How Much Deck Stain Do I Need?

Nobody has ever regretted the spare gallon on the shelf. Plenty of people have regretted the half-finished deck and the can from a different batch that dried two shades darker.

1. Measuring the Deck Properly

Almost every under-buy we hear about comes from the same place: someone measured the floor and stopped. The deck boards are the biggest single surface, but on a typical deck with a railing and a set of steps they are only about half of what you will actually stain. So measure everything, write it down, and add it up before you look at a single coverage number.

Deck Boards

Length times width, in feet, gives square feet. A 12 by 16 deck is 192 square feet. For an L-shaped or wrap-around deck, break it into rectangles and add them. Do not subtract for the gaps between boards; the stain that falls into the gaps is still used, and the board edges soak it up. Do not bother subtracting for the footprint of posts either — it comes back as post surface anyway.

Railings

This is the one everybody forgets, and it is the one that drinks the most stain per square foot of deck because there is so much surface packed into it. Picture a standard 36-inch code-height railing: a cap rail, a top rail, a bottom rail, 4-by-4 posts every six to eight feet, and 2-by-2 balusters on 4-inch centres. Every one of those balusters has four faces, and you stain all of them from both sides of the rail. When you add up the cap, the rails, the posts and both sides of every baluster, a running foot of that railing carries roughly 3 to 4 square feet of stainable surface. So the rule of thumb is simple: linear feet of railing times 3.5, and use 4 if the balusters are close or the posts are chunky. A 36-foot run of railing is therefore about 125 square feet, which is two-thirds of the deck floor on our example deck. A solid privacy screen or a horizontal-slat rail is easier: height times length times two for both faces.

Stairs

Each step has a tread and a riser, and the whole flight has two stringers on the outside, sometimes a third in the middle that you can ignore because nobody sees it. A 4-foot-wide tread made from two 2-by-6 boards is about 4 square feet. A riser board at 7 inches tall is a little over 2 square feet. Call each step 6 to 7 square feet including the exposed ends. Stringers are triangular — roughly half the total run times the total rise, times two for both sides — and on a four-step flight come to about 10 square feet. So four steps at 4 feet wide add up to around 35 square feet. Steps also take a beating, so many people give them a third coat, which is worth allowing for.

Fascia, Skirting and Rim Joists

The band of board around the outside edge of the deck — the fascia or rim joist — is usually a 2-by-10 or 2-by-12, so figure about 0.8 to 1 square foot per linear foot of exposed perimeter. On the 12 by 16 deck, with one long side against the house, that is 12 plus 16 plus 16, or 44 linear feet, and about 40 square feet. If the deck has lattice skirting or vertical board skirting down to the ground, measure it as height times length and add it; lattice in particular has a lot of surface for its footprint and takes stain badly, so use the rough-wood coverage figure for it.

The Underside

Most people do not stain the underside of a deck, and on a low deck you cannot. But on a raised deck with a patio or a walkout basement beneath it, the underside of the boards and the joists are visible and often get finished. If so, the board underside is the same area as the top, and the joists add roughly 1.5 square feet per linear foot of joist (both faces plus the bottom edge). It is a lot of surface and a lot of stain; price it before you commit to it.

2. Coverage Rates by Stain Type

Every can states a coverage range, and every range is wide because the product does not know what you are putting it on. Here is what we find those ranges mean in practice on real decks, first coat, applied by pad or brush.

Penetrating Oil Stains

Products like Ready Seal, Cabot Australian Timber Oil, TWP and Armstrong-Clark are designed to soak in, and how far they go depends entirely on how much the wood can drink. On rough-sawn, weathered, freshly sanded or freshly stripped softwood, expect 150 to 250 square feet per gallon on the first coat, and be prepared for the low end. On smooth, planed, new wood the same gallon stretches to 250 to 400 square feet because there is less open fibre to fill. Old pressure-treated pine that has been baking in the sun for a decade is the thirstiest surface we deal with; it is not unusual to see 150 or less.

Water-Based Semi-Transparent Stains

DEFY Extreme, Behr Premium Semi-Transparent, Olympic Elite and the like are acrylic or acrylic-alkyd hybrids that penetrate a little and film a little. They tend to be more predictable than oils because they do not disappear into the wood the same way. Plan on 250 to 350 square feet per gallon on the first coat, edging down to 200 on very rough wood.

Solid Stains

Solid-colour stains are essentially thin paints. They sit on the surface, they hide the grain, and they cover more consistently. Expect 300 to 400 square feet per gallon per coat. Because solids are a film, the second coat covers almost as far as the first, and on previously coated wood a single coat is often specified.

