Measure the deck floor (length × width), add the surface area of the railings and stairs, then divide the total by how many square feet a gallon of your stain actually covers on your wood. For a first coat of penetrating stain, that is roughly 200–300 square feet per gallon on new, smooth boards and 100–150 on weathered or rough-sawn wood. A second coat typically needs 30–50% less than the first. Read the coverage figure on your stain's label before anything else — it is the manufacturer's number for that product, and it overrides any general figure here.
In practice, a bare 12×16 deck floor needs about one and a quarter gallons for two coats on good wood and about two gallons on weathered wood. Add railings and the total can easily double, because a railing has two faces and dozens of balusters, and it is the part of the job people leave out of their sums.
The Four-Step Method
Every stain quantity calculation is the same four steps. The only judgement involved is picking a realistic coverage rate, and that is where most estimates go wrong.
Step 1: Measure the Deck Floor
Multiply length by width in feet. A 12×16 deck is 192 square feet. For an L-shaped or multi-level deck, split it into rectangles, measure each one, and add them together. Do not subtract the gaps between boards: the gaps remove perhaps 2–4% of the surface, but the board edges you inevitably stain along those gaps, and the end grain at the board ends, drink at least that much back. Length × width is the right number.
Step 2: Add Railings
Measure the total linear feet of railing, then multiply by the railing height, by 2 for both sides, and by a spindle factor that accounts for the balusters, rails, posts and the extra stain lost on narrow parts:
Railing sq ft = linear feet × height (ft) × 2 sides × spindle factor
Use a factor of about 1.25 for a standard railing with 2×2 balusters at roughly 4-inch spacing. The reasoning and the other factors are in the railings section below.
Step 3: Add Stairs and Trim
Allow about 2 square feet per step for every foot of stair width: a 4-foot-wide stair is about 8 square feet per step, including the tread, the riser and the exposed stringer faces. If you are staining the fascia or rim joist around the deck, add about 1 square foot per linear foot of exposed perimeter. Built-in benches and planters are measured like small decks, with every face you plan to stain counted.
Step 4: Divide by Realistic Coverage
Divide the total by the coverage rate for your stain and your wood's condition — the table below gives typical ranges — and repeat for each coat. Add the coats together, add a 10% margin, and round up to whole gallons. That margin covers end grain, a thirsty board or two, spillage and the stain left in the tray, and it is cheaper than a mid-deck trip to the store for a can that may not match.
Coverage by Stain Type and Wood Condition
Coverage varies more with the wood than with the brand. A new, smooth pressure-treated board has a relatively closed surface and takes a modest amount of stain. The same board after five years in the sun is grey, checked and open-grained, and it will soak up close to twice as much. Rough-sawn cedar and redwood behave like weathered wood from day one.
| Stain type | New / smooth wood, 1st coat | Weathered / rough wood, 1st coat | Second coat | Usual coats |
|---|---|---|---|---|
| Transparent / toner oil | 200 – 300 sq ft/gal | 100 – 175 sq ft/gal | Often skipped; if applied, 1.5–2× first-coat area | 1 – 2 |
| Semi-transparent oil (penetrating) | 150 – 300 sq ft/gal | 100 – 150 sq ft/gal | 250 – 400 sq ft/gal (wet-on-wet) | 1 – 2 |
| Semi-transparent water-based / hybrid | 150 – 250 sq ft/gal | 100 – 150 sq ft/gal | 200 – 300 sq ft/gal | 2 |
| Solid colour stain (acrylic) | 200 – 300 sq ft/gal | 125 – 200 sq ft/gal | 250 – 400 sq ft/gal | 2 |
| Hardwood oil (ipe, cumaru, etc.) | 300 – 500 sq ft/gal | 250 – 400 sq ft/gal | 400+ sq ft/gal | 1 – 2 thin coats |
| Deck resurfacer (thick coating) | 50 – 75 sq ft/gal | 40 – 75 sq ft/gal | Similar to first | 2 |
Penetrating stains are limited by what the wood will absorb. A penetrating oil works by soaking in, and once the wood is saturated, extra stain simply sits on the surface, stays tacky, and later peels or wears off in patches. That means the label coverage is close to a ceiling as well as a floor: on new, dense boards you genuinely will not get more in, and trying to is how people end up with sticky decks. It is also why transparent toners are frequently a single coat.
