About this tool
This calculator estimates how much stain or paint a deck needs. It adds three surfaces that most people forget to add up separately: the deck floor (length x width), the railings (linear feet x a 3 ft height, doubled if you coat both faces of the balusters and rails), and the stairs (about 12 sq ft of tread and riser per step). You pick the finish, because stain soaks into rough-sawn lumber at about 250 sq ft per gallon but goes much further on smooth or previously coated boards, while solid deck paint behaves like exterior house paint at about 400 sq ft per gallon. Coats, a waste allowance, and an optional price per can complete the estimate. It is for homeowners restaining a deck each spring, anyone building a new deck who wants the finish in the budget, and painters checking a quote.
How it works
Deck floor area = length x width. Railing area = linear feet of railing x 3 ft (the height of a code-minimum guard rail), and x 2 when 'both sides' is on, because balusters and rails have two faces to coat. Stairs = number of steps x 12 sq ft, which approximates an 11-inch tread plus a 7-inch riser on a 4 ft wide stair, both faces included. The total is multiplied by the number of coats and by 1 + waste%, then divided by the coverage rate for the chosen finish: 250 sq ft/gal for stain on rough or weathered wood, 350 for stain on smooth wood, 400 for solid deck paint, or any custom rate from the can. Gallons round up to whole cans and split into 5-gallon buckets plus 1-gallon cans. Metric mode uses 1 gallon = 3.785 L and 1 m2/L = 40.75 sq ft/gal.
Frequently asked questions
How much does a gallon of deck stain cover?
On smooth, dry wood a gallon of penetrating deck stain covers roughly 300 to 400 sq ft for the first coat. On rough-sawn, weathered, or very dry boards the wood drinks the stain and coverage drops to 150 to 250 sq ft. Second coats go further because the surface is already sealed. Manufacturers print a range on the can; this calculator defaults to 250 for rough and 350 for smooth wood and lets you type the label figure instead.
Should I stain or paint my deck?
Penetrating stain is the usual choice for a walking surface: it does not peel, it shows the grain, and recoating is a wash-and-reapply job every two to four years. Solid paint and solid-color stain hide old boards and mismatched repairs but form a film that can peel under foot traffic and standing water, and stripping it later is hard work. Paint is more common on railings and trim than on deck boards.
How do I count railing area?
Measure the total run of railing in feet and multiply by its height, usually 36 to 42 inches, so about 3 ft. That gives the area of one face. Because balusters, top rails, and bottom rails have two exposed faces, the calculator doubles it when 'coat both sides' is on. Chunky posts add a little more; if the deck has many 4x4 or 6x6 posts, add roughly 5 sq ft per post to the railing feet or raise the waste allowance.
How many coats of deck stain do I need?
Most penetrating oil and water-based stains call for one or two thin coats applied wet-on-wet within an hour, before the first coat dries, so the second coat is really extra material rather than a separate day. Solid stains and paints need two coats for even color and durability. Never leave puddles of penetrating stain to dry on top of the wood; wipe or back-brush them out or the surface will stay sticky.