Deck Stain Calculator

What most stain calculators miss is over half the surface. Four things move the answer past length times width, and on this deck they move it from 1.6 gallons to 3.6.

The railing is not trim, it is surface

A 40 ft railing carries about 87 balusters. Each is four faces, and four faces of a 2×2 at 36 inches is 1.5 sq ft — so the balusters alone come to 130.5 sq ft (12.1 m²). Add the posts and rails at 88 sq ft and the railing system totals 218.5 sq ft: more surface than the 192 sq ft floor it surrounds.

None of it reads as area. A floor looks like a job. A railing looks like trim. That is why the tin runs dry with one side still bare, and why the second trip to the store is for a different batch — which will not match.

The oil and the acrylic are not the same product

Two stains at the same pigment level behave nothing alike. A semi-transparent penetrating oil covers 350–400 sq ft per gallon in 1 coat; a semi-transparent stain and sealer covers 250–300 in 2. Same shelf, same colour description, 100 sq ft per gallon and a whole coat apart. Pigment level is how stain is sold and it does not predict coverage. Chemistry does, and on this deck the difference is 1.8 gallons against 3.6.

Weathered wood drinks more than the tin assumes

A published coverage rate assumes prepared wood in sound condition, and most decks are not that. Weathered grey pine takes this deck from 275 sq ft per gallon down to 186 — a third more stain than sound previously-stained boards, which need 2.7 gallons against 3.6. Rough-sawn boards go further again, to 4.5, and new pressure-treated stock runs the other way at 2.4 — a warning, not a saving: a finish that will not soak into mill glaze will not bond to it.

The second coat goes twice as far as the first

The second coat of a film-forming stain covers 2.0× the ground the first did, because the first coat filled the grain and the sheets instruct that the second go on thinner: 2.31 gallons for coat one here and 1.15 for coat two. The ratio is read off the sheets and stored per product: a penetrating oil publishes none, so extra coats on an oil are costed at the full rate.

To work out how much stain a deck needs, this calculator adds the surfaces up first and divides once at the end.

surface = floor + rails + balusters + stairs (+ underside)
coverage = published rate × wood condition
gallons = surface ÷ coverage, plus later coats at 2.0× coverage

Five inputs drive it: deck size, stain type, wood and condition, coat count, and which surfaces beyond the floor you are coating. Deck size takes length and width or a square-foot figure you already know — unlike a board count, coverage needs only the area.

The railing controls are worth setting carefully. Railing length is measured along the run and is usually three sides rather than four, because the house takes the fourth; baluster count is estimated from that length if you leave it blank. Cable or glass infill drops the railing to 2.2 sq ft per linear foot, posts and rails only.

A 5% allowance is added at the end for what stays in the tray, the roller and the brush. Every constant in the chain is published on our methodology page with its source.

A 16 × 12 ft deck with a 40 ft balustered railing and 4 steps comes to 428.5 sq ft. Coverage starts at the published midpoint and is adjusted twice before anything is divided:

275 sq ft/gal × 0.75 weathered × 0.90 pine = 186 sq ft/gal

The first coat is 428.5 ÷ 186 = 2.31 gallons. The second covers twice the ground, so it adds 1.15 rather than another 2.31. That is 3.46 gallons on the wood and 3.64 once the 5% allowance is added — 4 × 1 gallon, about $184 at the prices in our data.

The floor alone would have been 1.6 gallons and 2 × 1 gallon — the estimate a floor-area calculator hands you, and it runs out along the second side of the railing. Adding the underside and joists goes the other way, to 793.3 sq ft and 6.7 gallons: a spray job, since a brush does not reach between joists, which is why it is off by default.

What to buy is a set of containers, and 3.64 gallons is not one. Stain is sold in quarts, gallons and 5 gallon pails at three prices per gallon, so the cheapest way to cover a requirement is rarely the obvious one.