Solid stains behave more like paint. They form a film on the surface, so coverage depends more on film thickness and less on absorption. The second coat is there for opacity and durability, not penetration, and most manufacturers specify two. The difference between solid and semi-transparent goes well beyond quantity — our solid vs semi-transparent comparison covers how each wears and recoats.
Gallons for Common Deck Sizes
The table below is the deck floor only — no railings or stairs. It uses two planning figures: new or smooth wood at 250 square feet per gallon for the first coat and 400 for the second, and weathered or rough wood at 150 for the first coat and 250 for the second. Those are mid-range values for semi-transparent and solid stains; swap in your label's figures where you have them.
| Deck size | Area | New wood, 1 coat | New wood, 2 coats | Weathered, 1 coat | Weathered, 2 coats |
|---|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 0.4 gal | 0.65 gal | 0.7 gal | 1.1 gal |
| 10 × 12 ft | 120 sq ft | 0.5 gal | 0.8 gal | 0.8 gal | 1.3 gal |
| 12 × 12 ft | 144 sq ft | 0.6 gal | 0.9 gal | 1.0 gal | 1.5 gal |
| 12 × 16 ft | 192 sq ft | 0.8 gal | 1.25 gal | 1.3 gal | 2.0 gal |
| 14 × 16 ft | 224 sq ft | 0.9 gal | 1.5 gal | 1.5 gal | 2.4 gal |
| 16 × 16 ft | 256 sq ft | 1.0 gal | 1.7 gal | 1.7 gal | 2.7 gal |
| 16 × 20 ft | 320 sq ft | 1.3 gal | 2.1 gal | 2.1 gal | 3.4 gal |
| 20 × 20 ft | 400 sq ft | 1.6 gal | 2.6 gal | 2.7 gal | 4.3 gal |
These are the calculated quantities, not what to put in the cart. Add your railings and stairs from the next section, add 10%, and then round up to whole gallons. A 12×12 deck with no railing on new wood comes out at 0.9 gallons for two coats, which is one gallon on paper — but with the margin it is a gallon and a quart, and one gallon leaves you with no room for a thirsty board. On anything above about 300 square feet in total, a five-gallon pail usually starts to make sense on price as well as convenience.
Notice how much the wood's condition matters. A weathered 16×16 deck needs about as much stain for one coat as the same deck in new wood needs for two. If you are pricing the job as well as sizing it, our deck staining cost guide turns these quantities into budgets for DIY and professional work.
Railings, Stairs and Fascia
Railings are where stain estimates fall apart. A railing looks like a thin line on a plan, but it has two faces, and every baluster has four sides.
Why the Spindle Factor Is About 1.25
Take one linear foot of a typical 36-inch railing with 2×2 balusters at about 4-inch spacing. There are roughly three balusters in that foot. Each is 1.5 inches square and around 32 inches tall, giving about 1.3 square feet of surface on its four sides — roughly 4 square feet for the three. The top and bottom rails add close to another 1.7 square feet between them, and the posts, spread along the run, add perhaps 0.7. That is around 6–6.5 square feet of real surface per linear foot, close to the 6 you get from 3 feet high × 2 sides.
The factor of 1.25 on top of that is not extra wood; it is extra stain. Narrow parts waste material. A brush loaded for a deck board drips down a baluster, the corners take a second pass, and the end grain on rail cuts and baluster tops soaks up far more than flat grain. On vertical work, plan for using about a quarter more stain than the geometry suggests.
