Deck Stain Calculator — Gallons for Boards and Rails
The deck surface is the easy part. Railings, spindles and the fascia usually add half again, and stain on rough or weathered wood covers nothing like the figure on the can.
Take this list to the store
We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
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Measure the walking surface
Length times width. This is the number everybody starts with and it is usually less than half the job.
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Add the railing properly
A spindle railing has a top rail, a bottom rail, and four faces on every baluster. Measured honestly it comes to roughly four times the area of the rectangle it occupies. Forty feet of three foot railing is 120 sq ft of elevation and close to 480 sq ft of actual surface.
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Be honest about the wood
The can says 250 to 350 square feet per gallon. That is smooth, sealed wood. Rough sawn or weathered gray decking drinks it — 150 or even 125 square feet per gallon is realistic. Guess high on coverage and you will be back at the store mid-job with a half-stained deck.
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Buy it all in one batch
Stain varies between batches. Buy the whole quantity at once and, if it runs to several cans, box them — pour them all into one container and mix — before you start. A color change halfway across a deck does not blend out.
Coverage by surface
Square feet per gallon, one coat.
| Surface | Semi-transparent | Solid stain | Clear sealer |
|---|---|---|---|
| Smooth, previously sealed | 250–300 | 300–400 | 300–400 |
| Normal, a few seasons old | 200–250 | 250–300 | 250–300 |
| Rough sawn | 150–200 | 175–250 | 175–250 |
| Weathered gray, bare | 100–150 | 150–200 | 150–200 |
Manufacturer figures are for the best case. Assume the lower end of the range.
What a railing actually adds
40 ft of railing, 3 ft high — 120 sq ft of elevation.
| Style | Multiplier | Real surface | Extra gallons at 200 sq ft/gal |
|---|---|---|---|
| Spindles, 2 in. square | × 4 | 480 sq ft | 2.4 |
| Spindles, 1 in. round | × 3.5 | 420 sq ft | 2.1 |
| Solid panel | × 2.2 | 264 sq ft | 1.3 |
| Cable or glass infill | × 1.2 | 144 sq ft | 0.7 |
A spindle railing can easily need more stain than the deck it surrounds.
How much stain for a 16 × 20 deck?
The boards are 320 square feet, which at 200 square feet per gallon is 1.6 gallons. But put a forty foot spindle railing round it and you add roughly 480 square feet of surface — another 2.4 gallons. Add the steps and you are ordering five gallons, not two.
That is the whole reason this calculator exists. Almost every deck stain project that runs short ran short on the railing.
Why the can lies
It does not, quite — it quotes coverage on the surface the manufacturer tested, which is smooth, sound, previously finished wood. Almost no deck being stained is in that condition, because if it were you would not be staining it.
Weathered wood has opened up. The grain has raised, the surface is fibrous, and it takes up stain like a sponge. On genuinely gray, bare decking you can be down to 125 square feet per gallon on the first coat, and the second coat will then go much further because the first one sealed it.
One coat or two?
Most semi-transparent deck stains are designed for one coat, and a second coat on top of a dry first one often will not penetrate — it sits on the surface, stays tacky, and peels within a season. If you want more color, the answer is a darker stain rather than more coats.
Solid stains and bare new wood are the exceptions where two coats are normal. If you do go twice, apply wet-on-wet within the window the manufacturer specifies, usually less than an hour.
Preparation decides how much you use
A deck that has been cleaned and brightened takes stain evenly and uses less of it. A deck with mill glaze, old failing finish or ground-in dirt takes it unevenly, which means you use more trying to even it out and it still looks patchy.
Cleaning is not optional and it changes the arithmetic. Budget the day.
Do not stain in the sun
Stain on hot boards flashes off before it can penetrate, which leaves lap marks and a finish that has not soaked in. Work in the shade or on an overcast day, follow the sun around the deck rather than chasing it, and keep a wet edge along whole boards.
Stopping mid-board is how you get a visible line that no amount of subsequent work removes. Finish the board you are on.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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railing area = length × height × style multiplierSpindles have four faces each; a rectangle of elevation badly understates it. -
step area = steps × width × 1.5 sq ftEach step is a tread plus a riser. -
total surface = deck + railings + stepsThe railing is routinely the largest of the three. -
gallons = total surface × coats ÷ coverageCoverage depends on how thirsty the wood is, not on the product.
Common questions
How much stain do I need for a 500 sq ft deck?
About 2.5 gallons for the boards alone at 200 sq ft per gallon — but add the railing and it is closer to five. Work the railing out separately; it is usually the bigger number.
How many square feet does a gallon of deck stain cover?
200 to 250 on normal decking, 250 to 300 on smooth sealed wood, and as little as 125 on weathered gray boards. The high figures on the can assume ideal conditions that a deck due for staining will not be in.
Do I need to stain the underside of the boards?
On a new deck before it is laid, sealing all four sides is genuinely worthwhile — it slows cupping. On an existing deck it is impractical and not worth taking apart for. Do the edges and ends you can reach.
Can I stain over old stain?
Over the same type, yes, once it has been cleaned. Semi-transparent over solid will not work — the solid has sealed the surface so nothing penetrates. Going the other way is fine. If you do not know what is on there, test a small area first.
How long does deck stain last?
Two to three years for semi-transparent on a horizontal surface, three to five for solid, and longer on vertical surfaces like railings that do not take foot traffic or standing water. Anything that promises ten years on a deck floor is promising about a wall.
What is the multiplier for spindle railings?
About four times the area of the rectangle the railing occupies. Every baluster has four faces, plus the top and bottom rails and both sides of everything. Round spindles are slightly less at around 3.5.
Should I use a roller, brush or sprayer?
Spray to apply and then back-brush immediately to work it into the grain. Spraying alone leaves it sitting on top. On railings and spindles a brush is faster than it sounds, because overspray on a spindle railing mostly lands on the ground.
How long before it rains?
Check the can, but plan on at least 24 to 48 hours clear. Rain on stain that has not cured lifts it and leaves blotches. The forecast matters more than the calendar here — a dry week beats a convenient weekend.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- Manufacturer application instructions — coverage rates by surface condition
- Forest Products Laboratory, Wood Handbook — finish penetration and weathering
- Western Wood Products Association — exterior finishing guidance
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 15, 2026.