Boat Capacity Lab

For monohull boats under 20 feet without a capacity plate, the USCG rule of thumb is: number of persons = (boat length in feet × boat width in feet) ÷ 15, rounded down to whole persons. For example, an 18 ft × 6 ft boat gives 108 ÷ 15 = 7 persons. This is an estimate for good weather conditions — the manufacturer's capacity plate always governs and must never be exceeded.

The USCG publishes two levels. The simplified rule of thumb for boaters is persons = (length × width) ÷ 15 for monohulls under 20 ft. The full federal method (33 CFR 183 Subpart C) used by manufacturers is: maximum weight capacity = (maximum displacement − boat weight) ÷ 5 for outboard boats, then persons capacity in pounds = weight capacity − engine system weight, then persons count = (persons pounds + 32) ÷ 141, rounded to the nearest whole number.

A USCG capacity plate shows three ratings: (1) maximum persons capacity, stated as a number of persons OR a persons weight in pounds; (2) maximum weight capacity in pounds for persons, motor, and gear combined; (3) maximum horsepower for the motor. These come from the manufacturer's certified calculation, which may be lower than any rule-of-thumb estimate. The plate is the legal limit.

No. Never exceed the persons, weight, or horsepower rating on your boat's capacity plate. Overloading reduces freeboard and stability and is a leading factor in capsizing and swamping accidents. The calculators on this page produce estimates only — if your boat has a capacity plate, it overrides every number shown here.

The USCG simplified method: maximum weight capacity (persons + motor + gear) = (maximum displacement − boat weight) ÷ 5 for outboard boats, or ÷ 7 for inboard boats. Subtract the weight of the engine, controls, battery, and fuel to get the capacity left for persons and gear. Use the Weight module above with your boat's displacement figure.

Under 33 CFR 183.53: compute factor = boat length (ft) × transom width (ft). If the factor is 52.5 or less, look it up in the federal table (0–35 → 3 HP, 36–39 → 5, 40–42 → 7.5, 43–45 → 10, 46–52 → 15; flat-bottom hard-chine boats drop one step). If the factor is over 52.5: with remote steering and at least 20 in transom height, HP = (2 × factor) − 90; tiller-steered flat-bottom boats use HP = (0.5 × factor) − 15; other tiller boats use HP = (0.8 × factor) − 25. Results are raised to the next multiple of 5.

Jon boats are flat-bottom, hard-chine, usually tiller-steered boats. Compute factor = length × transom width. For factor 52.5 or less, use the federal table and drop one horsepower step (for example, a factor of 48 gives 15 HP on the table, reduced to 10 HP for flat-bottom). For factor over 52.5, use HP = (0.5 × factor) − 15, raised to the next multiple of 5. Example: a 14 ft × 5 ft jon boat has factor 70, so (0.5 × 70) − 15 = 20 HP.

ABYC H-35 is the standard for powering and load capacity of pontoon boats. Its worksheet computes: pontoon volume from diameter and length (π × R² × length ÷ 1728, plus nose-cone volume), displaced water weight (volume × 62.4 lb), maximum weight capacity = (displaced weight − boat weight) ÷ (√diameter − 2.35) for two pontoons or (√diameter − 1.35) for three pontoons, persons + gear = weight capacity − engine weight, and persons = (persons + gear) ÷ 141. It also gives a maximum horsepower estimate = (total pontoon volume × total pontoon length) ÷ pontoon diameter. The Pontoon module above runs the full worksheet.

It depends on tube diameter, tube length, and boat weight — there is no single L×W÷15 shortcut for pontoons. Use the ABYC H-35 worksheet: compute displaced water weight from the tubes, subtract boat weight, divide by (√diameter − 2.35) for two tubes or (√diameter − 1.35) for three tubes to get max weight capacity, subtract engine weight, then divide by 141 and round down for persons. The Pontoon module above does this step by step.

Start with thrust: the industry rule of thumb is 2 lb of thrust for every 100 lb of fully loaded boat weight (boat + passengers + gear + battery). Then check range: runtime (hours) = battery watt-hours × 0.8 ÷ motor watts at your throttle setting, and power follows the cube law — halving speed uses about one-eighth the power. Also respect hull speed (1.34 × √waterline length in feet, in knots): pushing a displacement hull past it wastes battery for almost no gain. The Electric module above combines all three checks.

There is no exact conversion, because thrust is a static force (pounds) while horsepower is work over time. The honest approximation is through electrical input: shaft-equivalent HP ≈ input watts × 0.85 ÷ 746. For example, a 55 lb thrust motor drawing 624 W is roughly a 0.7 HP shaft-equivalent motor. Be skeptical of marketing 'HP-equivalent' claims based on static thrust alone — they compare a trolling motor's best-case bollard pull against a gas motor, which is not how horsepower is rated.

Measure your boat's transom height: from the top of the transom straight down to the bottom of the hull (keel line), in inches. Then pick the smallest standard shaft length that is equal to or longer than that measurement — 15 in (short), 20 in (long), 25 in (extra-long), or 30 in (ultra-long). The anti-ventilation plate should end up roughly level with, or up to 1 inch below, the bottom of the hull. Sailboats and boats with high transoms usually need 25 in shafts.

No. The USCG simplified formulas (persons = L×W÷15, the 183.53 horsepower table) apply to monohull boats under 20 feet. Larger boats are rated by the manufacturer under ABYC standards or ISO 12217 (stability and buoyancy assessment), which use stability testing rather than a length-times-width shortcut. If your boat is 20 ft or longer, use only its capacity plate and owner's manual.

For boats under 20 ft (6 m), USCG, ABYC, Transport Canada, and ISO rate capacity the same basic way: capacity is derived from displacement weight (what it takes to sink the boat), divided by a safety factor — not from a fixed weight per person. The familiar 150 lb per person figure is a historical average used in the simplified rule of thumb; commercial ratings have moved toward 175 lb. The number that matters on a plate is the weight capacity, from which the persons count is derived.

The classic USCG rule of thumb assumes 150 lb per adult. Commercial passenger vessel ratings were updated to 175 lb per person, and some standards use 185 lb. Because real crews vary, the Persons module on this page lets you switch between 150, 165, 175, and 185 lb to see how the persons weight budget changes. When in doubt, use the higher figure — it is the conservative choice.

In everyday use the terms overlap: 'boat capacity' usually means the small-craft USCG plate ratings (persons, weight, horsepower) covered on this page, while 'vessel capacity' is the broader term that can also include commercial tonnage, cargo deadweight, or passenger-ship ratings. The calculators here address recreational boat capacity for monohulls under 20 ft and pontoon boats.