| Railing style | Spindle factor | Sq ft per linear ft (36 in rail) |
|---|---|---|
| Standard 2×2 balusters, ~4 in spacing | 1.25 | ~7.5 |
| Flat horizontal boards or solid privacy panel | 1.0 | ~6 |
| Turned, decorative or closely spaced spindles | 1.5 | ~9 |
| Metal balusters, cable or glass infill (wood rails and posts only) | 0.3 – 0.5 | ~2 – 3 |
| 42-inch rail with 2×2 balusters | 1.25 | ~8.75 |
Railing Add-On Quantities
For a standard 36-inch wooden railing with 2×2 balusters, per coat:
| Railing length | Surface area | New wood (250 sq ft/gal) | Weathered (150 sq ft/gal) |
|---|---|---|---|
| 20 linear ft | 150 sq ft | 0.6 gal | 1.0 gal |
| 30 linear ft | 225 sq ft | 0.9 gal | 1.5 gal |
| 40 linear ft | 300 sq ft | 1.2 gal | 2.0 gal |
| 50 linear ft | 375 sq ft | 1.5 gal | 2.5 gal |
| 60 linear ft | 450 sq ft | 1.8 gal | 3.0 gal |
Forty feet of railing — three sides of a modest 12×16 deck — has more surface than the deck floor itself. That is the single biggest reason people run out of stain.
Stairs
Each step has a tread of about 11 inches (two 5.5-inch boards), a riser of about 7.5 inches, and short lengths of exposed stringer at each side. Tread plus riser comes to roughly 1.5 feet of depth for each foot of width, and the stringers, nosing and end grain push it to about 2. Open-riser stairs with no riser boards come in nearer 1.25.
| Stairs | Sq ft per step | 3 steps | 5 steps | 8 steps |
|---|---|---|---|---|
| 3 ft wide | ~6 | 18 sq ft | 30 sq ft | 48 sq ft |
| 4 ft wide | ~8 | 24 sq ft | 40 sq ft | 64 sq ft |
| 5 ft wide | ~10 | 30 sq ft | 50 sq ft | 80 sq ft |
| Stair railing, per side | Measure along the slope (about 1.1 ft per step) and treat as railing at ~7.5 sq ft per linear ft | |||
Stairs themselves are a small part of the total. Stair railings are not: five steps with railings on both sides adds about 11 linear feet of railing, or over 80 square feet.
Fascia, Rim Joists and Undersides
If the rim joist and fascia board are part of the look, add about 0.75–1 square foot per linear foot of exposed perimeter; a 2×10 rim is about 9.25 inches tall, and with the top edge and the board ends it works out close to a foot. Joists and board undersides are usually left unstained; if you want an elevated deck's underside to match, measure it as a second deck floor plus joist faces, which roughly doubles the job.
Worked Example: A 16×20 Deck
Here is a complete calculation for a common layout: a 16×20 deck attached to the house along one 20-foot side, railings on the other three sides, a 4-foot-wide opening for five steps down to the yard, stair railings on both sides, and the fascia stained on the three exposed sides. The wood is five-year-old pressure-treated pine, weathered grey, and the plan is a semi-transparent oil-based stain applied in two coats wet-on-wet.
| Item | Calculation | Square feet |
|---|---|---|
| Deck floor | 16 × 20 | 320 |
| Deck railing | (16 + 20 + 16 − 4 opening) = 48 ft × 3 × 2 × 1.25 | 360 |
| Stair railings | 2 sides × ~6 ft along slope = 12 ft × 3 × 2 × 1.25 | 90 |
| Stairs | 5 steps × 4 ft wide × 2 | 40 |
| Fascia | 52 ft of exposed perimeter × ~1 | 52 |
| Total | 862 |
First coat on weathered wood at 150 square feet per gallon: 862 ÷ 150 = 5.7 gallons.
Second coat at 250 square feet per gallon: 862 ÷ 250 = 3.4 gallons.
Total 9.2 gallons, plus 10% = about 10 gallons. Buy two five-gallon pails from the same batch and one extra gallon in the same colour as a safety margin, kept sealed until you need it.
Run the same deck on new, smooth wood at 250 and 400 square feet per gallon and the total is 3.4 + 2.2 = 5.6 gallons, or about 6 with the margin. The deck has not changed; the wood has, and that alone moves the order by four gallons. Measured as floor-only — the most common shortcut — it would come to 3.4 gallons, about a third of what the job takes.
