Enter the boat's length overall and beam, the number of coats and the coverage printed on the can. The calculator gives the underwater area and the number of quarts or litres to buy.
The formula
Interlux gives a rough estimate of the underwater area:
Length overall × beam × 0.85 = underwater area
Then divide the area by the paint's coverage per quart or litre to get the amount for one coat, and multiply by the number of coats. Interlux's quick chart assumes two coats. Keep the units the same throughout: feet with square feet per quart, metres with square metres per litre.
Why it's an estimate
The 0.85 factor stands in for the hull's shape below the waterline, so a deep fin keel, a full keel and a planing motorboat all get the same treatment. Interlux calls it a rough estimate. For an exact figure, ask the paint maker or the yard, who can measure the hull.
Before you paint
Check the paint suits the hull and what's already on it: copper paint can't go on aluminium, and an ablative needs sanding or a tie-coat before a hard paint covers it. See bottom paint for boats. If the hull is out for a survey too, the surveyor reads the bottom before it's painted over: see osmotic blistering.