Maximum Boat Horsepower Calculator
Work out the most outboard horsepower a monohull under 20 feet can be rated for, using the same computation the US Coast Guard requires manufacturers to use for the capacity plate.
The maximum horsepower on a boat's capacity plate is not a guess by the builder. For most small outboard boats sold in the US, it comes from a formula in federal regulations: 33 CFR 183.53. If your boat has lost its plate, or you are weighing up a repower, the same formula tells you the rating the boat would carry.
The US Coast Guard Horsepower Formula
The rule applies to monohull boats less than 20 feet long that are designed for outboard motors. Sailboats, canoes, kayaks and inflatable boats are excluded. The calculation has two steps.
Step 2 depends on the size of the factor:
| Factor | Horsepower capacity |
|---|---|
| 0–35 | 3 HP |
| 36–39 | 5 HP |
| 40–42 | 7.5 HP |
| 43–45 | 10 HP |
| 46–52 | 15 HP |
| Over 52.5, remote steering and transom at least 20 in | (2 × factor) − 90 |
| Over 52.5, flat bottom hard chine, tiller or transom under 20 in | (0.5 × factor) − 15 |
| Over 52.5, other hulls, tiller or transom under 20 in | (0.8 × factor) − 25 |
For factors up to 52, the factor is rounded to the nearest whole number before reading the table. Flat bottom hard chine boats, typically jon boats, drop one step in that range, for example from 5 HP to 3 HP. Above 52.5, if the formula result is not a multiple of 5, the rule allows it to be raised to the next multiple of 5. That is why the calculator shows both figures.
Maximum Horsepower by Boat Size
The examples below apply the rule to common sizes. "Tiller" means tiller steering or a transom under 20 inches. "Remote" means wheel steering with a transom at least 20 inches tall. All figures are rounded up to a multiple of 5 where the rule allows it.
| Length × transom | Factor | Flat bottom, tiller | V-hull, tiller | Remote, 20 in transom |
|---|---|---|---|---|
| 12 × 4 ft | 48 | 10 | 15 | 15 |
| 14 × 4.5 ft | 63 | 20 | 30 | 40 |
| 14 × 5 ft | 70 | 20 | 35 | 50 |
| 16 × 5.5 ft | 88 | 30 | 50 | 90 |
| 16 × 6 ft | 96 | 35 | 55 | 105 |
| 18 × 7 ft | 126 | 50 | 80 | 165 |
| 19 × 8 ft | 152 | 65 | 100 | 215 |
The table shows how much steering matters. A remote-steered boat with a tall transom can be rated for roughly twice the power of the same hull with a tiller.
How to Measure Length and Transom Width
Use the boat's overall length in feet, the figure on the hull identification paperwork or the builder's specifications. For transom width, measure the widest point of the transom itself, not the beam of the boat. On many hulls the transom is narrower than the maximum beam, and using beam inflates the factor. Leave out handles, motor brackets, swim platforms and other attachments.
Transom height matters for the higher formula. The 20 inch threshold is the standard long-shaft mounting height. A 15 inch short-shaft transom puts the boat in the lower formula even if it has wheel steering.
Common Mistakes in Online Examples
The most repeated simplification is "(2 × length × width) − 90". That is only one of the three formulas, and it applies only to boats with remote steering and a transom of at least 20 inches. Applying it to a tiller boat overstates the safe power by a large margin. The 14 × 5 ft tiller V-hull in the table rates 35 HP under the correct formula, against 50 HP from the remote-steering formula.
We also found a well-ranked boat builder's guide that writes its worked example as "(2 × 18 + 8) − 90 = 198" and calls the 8 ft dimension the beam. Its answer of 198 is the correct result for 2 × 18 × 8 − 90, but the written expression evaluates to a negative number, and the rule uses transom width, not beam.
The Capacity Plate, Older Boats and Bigger Boats
Builders must mark the maximum horsepower on a capacity plate near the helm. If the plate is present, its figure is the rating to follow, because the builder may also have used an allowed performance test. That test applies only to boats 13 feet or shorter with remote wheel steering, a transom of at least 19 inches (or 15 inches with a 19 inch motorwell), a two-person maximum and a test motor of no more than 40 HP.
Boats without a plate are common: older boats, imports and home-built hulls. The formula gives the rating such a boat would carry. Boats 20 feet and longer, multihulls such as pontoons, and inboard or sterndrive boats are outside this rule, so their power limits come from the builder.
How Many People Can the Boat Carry?
State boating agencies teach a quick rule for small boats without a plate: persons capacity = length × width ÷ 15, rounded down. The Illinois Department of Natural Resources notes that the rule assumes 150 lb per person, adults and children averaged together. A 12 × 4 ft boat works out to 48 ÷ 15 = 3.2, so three people. A 16 × 6 ft boat comes to 6.4, so six people.
The rule is a rough guide only. A capacity plate's persons and weight figures take precedence, and heavy gear, a large motor or rough water all reduce what a small boat can carry safely. The calculator shows the rule-of-thumb figure alongside the horsepower result.
Is It Illegal to Exceed the Rating?
The federal regulation sets what builders must mark. Whether running more power than the plate shows is an offense depends on your state. Several states, including Ohio, Idaho, South Dakota and West Virginia, make it one. The practical risks are wider: many insurers decline claims on overpowered boats, and an overpowered hull can handle dangerously at speed. If you are repowering, stay at or below the plate figure.
Once you know the horsepower range, our horsepower to knots calculator estimates the speed a given engine will give your hull, and thrust to HP converts trolling motor ratings.
Frequently Asked Questions
Multiply the boat's length by its maximum transom width, both in feet, to get the factor. For factors up to 52 read the Coast Guard table (3 to 15 HP). Above 52.5 use (2 × factor) − 90 with remote steering and a 20 in transom, otherwise (0.8 × factor) − 25, or (0.5 × factor) − 15 for flat bottom hard chine hulls.
It depends on the transom width. A 14 × 4.5 ft flat bottom jon boat with tiller steering has a factor of 63, giving (0.5 × 63) − 15 = 16.5 HP, which may be rounded up to 20 HP.
A 16 × 6 ft V-hull is rated 55 HP with tiller steering and 105 HP with remote steering and a 20 in transom. A 16 ft flat bottom jon boat of the same width with a tiller is rated 35 HP.
Transom width. Measure the widest part of the transom itself, excluding handles and fittings. Only if the boat has no full transom do you use the broadest beam in the aftermost quarter of its length.
No. It covers monohull boats under 20 feet designed for outboard motors. Pontoons (multihulls), boats 20 feet and longer, and inboard or sterndrive boats are rated by their builders.
The plate. Builders can use a performance test instead of the formula for some small boats, and the plate reflects the rating the boat was certified with.
It is not recommended. Several states, including Ohio, Idaho, South Dakota and West Virginia, prohibit it, many insurers will not cover an overpowered boat, and the extra weight and power can make the hull unsafe.