If the manufacturer's label for your chosen stain says one coat only, use the first-coat figure and stop there: 5.7 gallons plus margin, so a five-gallon pail and two single gallons. For the full prep-to-cleanup process, our step-by-step deck staining guide covers cleaning, brightening, application and timing.
Why the Second Coat Uses Less
The first coat fills the most open pores, end grain and checks in the wood. The second coat lands on a surface that is already partly saturated, so it spreads further — typically 30–50% further, which is why the planning figures above step up from 250 to 400 on new wood and from 150 to 250 on weathered wood.
How the second coat is applied depends on the stain, and the label is specific about it. Penetrating oils are usually applied wet-on-wet: the second coat goes on while the first is still wet, often within 20–30 minutes, so that both soak in together. Wait until the first coat dries and the wood may refuse the second, leaving it tacky on the surface. Some products say one coat only on new or dense wood for exactly this reason. Water-based semi-transparent and solid stains generally want the first coat dry — often a few hours, sometimes overnight — before the second. Our deck stain drying time guide gives recoat windows by stain type and weather.
What Changes Your Real Coverage
Wood condition. The biggest factor by far, as the tables show. Grey, checked, weathered boards absorb far more than new ones. A deck that has just been stripped or sanded is also more open and thirstier than it was before — expect to sit toward the low end of the coverage range after stripping old stain.
New pressure-treated wood. Fresh treated lumber is often still wet from treatment and may carry a smooth "mill glaze" that resists absorption. It needs to dry before staining — our pressure-treated wood guide covers how long to wait and how to test it. Once it is ready, new treated wood usually sits at the higher end of the coverage range.
Application method. Pads and brushes put the most stain into the wood with the least waste. Rollers are fast on the floor but load heavily. An airless sprayer is quick on railings, but overspray and the back-brushing that should follow it mean planning for roughly 15–25% more stain than brushing the same area.
Weather. Hot, sunny, windy conditions make stain flash off quickly, so it goes on less evenly and people tend to over-apply to keep a wet edge. Staining in the shade at moderate temperatures, as covered in our guide to deck staining temperatures, gives better coverage as well as a better finish.
Buying Tips
Buy the whole job at once, from the same batch. Factory-coloured stains carry a batch or lot number on the can, and tinted stains are mixed at the store. Either way, two purchases a week apart can differ slightly in colour, and on a deck floor that shows as a line where you switched cans. If you end up with cans from different batches, or several separately tinted gallons, box them: pour them all into a five-gallon bucket and mix, then work from that.
Keep an extra quart for touch-ups. High-traffic spots at the door, the top step and around the grill wear first, and touching them up in a year or two with the same stain blends far better than a fresh purchase. Write the colour name, formula and date on the lid, and store it somewhere it will not freeze if it is water-based.
Check the return policy before tinting. Unopened factory-colour gallons are usually returnable; custom-tinted stain usually is not. If you are buying a generous margin, keep the extra in a stock colour, or accept that the margin is the price of not running out.
Price per gallon falls with pail size. Five-gallon pails generally cost 10–20% less per gallon than single cans, so on any job needing more than about three gallons, compare the pail price before buying singles. Product choice matters more than container size, though — our deck stain comparison covers how the main options differ in durability, coats required and coverage.
Do not stretch it. Thinning stain or spreading it beyond the label's coverage to make a can last does not save money. A starved coat wears through in a season, and the next recoat costs more prep than the stain you saved.
Dispose of oily rags safely. Rags and pads soaked in oil-based stain can heat up and ignite as they dry. Lay them flat outside to dry, or keep them submerged in a sealed metal can of water until disposal.
Fences, Benches and Other Wood
The same method works for any wood structure. For a fence, measure length × height for each side you plan to stain. The back side of a fence, with its posts and rails, carries a little more surface than the front — use a factor of about 1.1–1.2 for the rail side. Picket fences with gaps can be measured as solid and then reduced by about a third, though the extra edges and end grain eat into that saving.
