Deck Stain & Sealer Calculator

How the stain estimate works

gallons = total surface area × coats ÷ coverage per gallon. The arithmetic is the same as any paint job. What makes decks different is the surface area, which is almost never the number people start with.

The railing is not a rounding error

A 12 by 16 deck is 192 square feet of boards. Put a railing on three sides at 3 feet high and you add 44 linear feet of railing, and because you coat both faces that is 44 × 3 × 2, or 264 square feet. The railing is more surface than the deck. Total 456 square feet, which at 250 square feet per gallon and two coats is 3.65 gallons, so 4 cans.

Doubling the linear run for both faces is the working figure and it is deliberately a little generous, because it covers the balusters, the top cap, and the posts, none of which are flat. A railing with wide flat balusters uses more than this; a cable railing uses much less.

Stairs

Each step is a tread plus a riser, roughly 18 inches of surface across the width of the stair, so a 3 foot wide step is about 4.5 square feet. Four steps is 18 square feet, which is small next to the railing but not nothing. Stringer sides are extra, and if yours are exposed, add a few square feet a step.

Coverage, and why the can is optimistic

250 square feet per gallon is the figure to use on smooth, previously sealed wood. Rough-sawn boards, or any deck that has never had a finish on it, drink far more: closer to 150 square feet per gallon on the first coat. The second coat then goes much further, because the first one filled the wood.

If the deck has never been sealed, buy for 150 and expect to have some left over rather than the other way round. There is no worse place to run out than three quarters of the way across a deck in the sun, because a lap mark where wet stain met dry stain does not sand out.

New pressure treated wood has to wait

This is the single most common deck finishing mistake. New pressure treated lumber is still saturated with the treatment solution, and stain will not penetrate wood that is already full of something else. It sits on the surface, stays tacky, and peels within a season.

Give it a few months of weather, then test it: sprinkle water on a board. If it beads up, the wood is not ready. If it soaks in within a minute or two, it will take stain. This is a wood-moisture question, not a calendar one, so the test beats any rule about waiting six months.

Cans, and buying the right size

Stain is sold in gallons and 5 gallon pails, and this calculator rounds up to whole gallons. Once you are past about 4 gallons, price the pail: it is usually cheaper per gallon and, more usefully, it is one batch, so there is no color variation between cans. If you are buying separate gallons, box them together first by pouring them into one bucket and mixing.

Sizing the deck itself

If the deck is not built yet, the deck calculator gives board and joist counts, and the composite vs. wood decking comparison puts the cost of this staining cycle against what composite saves you, which is the real argument between the two materials.