Disclaimer: This calculator provides estimates for educational purposes only. Actual towing capacity, braking distance, tongue weight, and payload vary with your specific vehicle, hitch class, trailer, tires, load distribution, and road conditions. Always verify these figures against your vehicle manufacturer's owner's manual and the door-jamb certification sticker, which lists GVWR, GCWR, GAWR, and maximum tongue and payload ratings. Never exceed the lowest of these rated limits, regardless of what this calculator shows. Exceeding a weight rating or misjudging stopping distance can cause loss of vehicle control, brake failure, trailer sway, jackknifing, serious injury, or death. Always consult your manufacturer's documentation and a qualified technician before towing.
Frequently Asked Questions
Q
How do I calculate the bilge pump size I need?
Determine the worst-case water ingress rate using Torricelli’s theorem based on breach size and depth below waterline. Then select a pump rated at least 2x that flow rate to account for head loss, hose friction, and voltage drop under load.
Ingress flow = Cd × Area × sqrt(2 × g × depth) in cubic feet per second
Cd = 0.61 for sharp-edged orifice (standard marine fitting)
Convert cfs to GPH: multiply by 3,600 × 7.48
A 3/4-inch broken hose at 2 ft depth admits about 571 GPH
What is head loss and how does it affect bilge pump output?
Head loss is the reduction in pump output caused by lifting water above the pump. Every foot of vertical discharge height reduces rated GPH by approximately 8–10%. A pump rated at 2,000 GPH at zero head may only deliver 1,200 GPH at 5 feet of discharge height.
Each foot of lift reduces output by roughly 8% of rated GPH
3 ft discharge height: ~24% loss, pump delivers ~76% of rating
5 ft discharge height: ~40% loss, pump delivers ~60% of rating
Horizontal hose runs add friction equivalent to extra lift
Every 10 ft of horizontal run equals approximately 1 ft of vertical head
Q
How many bilge pumps should a boat have?
The ABYC recommends at least one primary bilge pump and one backup. The primary should handle worst-case scenarios. The backup provides redundancy and should be independently wired on its own circuit with a separate float switch.
Minimum 2 pumps: primary + backup on separate circuits
Larger boats (30+ ft) should have 3 pumps in separate bilge areas
Primary pump: sized for worst-case (this calculator helps)
Backup pump: at least 50–75% of primary pump capacity
Wire each pump to its own fuse and float switch
Q
What size bilge pump do I need for my boat length?
As a general guideline, boats under 20 ft need at least 1,000 GPH, 20–30 ft boats need 2,000 GPH, 30–45 ft boats need 3,500 GPH, and boats over 45 ft need 5,000+ GPH. These are minimum ratings at zero head.
Under 20 ft: 1,000 GPH minimum, 500 GPH backup
20–30 ft: 2,000 GPH primary, 1,000 GPH backup
30–45 ft: 3,500 GPH primary, 2,000 GPH backup
45–60 ft: 5,000 GPH primary, 3,500 GPH backup
These are manufacturer ratings at 0 ft head — actual output is lower
Boat Length
Min Primary GPH
Backup GPH
Typical Amp Draw
Under 20 ft
1,000
500
3–4A
20–30 ft
2,000
1,000
6–8A
30–45 ft
3,500
2,000
10–14A
45+ ft
5,000
3,500
16–20A
Q
How fast will my boat flood from a thru-hull failure?
A failed 1.5-inch thru-hull fitting 2 feet below the waterline admits about 2,284 GPH. A typical 28 ft boat has roughly 209 gallons of bilge volume, meaning it could flood in about 5.5 minutes without an adequate bilge pump.
1.5-inch thru-hull at 2 ft depth: ~2,284 GPH ingress
28 ft boat bilge volume: roughly 209 gallons
Time to flood: 209 / 2,284 × 60 = 5.5 minutes
Deeper hulls flood faster — depth increases pressure and flow
Always have seacocks (ball valves) on every thru-hull fitting
Example Calculations
128 ft Boat with Broken 3/4" Hose
Inputs
Boat Length28 ft
Breach SizeBroken Hose (3/4 in)
Depth Below WL2 ft
Discharge Height3 ft
Number of Pumps1
Result
Recommended Pump1,502 GPH
Water Ingress571 GPH
Head Loss24%
Min. Required751 GPH rated
Time to Flood21.9 min
Area = 0.4418 sq in / 144 = 0.00307 sq ft. Velocity = sqrt(2 × 32.174 × 2) = 11.34 ft/s. Flow = 0.61 × 0.00307 × 11.34 × 3600 × 7.48 = 571 GPH. Head loss at 3 ft = 24%. Required rated = 571 / 0.76 = 751 GPH. Recommended = 751 × 2 = 1,502 GPH.
235 ft Boat with Failed 1.5" Thru-Hull
Inputs
Boat Length35 ft
Breach SizeFailed Thru-Hull (1.5 in)
Depth Below WL3 ft
Discharge Height4 ft
Number of Pumps2
Result
Recommended Pump2,059 GPH each
Water Ingress2,798 GPH
Head Loss32%
Min. Required2,059 GPH rated (total)
Time to Flood5.6 min
Area = 1.7671 sq in / 144 = 0.01227 sq ft. Velocity = sqrt(2 × 32.174 × 3) = 13.89 ft/s. Flow = 0.61 × 0.01227 × 13.89 × 3600 × 7.48 = 2,798 GPH. Head loss 4 ft = 32%. Required per pump = 2,798 / (0.68 × 2) = 2,058 GPH. Recommended = 2,058 × 2 = 4,117 GPH total.
Formulas Used
Water Ingress Rate
GPH = Cd × A × √(2 × g × h) × 3600 × 7.48
Calculates gallons per hour entering through a hull breach using Torricelli’s theorem.
Where:
Cd= Discharge coefficient (0.61 for sharp-edged orifice)
Estimates actual pump output after accounting for head loss from discharge height.
Where:
Rated GPH= Manufacturer’s GPH rating at zero head
0.08= Approximate loss factor per foot of discharge height
Discharge Height= Vertical distance from pump to discharge fitting in feet
Understanding Bilge Pump Sizing
Your bilge pump is the last line of defense against flooding. Sizing it correctly means understanding the worst-case water ingress rate your boat could experience and selecting a pump that can outpace that flow even with the output losses from lifting water to the discharge point above the waterline.
Water ingress rate depends on two factors: the size of the breach and the water pressure at that depth. Torricelli’s theorem calculates flow velocity from water head, and the orifice coefficient (0.61) accounts for flow contraction through a sharp-edged opening like a failed fitting or cracked hose barb.
The most common mistake is relying on the manufacturer’s GPH rating, which is measured at zero head (no lift). Real-world installations require pumping water 2–5 feet above the bilge to reach the discharge fitting. This reduces actual output by 16–40%, so always select a pump rated well above your calculated ingress rate.
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.