Fence boards are often rough-sawn, which pushes coverage down: 100–150 square feet per gallon is realistic for rough cedar or dry, weathered pine, against 200–250 for smooth boards. A 100-foot run of 6-foot privacy fence stained on both sides is about 1,300 square feet with the rail factor, which is roughly 9 gallons per coat at 150 square feet per gallon. Fences take no foot traffic, so one coat of a semi-transparent stain is often enough. Our fence stain guide compares products made for vertical, rough-sawn surfaces.
The Short Version
Measure the deck floor, then add the railings — about 7.5 square feet per linear foot for a standard 36-inch railing — plus stairs and fascia. Divide by 250 square feet per gallon for a first coat on new, smooth wood or 150 on weathered or rough wood, then add 60% of that for a second coat. Add 10%, round up, and buy it all at once from one batch, with a quart set aside for touch-ups.
And before using any of these figures, read the label. The manufacturer's coverage range and coat instructions are specific to that product, and on penetrating stains in particular, putting on more than the wood can absorb causes more problems than running short.
Frequently Asked Questions
A 12×12 deck floor is 144 square feet. On new, smooth wood one coat needs about 0.6 gallons and two coats about 0.9 gallons; on weathered or rough wood one coat needs about 1 gallon and two coats about 1.5. With a 10% margin, that means one gallon for a single coat on new wood, and two gallons for two coats or for weathered wood. Railings change the answer completely: 30 feet of standard 36-inch railing adds about 225 square feet, more than the floor, so a railed 12×12 deck on weathered wood can need around four gallons for two coats.
Most deck stains cover 150–300 square feet per gallon for a first coat, and the wood's condition decides where in that range you land. New, smooth boards sit around 200–300; weathered, grey, checked or rough-sawn wood is closer to 100–150. A second coat typically goes 30–50% further than the first because the wood is partly saturated. The label on your can gives the manufacturer's range for that product and should be your starting point.
It depends on the stain, and the label tells you. Solid and most water-based semi-transparent stains specify two coats, with the first allowed to dry before the second. Many penetrating oil stains call for two coats applied wet-on-wet, the second going on within about 20–30 minutes of the first, and some say one coat only on new or dense wood because the wood cannot absorb more. Applying a second coat of penetrating stain when the wood will not take it leaves a tacky surface that peels or wears unevenly. Weathered, thirsty wood benefits most from a second coat.
Far more than most people expect. Use linear feet × railing height × 2 sides × a spindle factor of about 1.25 for standard 2×2 balusters. For a 36-inch railing that comes to about 7.5 square feet per linear foot, so 40 feet of railing is roughly 300 square feet — about 1.2 gallons per coat on new wood and 2 gallons on weathered wood. Turned or closely spaced spindles push the factor to about 1.5; metal or cable balusters, where only the wood rails and posts are stained, drop it to 0.3–0.5. On many decks the railings need as much stain as the floor.
A 20×20 deck floor is 400 square feet. On new wood that is about 1.6 gallons for one coat and 2.6 for two; on weathered wood about 2.7 for one coat and 4.3 for two. With a 10% margin, buy three gallons for two coats on new wood and a five-gallon pail for two coats on weathered wood. Add railings and stairs separately — a 20×20 deck railed on three sides has around 55 feet of railing, which is roughly another 400 square feet, close to doubling the order.
Yes. An airless or pump sprayer loses stain to overspray and wind, and most manufacturers recommend back-brushing sprayed stain to work it into the wood, which moves some of it around rather than adding to coverage. Plan on roughly 15–25% more stain when spraying than when brushing or padding the same area. Spraying still earns its keep on railings, where it saves hours.
Usually because the wood was thirstier than the label assumes. Label coverage ranges are written with the high end for smooth, sound wood; weathered, grey, checked, recently stripped or rough-sawn boards fall at or below the low end. Other common causes are application by roller or sprayer, which load more stain than a pad or brush; hot, sunny or windy conditions that make stain flash off and encourage over-application; and forgetting the end grain, edges and railings when measuring. A quick quart test on a measured area before buying the full amount gives you your deck's real coverage rate.