Second Coats

The first coat fills the open fibre; the second has much less to fill. On penetrating products, a second coat goes 30 to 50 per cent further than the first, so if the first coat ran at 200 square feet per gallon, the second will run at 260 to 300. On water-based semi-transparents the gain is similar. On solids it is small, so budget the second coat at the same rate as the first. This is why a two-coat job does not need double the stain — it needs roughly 1.6 to 1.75 times the first coat.

3. What Changes the Coverage

Porosity and Age

Wood is a bundle of straws. The more open the ends of those straws, the more stain goes in. New planed wood has compressed, partly closed fibre. Weathered wood has had its surface lignin broken down by ultraviolet light and its fibres opened by wetting and drying, so it is far more absorbent. Freshly sanded or stripped wood has had the closed surface cut away and is very absorbent. Wood that was previously stained and is being recoated still carries some of the old resin and absorbs less. As a rough guide: previously stained, smooth new, weathered, sanded — in increasing order of thirst.

Species

Southern yellow pine, the usual pressure-treated species, is moderately dense but has broad, open earlywood bands that soak up stain. Cedar and redwood are soft and porous and take a lot on the first coat but are consistent. Douglas fir is denser and takes less. Tropical hardwoods like ipe, cumaru and tigerwood are so dense that most stains barely penetrate, and coverage can exceed 400 square feet per gallon, but they also need a specific hardwood formula or the stain will never dry. If you are staining cedar, the specific product recommendations in our best deck stain for cedar guide matter more than the exact coverage figure.

Application Method

How you put it on changes how much you use. A stain pad on a pole applicator or a roller followed by back-brushing is the most efficient method; nearly every drop ends up on wood. A brush alone is efficient but slow. A pump-up garden sprayer wastes a little to drift. An airless or HVLP sprayer, which is the fastest way to do rails and lattice, loses 20 to 30 per cent to overspray, to the gaps between boards and to the fact that sprayers tend to lay product on heavier than a pad. If you plan to spray, multiply your total by 1.25. The trade-offs between methods are covered in detail in brush vs roller vs sprayer for deck stain.

Temperature and Sun

Hot boards in direct sun flash off the solvent before the stain can spread, so you end up laying it on thicker to get the same look and using more product. Staining in the shade or on a cool overcast day lets the stain flow further and soak in properly. It is better for the finish and better for the budget.

4. The Formula With Worked Examples

Here is the whole calculation in four lines. It is the same arithmetic we use in our driveway sealer calculator, just with more surfaces to remember.

Step one: total square feet equals deck boards plus (railing linear feet times 3.5) plus stairs plus fascia and skirting plus any underside.
Step two: first-coat gallons equals total square feet divided by the first-coat coverage rate for your product and wood.
Step three: second-coat gallons equals total square feet divided by (first-coat rate times 1.4), or the same as the first coat for a solid.
Step four: add the coats, multiply by 1.15 for margin, multiply by 1.25 again if spraying, and round up to whole gallons.

Worked Example: 12 by 16 Deck, 36 Feet of Railing, Four Steps

Start with the surfaces. The boards are 12 times 16, which is 192 square feet. The railing is 36 linear feet times 3.5, which is 126 square feet. Four steps at 4 feet wide, including risers and stringers, come to about 35 square feet. The fascia on three exposed sides is 44 linear feet at roughly 0.9 square feet each, call it 40. Add them: 192 plus 126 plus 35 plus 40 is 393 square feet. Round to 400 square feet. Notice that the boards you measured first are less than half of the total.

Example A, penetrating oil on ten-year-old treated pine. This is weathered, thirsty wood, so use 200 square feet per gallon for the first coat. 400 divided by 200 is 2 gallons. The second coat runs at about 200 times 1.4, or 280, so 400 divided by 280 is 1.4 gallons. Total 3.4 gallons. Add 15 per cent for margin and you are at 3.9. Buy 4 gallons, or a 5-gallon pail if the per-gallon price is better, which it usually is.

Example B, water-based semi-transparent on the same deck. Use 300 square feet per gallon for the first coat. 400 divided by 300 is 1.33 gallons. The second coat at 300 times 1.4 is 420, so 400 divided by 420 is 0.95 gallons. Total 2.3 gallons, plus 15 per cent is 2.6. Buy 3 gallons.

Example C, solid stain over a previously coated deck. Use 350 square feet per gallon and, because it is a film, the same rate for both coats. 400 divided by 350 is 1.15 gallons per coat, 2.3 gallons for two. Plus 15 per cent is 2.6. Buy 3 gallons. If the old finish is sound and the can says one coat is enough for a recoat, you could get away with 2, but the third gallon is your touch-up supply for the next five years.