On this deck the answer is 4 × 1 gallon at $184. One 5 gallon pail holds enough and costs $39.10 a gallon against $46 — cheaper per gallon, $196 in total, and 1.36 gallons you will not open. The larger container is not automatically the saving; it becomes one once the job needs most of 5 gallons. Quarts run the other way at $62.10 a gallon, worth buying only to top up a gallon-sized gap.

Buy it all in one trip. Stain is tinted in batches, and two tins of the same colour code bought weeks apart dry to visibly different shades. Where a job takes more than one container, pour them together and mix before starting — boxing, in the trade — so the difference goes into every board rather than into a line across the deck.

Penetrating or film-forming: ask this before you ask about colour

A penetrating oil soaks into the wood, goes on in one coat at 350–400 sq ft per gallon, and wears away as it ages, so the next refinish is a wash and a recoat. A film-forming acrylic cures to a layer on the surface, takes 2 coats at 250–300, and peels when it fails, so the next refinish means stripping. Both are sold as "semi-transparent" and look much the same on the shelf.

Clear sealer is not a stain with the pigment left out

A clear water-repellent sealer repels water and does essentially nothing about ultraviolet light, so the deck greys underneath it on schedule. It goes a little further than the semi-transparent acrylic — 300 sq ft per gallon against 275, or 2.2 gallons on this deck — and lasts the least time of anything here, 1–2 years. Reasonable on ipe, where a hardwood oil wiped back is the usual treatment. Close to a waste of money on pine.

Clear sealers vary more than any other category. Olympic WaterGuard publishes 250–350 sq ft per gallon; Thompson's Waterproofer Plus is rated 150–200. At the low end a gallon covers half what our figure assumes, so read the tin before you buy.

What pigment buys, and what it does not

Pigment buys ultraviolet protection, so the grades run transparent, semi-transparent, semi-solid, solid in the same order as how much grain they hide and how long they last — 4–6 years for a solid. Hiding more and lasting longer are the same axis. Coverage is not on it, and the sheets are blunt about that.

Behr's solid publishes 200–400 sq ft per gallon, a midpoint of 300 — above its own semi-transparent acrylic at 275. Film build and chemistry moved that number, not pigment load. Semi-solid is the one figure here with no sheet behind it, at 288: the mean of the two acrylics bracketing it, and a planning number rather than a reading.

Solid stain is close to a one-way door: it peels rather than fading, and the deck must be stripped or sanded before anything else will go on. Think hard before putting it on new wood.

Before you open the tin, wash the deck and let it dry. Stain over a dirty deck fails early whatever it cost, and a brightener reopens grain that cleaning closed. Allow at least 24 hours of drying afterwards, longer for boards that were soaked rather than damped.

The weather window is not one window

The weather window depends on the product, and the products disagree. Behr's penetrating oil applies at 40–90°F and resists rain 4 hours after application. Thompson's clear sealer wants air and surface above 50°F during application and for 48 hours after, with no rain forecast within 24 hours. Across those two sheets the floor sits between 40°F and 50°F and the rain-free stretch runs from 4 hours to 24 hours. Read the label on the tin you bought.

Air temperature is not surface temperature: a deck in direct sun runs far hotter than the air, and stain that skins before it penetrates leaves lap marks. Work in shade, keep a wet edge along whole boards rather than across them, and back-brush anything sprayed.

New pressure-treated lumber: wait 30 days

Wait 30 days before sealing new pressure-treated lumber, which is what Thompson's publishes for its clear sealer. Most advice says "wait until it is dry" and leaves you without a date. Kiln-dried-after-treatment boards may be ready sooner and wet ones later: sprinkle water on them, and if it beads rather than soaking in, the wood is not ready.

How long before you can walk on it

Allow 24 hours before foot traffic and 72 before full cure, on the Behr penetrating oil sheet. Dry to touch comes at 2–4 hours and is not the same thing. Acrylics differ; those figures are the oil's.