Painting & Finishing
Deck Stain Calculator
Enter the deck, the railing, and the stairs, and this tells you the gallons. Rough wood drinks far more than smooth.
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| Deck floor | — |
|---|---|
| Railing, both faces | — |
| Stair treads | — |
| Total surface | — |
| Divided by coverage | — |
| Gallons, rounded up | — |
What to buy
Paid links. If you buy through one, we earn a commission. Your price is the same. Nothing is listed here unless the calculator above says you need it.
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Exterior deck stain
The gallons above assume the coverage you entered. Semi-transparent and solid stains cover very differently, so read the can before you trust the number.
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Deck cleaner or brightener
Stain will not penetrate a dirty or greyed deck. Cabot's own instructions say to clean and let the surface dry 24 to 48 hours first.
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Stain pad or natural-bristle brush
Oil-based stains want a natural-bristle brush or a pad. The applicator affects how much you actually use.
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Drop cloths
Stain does not wash off siding, concrete, or plants the way latex paint does.
How to estimate deck stain
Deck stain has one variable that swamps all the others, and it is not the size of your deck. It is how rough the wood is. Get that right and the rest is addition.
square feet = (deck length × width) + (railing length × height × 2) + (treads × 3)
gallons = square feet × coats ÷ coverage per gallonRailing counts twice because you stain both faces. Treads are about 3 sq ft each including the riser.
The coverage number does everything
Cabot's data sheet for its Semi-Transparent Stain and Sealer 300 Series states coverage two ways: approximately 450 to 650 square feet per gallon on smooth surfaces, and 200 to 300 square feet per gallon on rough surfaces.
Read those numbers again. Same can, same product, and the rough figure is roughly a third of the smooth one. Nothing else on this page moves the answer that much. A deck that has been outside for a few seasons, or one built from rough-sawn lumber, is firmly in the rough band, which is why this calculator starts at 250 rather than something cheerier.
Smooth, sealed, previously finished decking sits at the high end. Weathered, fuzzy, or freshly sanded-open wood sits at the low end and can go below it. If you are not sure which you have, assume rough. The failure mode of guessing high is a half-stained deck and a second trip.
One coat, not two
This is where deck stain differs from paint, and where most people get it wrong.
Cabot's instructions say to apply one coat only, and the restrictions section warns that overapplication may result in a glossy appearance, poor drying, or sagging. A penetrating stain works by soaking into the wood. Once the wood has taken all it can, more stain sits on top as a film, and a film that was never meant to be a film peels.
So the default here is one coat. Only choose two if your specific product's label calls for it, which some solid-color stains do because they behave more like paint.
Do not forget the railing
The railing is the most commonly missed surface, and it is not a rounding error.
A 40 foot railing at 3 feet tall is 120 square feet on one face. You stain both faces, so that is 240 square feet. On a 16 by 12 deck, the floor is 192 square feet. The railing is larger than the deck.
And that is before spindles. A railing full of balusters has far more surface than a flat panel of the same dimensions, because each spindle has four sides. Counting the railing at full height on both faces is a deliberate over-estimate that roughly absorbs the spindles. If your railing is unusually ornate, add more.
Stairs
Each tread plus its riser face runs about 3 square feet on a standard 36 inch wide stair. Four steps is about 12 square feet, which is small next to the deck but not nothing. If your stairs are wider, scale it: a 48 inch stair is about 4 square feet a tread.
A quick cost check
Take a 16 by 12 deck with 40 feet of 3 foot railing and 4 steps. That is 192 plus 240 plus 12, so 444 square feet. At 250 square feet per gallon on rough wood, that is 1.78 gallons, so 2 gallons. At 500 on smooth wood, it is 0.89 gallons, so 1 gallon.
At $45 a gallon that is the difference between $90 and $45 on an identical deck. The wood decides, not the deck size.
Common estimating mistakes
- Using the smooth-surface coverage on a weathered deck. The single most expensive assumption on this page.
- Leaving the railing out. It is often bigger than the floor once you count both faces.
- Planning two coats by habit. Paint logic. Most penetrating stains explicitly want one.
- Staining new pressure-treated lumber immediately. Cabot says to test new treated wood for porosity first. If sprinkled water beads instead of soaking in, it is not ready.
- Skipping the cleaning step. Cabot's surface prep calls for a sound, dry, clean surface, and 24 to 48 hours of drying after washing. Stain over dirt fails early no matter how much you bought.
When to stop and call someone
The lead warning on Cabot's own data sheet is the honest one to repeat. If you scrape, sand, or remove old paint or finish, you may release lead dust. On anything built or finished before 1978, test before you sand, and treat a positive result as work for a certified contractor rather than a weekend.
The other boundary is structural. While you are down there measuring, look at the ledger board where the deck meets the house, and at the post bases. Soft wood, rusted fasteners, or movement are not staining problems. A deck that fails does so at those connections, and no finish will hold it together.
Questions
How much does a gallon of deck stain cover?
It depends enormously on the wood. Cabot's data sheet for its Semi-Transparent Stain and Sealer 300 Series states approximately 450 to 650 square feet per gallon on smooth surfaces, but only 200 to 300 square feet per gallon on rough surfaces. That is a three-fold difference for the same product, which is why this calculator defaults to 250 and asks you to check the can.
Do I need one coat of stain or two?
One, on most penetrating stains. Cabot's instructions for its semi-transparent stain say to apply one coat only, and warn that overapplication can cause a glossy appearance, poor drying, or sagging. Stain works by soaking into the wood, so a second coat that cannot penetrate sits on the surface and fails early. Solid-color stains behave more like paint, so follow that specific label.
Why does the railing take so much stain?
Because you stain both faces, and the spindles have far more surface than their length suggests. A 40 foot railing at 3 feet tall is 120 square feet per face, so 240 square feet counted both ways. That is often more area than half the deck floor, and it is the single most common thing people leave out of an estimate.
Does new pressure-treated wood need to weather first?
Usually yes. Cabot's sheet says new treated wood should be tested for porosity before application. Fresh treated lumber is often too wet and too dense to absorb stain properly. The standard test is to sprinkle water on it: if it beads rather than soaking in, the wood is not ready.
How long does deck stain last?
Cabot's data sheet says finish durability depends on weather, wear, and local conditions, and that its finish typically lasts 2 to 5 years on decks and 4 to 8 years on siding and fences. Decks get the short end because they take foot traffic and full sun. Plan on recoating a deck far more often than a fence.
Every constant on this page is logged with its source and the date it was checked in data/sources.md.