Deck stain calculator

The railing is not a detail

A 12 ft × 16 ft deck is 192 sq ft of floor. Put a wood-baluster railing round the three open sides — 40 linear ft — and you have added roughly 180 sq ft of railing, plus another 30 sq ft of fascia and 18 sq ft of stairs. The floor is less than half the job.

That railing figure is geometry, not a guess, and you can check it. For a 36 in guard with 2x2 balusters (1-1/2 in actual) spaced to code, there are about 2.4 balusters per foot, each with four faces 1-1/2 in wide and roughly 30 in long: 2.4 × 4 × 0.125 ft × 2.5 ft = 3.0 sq ft. The top and bottom rails contribute about 1.2 sq ft per foot between them, and 4x4 posts every 6 ft add another 0.6. Total ≈ 4.5–4.8 sq ft per linear foot — call it 4.5. Swap to metal balusters and only the rails and posts get stained, which drops it to about 1.8.

Stairs are simpler: 11 in of tread plus a 7 in riser is 18 in, so 1.5 sq ft per foot of stair width, per step. Skirt boards and exposed stringers are extra.

If you buy stain for the floor alone, you will finish the boards and stop. There is no way to hide the join where you ran out.

Coverage swings by a factor of two

Every figure here is off a manufacturer's data sheet, at the low end of the published range:

• Cabot Australian Timber Oil #3400 — "On smooth surfaces, approximately 400–600 sq. ft. per gallon. On rough surfaces, 250–350 sq. ft. per gallon."
• BEHR PREMIUM Semi-Transparent Waterproofing Stain & Sealer No. 5077 — 250–300 sq ft/gal, with a thinner second coat at 500–600.
• BEHR PREMIUM Solid Color Waterproofing Stain & Sealer No. 5011 — 200–400 sq ft/gal, "depending on application method and substrate porosity".
• Benjamin Moore ARBORCOAT N638 — 200–400 sq ft/gal "depending on surface texture and porosity", and never thinner than 200.

Rough-sawn, weathered or newly stripped wood sits at the bottom of those ranges. Smooth, tight, previously sealed boards sit at the top. A single figure for all of them — 300 is the usual compromise — is wrong for half the decks it is applied to, and wrong in the direction that leaves one half done. This page asks which you have instead.

Do not use interior or exterior paint coverage here. Paint sits on the surface at 350–400 sq ft/gal; stain is absorbed, and the wood decides how much.

The second coat is cheap, the third is a mistake

Behr's sheet asks for two coats and prices the second at half the first (500–600 sq ft/gal against 250–300), which is why two coats above costs 1.5 gallons per gallon of first coat rather than 2. Penetrating oils are the opposite: Cabot's sheet says "Apply 1 coat only". A second coat of oil has nothing left to soak into and stays sticky for weeks.

Both sheets warn against over-application, and Behr is blunt about the consequence — "over application of stain will lead to surface failure including peeling & cracking". More stain is not more protection.

Buy it all at once and box the cans

Cabot's instruction is worth repeating: "If project requires more than 1 container, blend all containers together to ensure uniformity." Tinted stain varies between batches, and on a deck the seam between two cans lands as a visible band across the boards. Pour them into a 5-gallon bucket, stir, and work out of that.

What the surface figures assume

The railing factor is the largest number on this page after the floor, and it is geometry for one particular railing: a 36 in guard, 2x2 balusters at code spacing, a top and a bottom rail, 4x4 posts about every 6 ft. That is the arithmetic set out above, and the metal-baluster figure is the same derivation with the balusters removed. A 42 in guard, tighter baluster spacing, built-up corner posts or a horizontal-slat design all move it and none of them have a field here. The solid-panel figure is not derived on this page — it is an allowance for coating both faces of a privacy panel. If that is what you are staining, measure the faces and enter them as deck area instead.

The floor is a flat rectangle, top face only. Board edges, the gaps between them, the underside of the deck and the joists below are outside it, as are landings and anything you can see of the frame from the garden. Cleaners, brighteners and strippers are separate purchases and are not counted at all.

One spread rate is applied to every surface — floor, riser, rail and fascia all get the same square feet per gallon. And it is worth knowing what the surface-condition switch does: on penetrating oil it moves coverage from 400 to 250 and drives the answer harder than the deck size does, but on solid stain it changes nothing, because 200 sq ft/gal is already the bottom of Behr's published range for No. 5011 and there is nowhere below it to go.

The three products are classes standing in for a whole shelf, each set at the low end of a named data sheet. Where the can in your hand prints its own rate, that rate wins.