Example D, Example A but sprayed. Take the 3.9 gallons from Example A and multiply by 1.25 for spray loss: 4.9 gallons. Buy a 5-gallon pail and have a brush ready to back-brush every board, because sprayed penetrating stain that is not brushed in is the leading cause of the sticky deck we describe in deck stain not drying.

A Note on Smooth New Wood

Run the same deck as brand-new smooth cedar with a penetrating oil at 325 square feet per gallon: 1.23 gallons for the first coat, about 0.9 for the second, 2.1 total, 2.45 with margin. Buy 3 gallons. New wood is cheaper to stain than old wood, but it is also the case where you are most likely to over-apply, because the stain does not disappear the way it does on weathered boards. Put on what the wood will take and wipe the rest.

5. Buying Extra and Why Colour Lots Matter

Buy 10 to 15 per cent more than the formula says, and buy all of it in one trip. There are two reasons, and the second is the one people learn the hard way.

The first reason is that coverage figures are optimistic and wood is unpredictable. A section of deck under a tree that has stayed damp and grey will absorb more than the section in the sun. The board ends, which are the most absorbent part of every board, are never included in anyone's square footage. Steps get a third coat. You will spill some. Fifteen per cent absorbs all of that.

The second reason is batch colour. Stain is made in batches, and the pigment load varies slightly from one batch to the next. On a can this shows up as a lot number or batch code stamped on the lid or the bottom. Two gallons of the same colour from different lots can dry visibly different — usually a shade or half a shade apart, which sounds trivial until you see the line across the middle of your deck where the first can ran out. Tinted-to-order stains from a store mixing machine have the same problem, made worse by the fact that the second can may have been tinted by a different person on a different day.

The cure has two parts. First, buy every can you need at the same time and check that the lot numbers match; the store will usually swap a can from the back if one is odd. Second, if you are working from multiple gallon cans, box them: pour all of them into a clean 5-gallon bucket, stir thoroughly, and work from the bucket. Then any batch variation is averaged across the whole deck instead of concentrated at a line. If your total comes to more than about 3.5 gallons, simply buying a 5-gallon pail solves this for you; it is one batch, and the per-gallon cost is lower.

If you do run out mid-job and have to go back, at the very least stop at a natural break — the end of a board run, a change in direction, the top of the stairs — and mix the last of the old can into the first of the new so the transition is blended rather than sharp.

6. One Coat, Two Coats and Leftovers

Single-Coat Versus Two-Coat Products

The number of coats is not a matter of taste; it is written on the can and it changes your maths. Most penetrating oils and most water-based semi-transparents are two-coat products, usually applied wet-on-wet or within a short window so the second coat can still penetrate. Several products are explicitly single-coat: Ready Seal, for instance, is formulated to be applied until the wood stops absorbing and then left alone, and putting a conventional second coat on it a day later produces a surface that never fully dries. Many solid stains specify one coat for maintenance recoats and two for bare wood. Our guide to how many coats of deck stain goes through the major products; the short version is that a single-coat product needs about 60 per cent of the stain a two-coat product does, but you cannot make a two-coat product into a one-coat product by applying it thicker.

What to Do With the Leftover

A partly used gallon of oil stain, with the lid cleaned and hammered down tight and the can stored upside down for a moment to seal the rim with product, will keep for two to three years in a dry space that does not freeze. Water-based stains are less forgiving of freezing and should come indoors for the winter. Label the can with the deck it went on and the date. When you need to touch up a scuffed step or a replaced board, having the original batch on hand is the difference between an invisible repair and a patch.

If you boxed several cans into a bucket, decant the leftover back into a clean can rather than storing it in the bucket, whose lid will not seal well enough. A quart-sized can from the paint aisle is ideal. Never leave stain in a sprayer or a tray overnight.

7. Quick Reference Table

Everything above, condensed. Use the low end of each range for rough or weathered wood and the high end for smooth new wood.

Frequently Asked Questions

Related Deck Staining Guides

• How Many Coats of Deck Stain? — the product-by-product answer that decides whether you double the maths.
• Brush vs Roller vs Sprayer — the application method that changes coverage by a quarter.
• Best Deck Stain Reviews — the products behind the coverage figures above.
• How to Stain a Deck — putting the gallons you bought onto the wood properly.
• Oil-Based vs Water-Based Deck Stain — why the two chemistries cover and behave so differently.
• Do You Need to Sand a Deck Before Staining? — and why a sanded deck drinks more stain than a cleaned one.