Enter your deck's length and width, add the railing footage and how many coats you're
applying, and get the number of gallons to buy. The default coverage is 250 square feet
per gallon — a typical figure for semi-transparent stain — and you can change
it for clear sealers (more) or solid stains (less). Wood condition matters a lot, so the
rule is simple: buy a little extra.
Gallons from deck size & coats·Counts railings and spindles·Stain or sealer coverage
Read this first
This is an estimate based on a typical coverage figure, not a substitute for the number
printed on your can or for your own measurements. Real coverage swings widely with wood
condition: old, rough, sun-bleached, or freshly pressure-treated boards drink up far more
than a smooth test panel. Always buy a little extra — a second store run mid-project
is worse than a partly used can on the shelf — and follow the directions on the
product you buy.
The calculator
Estimate gallons of stain or sealer
Enter the deck's length and width in feet, the total railing footage if any, the number of coats, and the coverage your product claims per gallon. You'll get the total surface area, the coated area across all coats, and the gallons to buy — rounded up to whole cans.
Optional. Counts about 2 sq ft of surface per linear foot for rails & spindles.
2 is standard for most stains; clear sealers are often 1.
Editable. Thicker or closer spindles use more; cable/glass rail uses less.
Deck floor area
Railing surface
Total surface area
Coated area (all coats)
Coverage used
Gallons (exact)
The math, honestly
How the gallon count is figured
It's all surface area times coats, divided by coverage. First, the deck floor:
length × width in feet gives the floor area in square feet. Then the
railing: each linear foot of railing run carries roughly 2 square feet
of stainable surface once you count the top rail, bottom rail, and the spindles between,
so railing feet × 2 gets added on. That's your
total surface area.
Multiply the surface by the number of coats — you're coating the
same wood twice for a two-coat job — then divide by the product's
coverage per gallon and round up, because you buy whole cans:
gallons = ceil(surface × coats ÷ coverage). For a
12 × 16 deck with no railing and two coats at 250 sq ft/gal,
that's 192 × 2 ÷ 250 = 1.54, which rounds up to
2 gallons.
Why coverage is the slippery number: the 250 sq ft/gallon default is a
middle-of-the-road figure for semi-transparent stain. Clear sealers are thinner and
stretch to 300–400; solid stains are thicker and cover closer to 200. And no
matter the product, rough or weathered wood drinks far more than the label promises.
Calculate from the can's number, then buy a little extra.
Coverage by product type
Typical coverage per gallon for the main deck-finish types. These are starting points for
smooth, sound wood — the figure on your specific can is the one to trust, and rough
or weathered boards will lower every number here.
Product type
Typical coveragesq ft per gallon
Noteswhy it differs
Clear sealer
300–400
Thin, penetrating, little to no pigment — stretches the furthest.
Semi-transparent stain
250
The calculator default. Some pigment, still penetrates the grain.
Semi-solid stain
200–250
More pigment and solids than semi-transparent; covers a bit less.
Solid / opaque stain
200
Thickest, most pigment, forms a film — covers the least.
Figures are typical published ranges for smooth, sound lumber and assume normal
application. Old, rough-sawn, or sun-dried wood absorbs more and can cut real coverage
well below these numbers. Always confirm against the coverage printed on the can.
Common deck sizes
Worked examples for a few common deck footprints, computed with the same formula at the
250 sq ft/gallon default — so these match what the calculator gives you. Gallons are
shown with two coats and no railing, then rounded up to whole cans.
Deck
Floor areasq ft
Coated area2 coats
Gallons2 coats, no railing
10 × 10
100
200
1
12 × 16
192
384
2
16 × 20
320
640
3
20 × 24
480
960
4
Coated area is floor area × 2 coats; gallons are that figure ÷ 250 sq ft per
gallon, rounded up. Add railing and the gallon count climbs — 40 ft of railing adds
about 80 sq ft of surface, or 160 sq ft across two coats.
Reading the result well
A gallon number is only useful if you act on it sensibly. Four things worth knowing before
you buy.
Treat coverage as a best case, not a promise
The 250 sq ft/gallon default and the can's printed figure both assume smooth, sound, moderately absorbent wood. Your deck is probably none of those after a few seasons outside. Sun, rain, and foot traffic open the grain so the boards drink more, especially on that first coat. If your deck is old or rough, plan on lower real coverage and buy extra rather than calculating to the exact gallon.
Don't forget the railings
Railings and spindles are easy to under-estimate because each baluster has four faces and there are a lot of them. The 2 sq ft per linear foot rule of thumb usually lands close, but if your spindles are thick, ornate, or tightly spaced, raise that figure. An open cable or glass rail uses far less. Either way, leaving the railing out of the math is the most common reason people run short.
Two thin coats, not one thick one
For penetrating stains, the wood can only absorb so much at a time. Flooding it with one heavy coat leaves excess sitting on the surface that never cures, staying tacky or peeling later. Apply a thin first coat, let it soak in, then add a second thin coat where the wood still wants it. That's why the coat count multiplies the area rather than the can: you're covering the same surface twice, lightly.
Buy whole gallons — and round up
You can't buy a fraction of a can, so the calculator rounds every gallon count up. Keep the leftover until the job is done; a partly used can is cheap insurance and useful for touch-ups. Running short mid-project means a store run, a possible color batch mismatch, and a lap line where the dried edge meets the fresh coat. When in doubt, round up again.
