Deck Stain Calculator: How Much Stain Do I Need?

How to use the deck stain calculator

1. Enter the deck floor as length × width.
2. Add the railings. Enter the total length of railing and its height. The calculator doubles the face area to account for both sides of the balusters and rails.
3. Add the stairs. Enter the number of steps and the stair width, and choose whether you will stain the risers.
4. Pick the stain. The presets carry each product’s published spread rate and number of coats. For anything else, choose Other and type the range from the can.
5. Pick the wood condition. New, smooth boards take the least stain, and old, weathered or rough boards the most.

The result is whole gallons, with the exact figure and the spread rate used shown beneath it. It also breaks the area into floor, railings and stairs, so you can see where the stain goes.

How much deck stain do I need? The formula

Stain makers print spread rates as a range, in square feet per gallon. The low end is for wood that soaks it up and the high end for wood that does not. Sherwin-Williams puts it plainly on its SuperDeck data page: new wood normally needs less product than old, weathered wood, because older wood is more porous. So the calculator uses the low end of the range for weathered, rough or very porous wood. It uses the midpoint for average wood and the high end for new, smooth boards.

The railing and stair rules are ours. A railing with spaced balusters has much more surface than its outline, with four sides on every baluster plus the top and bottom rails. Doubling the outline is a fair middle estimate. Each step is counted as an 11¼ in tread plus a 7¼ in riser.

Deck stain coverage by product

Notice how wide the oil semi-transparent range is: the same gallon can cover 150 sq ft or 400 sq ft depending on the wood. That is the main reason people run short. Coat counts matter too. Sherwin-Williams says not to apply more than one coat of SuperDeck semi-transparent oil, because a second coat will not penetrate and leaves a glossy, tacky surface. Behr’s DeckPlus acrylic, by contrast, needs two thin coats for the look and durability it is rated for.

Worked examples

A 16 × 12 ft deck with 40 ft of 3 ft railing and four 4 ft steps. The floor is 192 sq ft, the railings 240 sq ft and the stairs about 25 sq ft, for 457 sq ft in total, or 503 sq ft with 10% extra. With SuperDeck oil at an average 275 sq ft per gallon, that is 1.8 gal, so 2 gallons. If the boards are badly weathered, the rate drops to 150 sq ft per gallon and the job needs 3.4 gal, so 4 gallons.

The same deck with Behr DeckPlus. The first coat at an average 275 sq ft per gallon is 1.8 gal, and the second coat at 550 is 0.9 gal. That is 2.7 gal in total, so 3 gallons.

Floor only, 300 sq ft, new wood, Cabot semi-solid. 330 sq ft with extra at 350 sq ft per gallon is 0.94 gal, so a single gallon, just.

Prep that changes how much you use

• Clean and strip first. Behr’s data sheets call for a wood cleaner to remove mill glaze and dirt, or a stripper over failing coatings, then rinsing and drying before staining. Clean wood soaks up stain more evenly.
• Do not stain over a sealed surface. Sherwin-Williams says not to apply SuperDeck semi-transparent over sealed or previously coated wood. Let it weather until water no longer beads up.
• Mind the weather. SuperDeck oil wants surface temperatures between 50 and 90°F and no rain for 24 hours. Check the label of your product.
• Box the cans. Mix all the gallons together in one larger container before you start, so the color is the same from board to board.
• Test first on a hidden board to check the color before you do the whole deck.

Building or repainting too?

If the deck is not built yet, the deck calculator counts boards, joists and screws, and the deck joist span calculator checks the framing. For the house siding or trim next to it, use the paint calculator, which works from wall area and coats the same way.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Area = deck floor (length × width) + railings (length × height × 2) + stairs (steps × width × (11¼ in tread + 7¼ in riser if stained)). The railing factor of 2 counts both faces, which roughly covers all sides of spaced balusters and rails. This and the step sizes are our assumptions.
• Gallons = area × (1 + extra) ÷ spread rate for each coat. The default 10% extra is our allowance for board edges and touch-ups.
• Wood condition picks the point in the maker’s range: the low end for weathered, rough or porous wood, the midpoint for average wood and the high end for new, smooth wood. Sherwin-Williams notes that new wood normally needs less product than old, weathered wood; the exact mapping is ours.
• Coats follow each data sheet: SuperDeck oil semi-transparent and Cabot semi-solid are one-coat products, and Behr DeckPlus calls for two, the second at 500–600 sq ft per gallon.

Sources

1. Sherwin-Williams, SuperDeck Exterior Oil-Based Semi-Transparent Wood Stain 550 data page. 150–400 sq ft per gallon depending on porosity and texture; one coat only; new wood needs less than weathered wood.
2. Behr, DeckPlus Semi-Transparent Waterproofing Wood Stain (No. 3077) technical data sheet. 250–300 sq ft per gallon first coat, thinner second coat at about 500–600; 2 coats required.
3. Cabot, Semi-Solid Deck and Siding Stain #17400 Series data sheet. Approximately 250–350 sq ft per gallon; 1-coat application.

Last reviewed October 26, 2026. Product figures vary by brand; the label on what you buy wins.