Marine Boat Bilge Pump Sizer: GPH Flow Rate & Head Loss Calculator
A bilge pump's advertised GPH rating is measured with zero head lift and no hose — real installations lose significant flow to vertical lift, hose friction and bends. This calculator estimates your boat's minimum recommended pump capacity and the realistic flow rate you'll actually get after installation losses.
Your 1,500 GPH pump only delivers 930 GPH after head loss — below the 2,640 GPH minimum, consider a larger pump.
| Measure | Value |
|---|---|
| Boat length × beam | 22ft × 8ft |
| Hull factor | 1.00 |
| Base min GPH | 2,640 |
| Head lift height | 2.0 ft |
| Hose length | 6.0 ft |
| Elbows/bends | 2 |
| Head loss factor | 62% |
| Candidate pump rated GPH | 1,500 |
| Real-world effective GPH | 930 |
| Recommended hose diameter | 1-1/2" |
- 1.
Base_GPH = Length × Beam × Hull_Factor × 15 - 2.
Head_Loss_Factor = 1 − (Head_Ft×0.08 + Hose_Ft×0.02 + Elbows×0.05) - 3.
Real_World_GPH = Pump_Rated_GPH × Head_Loss_Factor
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
Formula & step-by-step maths
- Hull_Factor
- Multiplier by hull/boat type
- Head_Loss_Factor
- Combined friction/lift flow reduction
Why rated GPH overstates real performance
Manufacturers test pumps with the discharge port wide open at zero lift. Add a few feet of vertical head and a hose with bends, and effective flow can drop 30-50% below the sticker rating, which is why sizing with margin is critical for safety.
Understanding the hull factor
Open bow boats generally have simpler bilge geometry and lower water intrusion risk, sailboats often have deeper, harder-to-access bilges and slower natural drainage, so this calculator applies different multipliers by hull type to reflect real-world risk.
Head lift and hose length losses
Every foot of vertical lift the pump must push water against subtracts meaningfully from flow, and every foot of hose plus each 90-degree elbow adds friction — a long, twisty discharge run can cut a pump's real output by half.
Choosing the right discharge hose diameter
Undersized discharge hose creates a bottleneck regardless of pump capacity — larger boats or high-GPH pumps need at least 1-1/8" or 1-1/2" hose to avoid throttling flow before it reaches the thru-hull.
Minimum recommended bilge pump GPH by boat length
| Boat length | Typical min GPH |
|---|---|
| 16 ft | 500 GPH |
| 20 ft | 800 GPH |
| 24 ft | 1,100 GPH |
| 28 ft | 1,500 GPH |
| 32 ft | 2,000 GPH |
| 36 ft | 2,500+ GPH |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: boat length 17.16 ft, hull type "Open Bow"
On the lower / more conservative end. Your 1,500 GPH pump only delivers 930 GPH after head loss — below the 1,647 GPH minimum, consider a larger pump.
Example 2: boat length 22 ft, hull type "Cruiser"
A typical middle-of-the-road setup. Your 1,500 GPH pump only delivers 930 GPH after head loss — below the 2,640 GPH minimum, consider a larger pump.
Example 3: boat length 28.6 ft, hull type "Sailboat"
On the higher / more demanding end. Your 1,500 GPH pump only delivers 930 GPH after head loss — below the 4,118 GPH minimum, consider a larger pump.
Quick answers about the Bilge Pump GPH
What exactly does the Bilge Pump GPH work out?
A bilge pump's advertised GPH rating is measured with zero head lift and no hose — real installations lose significant flow to vertical lift, hose friction and bends. You enter boat length, boat beam width, hull type and vertical head lift height (plus 3 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 7 fields: boat length, boat beam width, hull type, vertical head lift height, discharge hose length, number of elbows/bends and candidate pump rated GPH. Nothing else is needed and nothing is stored.
How is it calculated — why rated GPH overstates real performance?
Manufacturers test pumps with the discharge port wide open at zero lift. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for bilge Pump GPH?
Open bow boats generally have simpler bilge geometry and lower water intrusion risk, sailboats often have deeper, harder-to-access bilges and slower natural drainage, so this calculator applies different multipliers by hull type to reflect real-world risk. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Minimum recommended bilge pump GPH by boat length" table on this page tell me?
It is the reference range this tool works against — 6 rows from "16 ft" (500 GPH) up to "36 ft" (2,500+ GPH). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which hull type should I pick?
The dropdown offers 3 choices — Open Bow, Cruiser and Sailboat. Pick the one that matches your real situation rather than the one you would like to be true; hull type usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Do I have to press a button or reload the page to see the result?
No. Bilge Pump GPH runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Bilge Pump GPH leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
For a typical 22ft cruiser with an 8ft beam, expect a baseline recommendation around 2,640 GPH using the length×beam×hull-factor formula, though many boats run dual pumps to split this load. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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