Where to buy
Got your numbers? Here's where to pick up what you need:
The terms behind the calculator, in plain English. These are background definitions, not
manufacturer specifications — follow the directions on the product you buy for the
real thing.
Coverage (spread rate)
How much area one gallon of a finish covers, stated in square feet per gallon. It's the number that drives the whole calculation. Semi-transparent stain runs about 250 sq ft/gal; thinner sealers cover more and thicker solid stains cover less. The figure on the can is a best case for good wood.
Semi-transparent stain
A penetrating finish with enough pigment to add color and UV protection while still letting the wood grain show through. It's the most common choice for residential decks and the basis for the calculator's 250 sq ft/gallon default.
Clear sealer
A thin, often water-repellent finish with little or no pigment. Because it penetrates rather than building a film, it stretches the furthest — roughly 300 to 400 sq ft/gallon — but offers the least UV protection and needs reapplying more often.
Solid (opaque) stain
A heavily pigmented, film-forming finish that hides the grain like a thin paint. It carries the most solids, so it covers the least — around 200 sq ft/gallon — and is more prone to peeling on horizontal surfaces than a penetrating stain.
Surface area
The total stainable area: the deck floor (length × width) plus the railing surface. It's measured before coats are applied; multiplying by the coat count gives the coated area the gallons are figured from.
Railing factor
A rule of thumb for how much stainable surface a railing adds per foot of run — about 2 square feet per linear foot, accounting for the top rail, bottom rail, and spindles. It's editable in the calculator because ornate or tightly spaced spindles use more and open cable or glass rails use far less.
Coat
A single application of finish over the whole surface. Most penetrating stains take two thin coats; clear sealers are often one. Coats multiply the area you have to cover, which is why a two-coat job needs roughly twice the gallons of a one-coat job.
Pressure-treated lumber
Wood chemically treated to resist rot and insects, common in deck framing and boards. Fresh pressure-treated wood is often too wet to accept stain and must dry out for weeks to months first. Once dry it can still be inconsistently absorbent, which is one more reason real coverage drifts from the label.
Frequently asked
Take the deck floor area (length × width in feet) and add about 2 square feet for every linear foot of railing. Multiply that surface by the number of coats — usually two for stain — then divide by the coverage on the can. A common figure for semi-transparent stain is 250 sq ft/gallon. A 12 × 16 deck is 192 sq ft, so two coats at 250 is 192 × 2 ÷ 250 = 1.54, which rounds up to 2 gallons. Rough or weathered wood soaks up more — buy a little extra. Try it in the calculator.
Two coats is the standard for most penetrating and semi-transparent stains, and it's the default here. The second coat evens out color and improves protection, especially on the floor that takes the most sun, water, and foot traffic. But don't over-apply — if the wood won't absorb a second coat, stop, because excess stain sitting on top can get tacky or peel. Clear sealers are often a single coat. Always follow the directions on the can.
It comes down to how much pigment and solids the product carries. Clear and water-repellent sealers are thin and penetrate, so they stretch furthest — roughly 300 to 400 sq ft/gallon. Semi-transparent stains sit in the middle at about 250. Solid (opaque) stains are thickest and cover the least, around 200. These are typical figures; the number on your specific can is the one to trust. The coverage table lays them out side by side.
Railings are deceptively thirsty because every baluster has four sides. A reasonable rule of thumb is about 2 square feet of stainable surface per linear foot of railing run, covering the top rail, bottom rail, and the spindles between. This calculator uses that figure and lets you change it. If your spindles are thick or closely spaced, bump it up; an open cable or glass rail uses far less. Forgetting the railing is the most common reason people run short.
Weathered, old, and rough-sawn wood uses noticeably more. Sun and rain open up the grain and dry out the surface, so it acts like a sponge and the first coat can vanish into the boards. New, smooth, or previously sealed wood is less absorbent and the label coverage is closer to reality. Pressure-treated lumber also needs weeks to months to dry before it will accept stain at all. When in doubt, assume your real coverage is lower than the can claims and buy extra.
For penetrating stains, yes — two thin coats almost always beat one heavy one. The wood only absorbs so much at once; flooding it leaves excess sitting on top that never cures properly and can stay tacky or peel. Apply a thin first coat, let the wood drink it in, then add a second thin coat where the wood still wants more. It's also why the coat count multiplies the area in the math: you're coating the same surface twice, not piling on twice the pigment at once.
A 12 × 16 ft deck floor is 192 square feet. With no railing, two coats of semi-transparent stain at 250 sq ft/gallon is 192 × 2 ÷ 250 = 1.54 gallons, which rounds up to 2 gallons. Add railing and it climbs: 40 ft of railing adds about 80 sq ft of surface, pushing the two-coat total to (192 + 80) × 2 ÷ 250 = 2.18 gallons, or 3 gallons rounded up. Buy a little extra for thirsty or weathered boards.
Because real wood isn't a flat test panel. Label coverage assumes smooth, sound, moderately absorbent lumber. Your deck might be rough-sawn, sun-bleached, partly rotted, or freshly pressure-treated — each changes how thirstily it drinks. Application matters too: a roller wastes more than a pad or sprayer, and a hot, windy day flashes the surface off faster. Treat the can's number as a best case, calculate from it, then buy a little extra so you aren't making a second store run mid-project.