Stain Calculator

How the stain calculator works

A 16 × 12-ft deck — 192 sq ft — needs 3 gallons of semi-transparent stain for two coats. At 200 sq ft per gallon on average-condition wood, and 211 sq ft after a 10% waste factor, each coat takes just over 1 gallon, so two coats is 2.1 gallons and you buy 3. Rough or weathered boards drink far more: the same deck jumps to 4 gallons at 130 sq ft per gallon.

Wood is way more porous than drywall, so stain coverage is roughly half of paint. We compute the area, multiply by coats, divide by coverage, add a waste factor, and round up to whole containers.

Coverage by surface

• Smooth (planed) wood — 250–350 sq ft / gallon (~6–8 m²/L). Recently milled lumber, freshly sanded decks. The calculator applies a 1.5× multiplier to your base coverage figure.
• Average — 150–250 sq ft / gallon (~4–6 m²/L). Default for most projects; the multiplier is 1.0.
• Rough / weathered — 100–150 sq ft / gallon (~2.5–4 m²/L). Old fences, rough-sawn cedar, heavily weathered. The multiplier is 0.65, so a 200 sq ft/gal product is treated as 130.
• First coat on bare wood — even rougher absorption; budget +20% for it specifically.

Species matters as much as surface finish. Southern yellow pine, the usual pressure-treated species, is dense and resinous and takes stain grudgingly when new; western red cedar and redwood are light and open and absorb quickly; ipê and other tropical hardwoods are so dense and oily that penetrating stain barely enters at all and coverage can exceed 500 sq ft per gallon. If you are staining a hardwood deck, use a product formulated for dense hardwoods and expect to apply very little of it — flooding it is the classic failure.

Stain types — pick by use

The trade-off across the whole range is a straight line: the more pigment a stain carries, the longer it protects and the less wood you see. Pigment is what blocks UV, and UV is what destroys the lignin at the surface of exposed wood and turns it grey. A clear sealer is therefore a water repellent, not a UV protectant, no matter what the label implies.

Two things that table hides. First, horizontal surfaces fail two to three times faster than vertical ones, because they hold standing water and take the sun square-on. That is why the same product is quoted at 2–3 years on a deck and 5+ years on a fence, and why a deck's floor may need recoating while its railings still look new. Second, a solid stain forms a film, and films peel. A solid stain on a horizontal deck floor that gets foot traffic and standing water will eventually flake rather than fade — which means the next refinish is a strip-and-sand job, not a wash-and-recoat. Solid stain is an excellent choice on a fence and a decision you may regret on a deck floor.

Prep by condition — the part that decides how long it lasts

Stain adhesion is almost entirely about whether the wood can accept it. Test with a splash of water: if it beads and sits, the surface is closed and stain will not penetrate; if it soaks in within about 30 seconds, the wood is ready. Do this before you buy anything.

Sanding finer than about 100 grit is counterproductive on exterior wood — it burnishes the surface closed and reduces absorption, which is the opposite of what you want. And a wood brightener (usually oxalic or citric acid) is not an optional cosmetic step after using an alkaline cleaner: the cleaner raises the surface pH and darkens some species, and the brightener returns it to a pH where the stain will behave predictably.

Timing and recoat windows

Stain is far more weather-sensitive than interior paint, and most premature failures trace back to the day it was applied rather than the product.

The second-coat row is the one that catches people. A penetrating stain — transparent through semi-solid — is meant to soak in, and the wood will only take what it can hold. The correct technique is wet-on-wet: apply, and while the first pass is still wet, go back over any area that has already gone dull and absorbed everything, then wipe off anything still sitting on the surface after 20–30 minutes. Coming back four hours later and applying a full second coat over a saturated deck leaves stain sitting on top with nowhere to go; it stays tacky, collects dirt, and peels. Solid and semi-solid stains, which form a film, genuinely do take two coats.

So use the coats field deliberately. Two coats is the right assumption for a fence, for solid stain, and for very absorbent rough or weathered wood. Set it to 1 for a penetrating stain on smooth, dense, or previously stained wood, and let the wet-on-wet technique use whatever the wood takes.

Worked example: a 16 × 12-ft deck with railings

The deck floor is 16 × 12 = 192 sq ft. Ticking "include railings" applies a 1.3× factor for the balusters, rails, and posts, giving 250 sq ft. A 10% waste factor brings that to 275 sq ft. On average-condition wood the coverage stays at the 200 sq ft/gal default, so one coat is 275 ÷ 200 = 1.37 gallons, and two coats is 2.75 → 3 gallons.

That 1.3× railing factor is a reasonable average and it is worth sanity-checking against your actual rail. A 2×2 baluster has four faces, so a run of closely spaced balusters can carry more surface area per linear foot than the deck boards beneath it, and it is all brush work. If your railing is heavily turned, latticed, or double-sided, use the Custom area mode and add the railing area explicitly rather than relying on the multiplier. The other direction: a cable or glass railing adds almost nothing, so leave the box unticked.

Worked example: a 60-ft privacy fence, both sides

At 6 ft tall, one face is 60 × 6 = 360 sq ft; staining both sides doubles it to 720 sq ft. With 10% waste that is 792 sq ft. On average-condition wood at 200 sq ft/gal, one coat is 3.96 gallons and two coats is 7.92 → 8 gallons. Buy that as a 5-gallon pail plus three single gallons, and box them together before you start so the tint is uniform across the whole run.

Now switch the surface to Rough / weathered — an unsealed cedar fence a few years old. Coverage drops to 200 × 0.65 = 130 sq ft/gal, one coat becomes 6.09 gallons, and two coats is 12.2 → 13 gallons. That 60% jump is the single biggest variable in the whole estimate, which is why it is worth walking the fence with a water bottle before ordering rather than guessing. It is also the argument for spraying and back-brushing a fence rather than brushing it: 720 sq ft of pickets is many hours with a brush, and back-brushing behind a sprayer gets the penetration a sprayer alone cannot.

Safety: oily rags are a real fire risk

This is the one item on this page that can hurt you. Oil-based stains cure by oxidation, an exothermic reaction. A wadded rag soaked in oil stain holds that heat in, and a pile of them in a bin or a corner of the garage can reach ignition temperature on its own with no spark, no flame, and nobody in the room. It is a documented cause of house fires every year and it typically happens hours after the work is finished.

Handle it the way the manufacturers instruct: spread used rags out flat and singly on a non-combustible surface outdoors until they are fully dry and stiff, or drop them into a sealed metal container filled with water. Do not bunch them, do not bin them indoors, and do not throw them in the back of a truck. The same applies to used steel wool, sanding dust mixed with oil finish, and the rags used to wipe off excess.

When to call a pro

Chemical stripping of a failed solid stain is the main one — the strippers involved are caustic, the runoff kills plants and stains concrete, and doing it properly means containment and neutralising. Pressure washing is the second: it is the fastest way to ruin a deck, because a tip that is too narrow or held too close carves the soft spring wood out from between the growth rings and leaves permanent furring that no amount of stain will hide. If you do it yourself, use a 25° or 40° tip, keep 12 inches back, work with the grain, and treat the washer as a rinsing tool for a chemical cleaner rather than a cleaning tool in its own right. Structural repairs — soft or rotted joists, loose ledger flashing, wobbly rail posts — should be fixed before any refinishing, and a ledger that was never flashed is a safety issue rather than a maintenance one.

For exterior walls (siding) use the paint calculator with a 250 sq ft / gal coverage assumption.

Frequently asked questions