If your boat already has a capacity plate riveted to the transom, that number is the legal one — full stop. This calculator exists for the boats that don't: homebuilt rigs, older boats that lost their plate, or anyone curious how the Coast Guard actually arrives at the number stamped on every new aluminum boat sold in the US.
Outboard Specs & Transom Safety
USCG max-horsepower formula (33 CFR 183.53) for boats under 20 ft without a capacity plate, plus a rough persons-capacity check.
33 CFR 183.53
Hull Inputs
This is the CFR's generic computation method — used when a boat has no existing capacity plate. If your boat already has one riveted to the transom, that number is the legal one; it overrides this estimate.
USCG Max Rating
90
max horsepower (legal ceiling)
Length × width factor104.0
Rough persons capacity6 people
Persons capacity here uses the simplified length × width ÷ 15 approximation — your actual capacity plate also weighs in motor weight and gear, and the weight limit always wins if it's lower than the headcount.