate is convention: a recirculating livewell is normally sized to move the whole volume every 15 to 20 minutes.
The answer surprises people in the opposite direction from most sizing questions. Factory pumps are usually far larger than the turnover convention calls for, which is why the useful upgrade is generally a timer rather than a bigger pump.
Measure your own well
A 34.9 gallon well needs about 140 GPH to recirculate and around 1,047 GPH to fill quickly, which is why one pump plus a timer is the usual answer.
| Figure | Value |
|---|---|
| Well volume | 34.9 gallons |
| Flow to turn over every 15 minutes | 140 GPH |
| Nearest stocked recirculation pump | 360 GPH |
| Flow to fill in 2 minutes | 1,047 GPH |
| Weight of water when full | 291 lb |
| Every 10 minutes instead | 209 GPH |
Shown with the example figures above. Change any field to run your own numbers.
The working
Published figuregallons = ( length x width x depth in inches ) / 231 flow needed = gallons x ( 60 / turnover minutes ) fill rate = gallons x ( 60 / fill minutes )
The 231 cubic inches per gallon is published. The 15 to 20 minute turnover interval is convention rather than a published standard. A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
Pumps and timers
Flow rates are manufacturer published figures. The useful upgrade on most boats is a timer rather than a larger pump, because the fitted pump is already sized for filling rather than recirculating.
Rule RM1100B auto submersible bilge pump with AutoSense
$120.53 Published specsSenses water electronically rather than with a float, which removes the mechanical part that jams under a crisp packet.
Best for: A boat left in the water or on a mooring
Check price on Amazon
Flow-Rite Livewell Timer Pro
$79.73Flow-Rite builds livewell plumbing as its actual business, and the Timer Pro is the one tournament boats standardise on.
Check price on Amazon
Rule 603C livewell pump, 800 GPH angled thru-hull
$74.99 Published specsThe angled outlet version, which is purely about which way the plumbing leaves the pump in your own bilge.
Check price on Amazon Rule 603STC livewell pump, 800 GPH straight thru-hull
$74.86 Published specsRule is the default livewell pump on most factory boats, and 800 GPH covers a typical thirty gallon well comfortably.
Best for: A standard bass boat livewell
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Attwood Tsunami MK2 aerator pump T1200, 1200 GPH
$64.19Twelve hundred GPH for a large or twin livewell, which is where 800 starts to run out.
Check price on Amazon Rule 601FC livewell pump, 500 GPH straight seacock
$63.39Five hundred GPH for a smaller well, where an 800 turns the water over faster than the fish appreciate.
Check price on AmazonEvery figure shown is the manufacturer own published specification. The rating on the unit you receive is what governs, and the capacity plate on your own boat governs everything above it.
Turnover rate by target
Turning a 35 gallon well over every 15 minutes needs about 140 GPH. Every 10 minutes needs 210, and every 20 needs 105.
Rule of thumb Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
| Turnover interval | Flow needed | Nearest stocked pump | When it suits |
|---|---|---|---|
| Every 20 minutes | 105 GPH | 360 GPH | Cool water, or delicate fish that bruise easily |
| Every 15 minutes | 140 GPH | 360 GPH | The normal figure for most fishing |
| Every 10 minutes | 209 GPH | 360 GPH | Hot water, a full well, or a long run home |
The 15 to 20 minute turnover figure is convention rather than a published standard. Nothing publishes it and it is repeated across the tournament fishing world, which is a different thing from a specification.
Fill rate is a different question
The turnover figure above assumes the well is already full and you are recirculating or exchanging water. Filling a well from empty is a separate requirement, and it is usually what the factory pump was sized for: nobody wants to stand at the ramp for ten minutes.
Filling a 35 gallon well in two minutes needs around 1,050 GPH, which is why boats carry pumps far larger than the recirculation convention calls for. On most boats the same pump does both jobs and a timer manages the difference, running the pump in cycles rather than continuously once the well is full.
Why more flow is not better
Past a point, a fish in a small well with a large pump spends the day in a washing machine, using energy it needs to survive being released. That is the practical reason the turnover convention exists rather than a rule of thumb about oxygen alone.
The other reason to cycle rather than run continuously is the battery. A pump running flat out all day is a meaningful load on a boat that is also running electronics and a trolling motor, and a timer that runs it a few minutes in each cycle removes most of that without the fish noticing.
Hot water is the real problem
Warm water holds less dissolved oxygen, so the day when fish are hardest to keep alive is the day the well is warmest. Raw water exchange stops helping when the lake itself is the problem, which is where a recirculating kit earns its place: it aerates the water already in the well rather than pumping more warm water in.
The other measures people take on hot days, ice and additives, are outside what this calculator can size, and they are genuinely what tournament anglers do rather than an upsell.
Frequently asked questions
How do I work out my livewell volume?
Length times width times depth in inches, divided by 231, which is the published number of cubic inches in a US gallon. Measure to the waterline you actually fill to rather than the top of the well, because most livewells are filled several inches below the rim and using the full depth overstates the volume substantially.
What GPH pump do I need for my livewell?
For recirculation, enough to turn the volume over every 15 to 20 minutes, which for a 35 gallon well is about 105 to 140 GPH. That is far less than most factory pumps deliver. For filling from empty in a couple of minutes you need roughly 1,000 GPH, which is why the fitted pump is usually much larger and a timer manages the difference.
Is the 15 minute turnover a real standard?
No, it is convention. It is repeated across the tournament fishing world and published by nobody, which is why this site labels it as a rule of thumb rather than a specification. It is a good working figure, and the right interval on any given day depends on water temperature, how full the well is and what species you are holding.
Why do I need a timer?
Because the pump that fills the well quickly is far larger than the one that should run continuously once it is full. A timer cycles it, which saves a meaningful amount of battery on a boat also running electronics and a trolling motor, and stops the fish spending the whole day being churned around a small box.
Should I recirculate or exchange water?
Exchange is fine when the lake is cooler than the well. Recirculation earns its place when the lake itself is the problem, which is any hot day, because warm water holds less dissolved oxygen and pumping more of it in does not help. A recirculating kit aerates the water already in the well instead.
Can a bilge pump be used as a livewell pump?
They are different products for a reason. A livewell pump is designed to run for long periods and frequently has an aerating outlet, while a bilge pump is designed for intermittent high volume. Some models share a motor cartridge, but check the manufacturer intended duty rather than assuming, because continuous running is what kills the wrong pump.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.