How the deck stain estimate works
- Floor. The deck's length times its width.
- Railings. Railing length × height × 2. Balusters have four faces and the rails and posts add more, so treating the railing as a solid panel stained on both sides is a reasonable stand-in.
- Stairs. Each step counts as the stair width × 1.5 ft, which covers a tread and a riser. Stringers and the edges of the deck add a little more, covered by rounding up.
- Coats and coverage. Multiply by coats and divide by the coverage for your wood's condition.
gallons = (floor + rail length × height × 2 + steps × stair width × 1.5) × coats ÷ coverage
Worked example
A 16 × 12 ft deck is 192 sq ft. It has 40 ft of 3 ft railing, which counts as 240 sq ft, and 4 steps 4 ft wide, which add 24 sq ft. That is 456 sq ft in all; the railing is more than the floor. Two coats on new wood at about 250 sq ft per gallon need 3.6 gallons (13.8 liters), so buy 4 gallons. If the wood is weathered and drinks stain at about 150 sq ft per gallon, the same deck needs 6.1 gallons: buy 7.
The floor alone, with no railings or stairs, would need only 1.5 gallons for two coats on new wood.
Coverage by wood and stain type
| Situation | Typical coverage per gallon |
|---|---|
| New, smooth, dry lumber, first coat | About 200 to 300 sq ft |
| Weathered, rough or very dry wood, first coat | About 100 to 175 sq ft |
| Second coat on any wood | Often noticeably more than the first |
| Solid-color stains on smooth wood | Often similar to or a little better than semi-transparent |
These are rules of thumb. Read the can: many deck stains list a coverage range and say how many coats to apply, and some are meant to go on as two thin coats wet-on-wet.
Stain types
| Type | Look | What to expect |
|---|---|---|
| Clear or toner | Wood grain and color fully visible | Least UV protection; usually needs recoating most often |
| Semi-transparent | Tints the wood, grain shows | Soaks in rather than forming a film; a common deck choice |
| Semi-solid | More color, grain partly visible | More UV protection than semi-transparent |
| Solid color | Opaque, like paint | Hides worn wood; forms a film that can wear in traffic areas |
Switching from a solid stain back to a semi-transparent one usually means stripping the old coating first, so choose with the long run in mind.
Preparing the deck
- Clean it. Use a deck cleaner to remove dirt, mildew and gray surface fibers. Rinse well.
- Strip or brighten. Old film-forming finishes that are peeling need stripping. A wood brightener after cleaning can restore color and help penetration.
- Let it dry. Sprinkle water on the boards. If it soaks in within a few minutes, the wood is ready for a penetrating stain. If it beads, wait or clean again.
- New pressure-treated lumber is often too wet to stain right away. Use the same water test, and follow the lumber and stain makers' advice.
Safety
- Rags soaked in oil-based stain can heat up and catch fire on their own as they dry. Lay them flat outdoors to dry, or put them in a sealed metal can filled with water, then dispose of them as your local rules require.
- Cleaners and strippers can burn skin and eyes. Wear gloves and eye protection, and protect plants nearby.
- Check loose boards, railings and stairs while you prep. If a railing wobbles or posts are rotten, fix them before anyone leans on them.
Building a new deck first? The deck calculator counts the boards, joists and screws.
Frequently asked questions
How much stain do I need for a 12 × 16 deck?
The floor alone is 192 sq ft, which takes about 1.5 gallons for two coats on new wood at 250 sq ft per gallon. With 40 ft of railing and four steps, it rises to about 3.6 gallons.
Why do railings take so much stain?
Balusters, rails and posts have many faces. Counted as a solid panel on both sides, 40 ft of 3 ft railing is 240 sq ft, more than a 16 × 12 ft floor.
Does weathered wood need more stain?
Yes. Dry, gray wood soaks up stain, so the first coat may cover only about 150 sq ft per gallon compared with about 250 on new wood. The same 456 sq ft deck needs 6.1 gallons instead of 3.6 for two coats.