Quick answer: deck sealer or stain is bought by the gallon, and one gallon usually covers roughly 150 to 300 square feet per coat. Gallons needed = deck area × coats ÷ coverage per gallon. Always check the coverage printed on the can.
Example: a 12 ft by 12 ft deck, two coats
- Deck surface: 12 × 12 = 144 sq ft
- Two coats: 144 × 2 = 288 sq ft to cover
- Coverage assumed: 200 sq ft per gallon
- Gallons: 288 ÷ 200 = 1.44, so buy 2 gallons
Do not forget
- Railings, stairs and balusters add area. Measure them separately and add them.
- Rough or weathered wood soaks up more, so coverage can fall near the low end.
- Many products call for one coat only, so read the label.
Quick table for a 200 sq ft per gallon product
| Deck size | Area | Two coats | Gallons to buy |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 200 sq ft | 1 |
| 12 × 12 ft | 144 sq ft | 288 sq ft | 2 |
| 16 × 20 ft | 320 sq ft | 640 sq ft | 4 (3.2 rounded up) |
Building the deck first? See how many deck boards for a 12×12 deck.
Prepare the wood first
Stain and sealer only work well on clean, dry wood. Before you buy, plan for these steps:
- Sweep the deck and clear out the gaps between boards.
- Clean with a deck cleaner to remove dirt, mildew and gray, weathered fibers. Strip old finish if it is peeling.
- Let the wood dry fully, often 24 to 48 hours after cleaning or rain.
- Do a water test: sprinkle a little water on the boards. If it soaks in, the wood is ready. If it beads up, wait longer or clean again.
- New pressure-treated lumber may need to dry out before finishing. Follow the product label for how long to wait.
Measuring stairs and railings
Railings, stairs and balusters can add a surprising amount of surface. Use these rough methods and add the results to your deck area:
- Stairs: tread width × tread depth × number of treads, plus the risers if you are staining them.
- Solid railings: length × height × 2 for both sides.
- Baluster railings: a common shortcut is to treat the railing as a solid panel, length × height, which roughly allows for all four sides of each baluster.
For example, 30 ft of railing at 3 ft high counts as about 90 sq ft. Two coats would add 180 sq ft of coverage, or about another gallon at 200 sq ft per gallon.
Transparent, semi-transparent or solid stain?
- Clear sealer or transparent stain: shows the most wood grain, but usually needs reapplying most often.
- Semi-transparent stain: adds color while letting grain show through. A popular middle ground.
- Solid stain: looks more like paint, hides wear and usually lasts longest, but hides the grain.
Coverage is different for each product, so always use the figure on the can for your final calculation.
When to apply
Choose a dry spell with mild temperatures. Many products recommend roughly 50°F to 90°F and no rain for at least a day after application. Avoid staining in strong direct sun, which can make the finish dry too fast and leave lap marks. Work a few boards at a time, along their full length.
Common questions
How many gallons of stain for a 12×12 deck?
About 1 to 2 gallons, depending on coverage and the number of coats. The example above needs 2 gallons for two coats.
Is one coat of deck stain enough?
Some products are designed for one coat, while others need two. Check the label, because an extra coat on a one-coat product can cause peeling.
How often should I re-stain my deck?
It depends on the product, climate and sun exposure. Transparent finishes may need attention every year or two, while solid stains often last longer.
Estimates only. Always follow the product label and local guidance.
Related: the DIY Project Budget & Materials Planner turns your measurements into quantities, costs and a shopping list.
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