Deck stain calculator

Gallons of stain for a deck, fence, or run of siding. Semi-transparent or solid, smooth or weathered wood, with the second-coat-covers-more rule that keeps you from buying a gallon you don't need.

01 How the math works

Stain is bought by coverage, not by volume. Gallons is the coated area divided by how many square feet a gallon covers. The trick is that a penetrating stain doesn't cover the same on both coats.

Semi-transparent stain soaks into the grain. The first coat seals the wood, so the second coat has far less to soak into and stretches much further. Behr's DECKplus data sheet puts the first coat at 250–300 ft²/gal and the second at 500–600 ft²/gal — roughly double. Solid color stain is different: it sits on the surface like thin paint, so both coats cover at the same rate. That single difference is why the same deck can need two gallons of one and three of the other.

02 Worked example

Take a 15 ft × 20 ft deck floor (4.57 m × 6.10 m) — 300 ft² (27.87 m²) of smooth, new pressure-treated boards, two coats of semi-transparent stain.

First coat at 250 ft²/gal: 300 ÷ 250 = 1.2 gal. Second coat at 500 ft²/gal: 300 ÷ 500 = 0.6 gal. Total 1.8 gal — round up to 2 US gallons (7 L).

Order the flat way — treat both coats like the first — and you get 300 × 2 ÷ 250 = 2.4 gal, which rounds up to 3 gallons. That third gallon sits unopened in the garage because the second coat never needed it. Switch the same deck to a solid stain and three gallons is the right answer: solid covers at 250 ft²/gal on every coat, so 2.4 gal really is what you buy.

03 When this calculator is wrong

The result assumes the wood accepts stain at the manufacturer's rate. Two things routinely break that, and both push the number up.

• Weathered or rough-sawn wood drinks stain. Open, grayed grain on an older deck takes a penetrating stain at the Thompson's WaterSeal figure of 150–200 ft²/gal on the first coat — roughly half the smooth-wood rate. Switch the condition toggle to "rough · weathered" and the 300 ft² example above jumps from 2 gallons to 3. A smooth-wood estimate on weathered boards under-orders by about a third.
• Railings and stairs carry more surface than the floor. A deck's balusters have far more coated area per linear foot than the floor has per square foot. A 40 ft railing of 2×2 balusters (1.5 in actual) at 4 in on-center, 36 in tall, is 120 balusters × 1.5 ft² each = 180 ft² of surface — before the top rail, bottom rail, and posts. That is more than half of a 300 ft² floor, and the floor-only footprint never sees it. Measure railings and stairs and add them to the "extra surfaces" field.
• Sprayer overspray and back-brushing change the rate. Spraying a deck loses material to overspray and to the gaps between boards; back-brushing works it in but doesn't recover the loss. Coverage on a sprayed-and-back-brushed deck runs 10–15% below a brushed-and-rolled rate on the same wood.

04 What to do with the result

Buy the rounded-up gallon count, but on stain there's a second reason beyond running short: batch matching. Stain color is mixed in lots, and two cans from different lots can read as slightly different shades once they're on the wood and dry. On a continuous surface like a deck floor, that line shows.

So on stain, the rule bends. Buy the rounded-up amount in one trip, from one shelf, checking that the lot or batch numbers match — the same discipline a matching tile or paint lot needs. If the job spans the boundary between two- and three-gallon territory, buy the third can now rather than chase a matching lot later. An extra can of stain returns or stores; a mismatched seam gets sanded and redone.

05 Common questions

How much stain do I need for a deck?

Divide the coated area by the coverage rate for each coat. A 300 ft² (27.87 m²) smooth deck in semi-transparent stain, two coats, works out to 1.8 gal — order 2 gallons. The same deck in solid stain needs 3, because solid covers the same on both coats.

Does a second coat of stain use as much as the first?

Not for penetrating semi-transparent stain. The first coat seals the grain, so the second covers roughly twice as much — Behr rates it at 500–600 ft²/gal versus 250–300 on the first. Solid stain is the exception: it films over the surface, so both coats cover the same.

Semi-transparent or solid — which covers more?

Per coat on new wood they start close, at 250–300 ft²/gal. Over two coats semi-transparent covers more total area because the second coat stretches. Solid stains hide the grain and hold up longer on horizontal wear surfaces, but they cost more gallons and can peel rather than fade.

How much does old, weathered wood change the estimate?

A lot. Grayed, open-grain wood takes penetrating stain at 150–200 ft²/gal on the first coat instead of 250–300 — close to half. Plan on roughly a third more stain for a weathered deck than a new one of the same size.

Do I count the railings in the square footage?

Yes, separately. Balusters and rails hold more surface per foot than the floor: a 40 ft railing can add 180 ft² or more. Measure them and put the total in the "extra surfaces" field so it's staged into the coverage math.