How to measure a deck for stain
Measure the deck boards' length and width, and multiply. Then go after what people forget: railings, stairs and the rim around the edge. Rails eat stain because of all those faces. We count each as a solid panel, two faces wide, as a reasonable stand-in for the spindles. For stairs, add up the treads and the risers if you plan to stain them.
A fence needs its height and its length, and you need to decide if you are staining one side or both. A 50 foot fence stained on both sides is twice the job. Do not skip the can, because that is where the most important number is. Most labels print a rough wood figure and a smooth wood figure, and the two are often about twice apart.
Before you buy, check that the wood will take it. Thompson's gives a good test: splash some water on the boards, and if it soaks in within five seconds, the wood is ready. If it beads up, wait or clean it first, since new or sealed wood may not absorb anything.
A worked example
Here is a 16 by 12 foot deck with 40 feet of railing, 40 square feet of stairs, and two coats: the first at 150 square feet a gallon and the second at 300. These lines come from the same formula the calculator above runs.
16 by 12 ft deck with rails, stairs and two coats
- Deck: 16 × 12 = 192 sq ft
- Rails: 40 × 3 × 2 = 240 sq ft
- Total with stairs: 472 sq ft
- First coat: 472 ÷ 150 = 3.15 gal
- Second coat: 472 ÷ 300 = 1.57 gal
- To buy: 5 gal
Notice that the rails add more area than the deck itself. If the deck is still on paper, the decking calculator sizes the boards, and the fence calculator does the same for a boundary.
Gallons per coat for common decks
Three columns, because coverage matters more than anything else on this page. Rough, thirsty wood may be near the left column, and smooth or a second coat may be near the right.
| Deck | Area | At 100 sq ft/gal | At 150 sq ft/gal | At 300 sq ft/gal |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 1.00 | 0.67 | 0.33 |
| 12 × 12 ft | 144 sq ft | 1.44 | 0.96 | 0.48 |
| 16 × 12 ft | 192 sq ft | 1.92 | 1.28 | 0.64 |
| 20 × 12 ft | 240 sq ft | 2.40 | 1.60 | 0.80 |
| 20 × 16 ft | 320 sq ft | 3.20 | 2.13 | 1.07 |
| 24 × 16 ft | 384 sq ft | 3.84 | 2.56 | 1.28 |
Mistakes that waste stain
- Doubling for a second coat. Some labels show it going about twice as far. Use what the can says.
- A single coverage figure. Across products we read, it ran from about 100 to 450. Anyone quoting one number is guessing.
- Overapplying. Ready Seal says so in capital letters. Stain that cannot soak in stays sticky on top.
- Staining too soon. New pressure-treated lumber may need time to dry. Do the splash test and read the label.
- Forgetting the fence's other side. A neighbor's side of a fence may be part of your job, so decide before you buy.
Questions
How many gallons of stain for a 12x16 deck?
About 1.3 gallons per coat for a 12 by 16 foot deck, at 150 square feet per gallon. That is 192 square feet. A smoother or newer board goes further and a rough one less, so read the coverage on your own can.
How much stain do I need for a 10x10 deck?
A 10 by 10 foot deck is 100 square feet, which takes about two thirds of a gallon per coat at 150 square feet per gallon. Railings and stairs are extra, so add them in the further section.
How much does 1 gallon of deck stain cover?
Anywhere from about 100 to 450 square feet, depending on the product and the wood. Wolman prints 150 to 200 on rough wood and 300 to 350 on smooth. Ready Seal's table runs from 125 to 170. Use your own can's figure.
How much stain for a fence?
Multiply length by height by the sides you are staining, then divide by your coverage. A 50 foot, 6 foot high fence stained on both sides is 600 square feet, or about 4 gallons at 150. Rough fence boards can use more.
Does old wood need more stain?
Yes, usually, because weathered wood absorbs more. No source publishes a percentage, so use the rough wood figure from your label. Thompson's says water soaking in within five seconds means the wood is ready to take a finish.
How do I adjust for multiple coats?
Add each coat separately, using its own coverage, rather than doubling. Some labels show a second coat going much further than the first. Ready Seal warns not to overapply, and says dense, new or smooth wood will not take a second coat.
This stain figure is a planning estimate from the coverage you enter, and products run from about 100 to 450 square feet a gallon. Thirsty, weathered boards drink more than the can suggests, and overapplying can leave a tacky mess. Read the label for coats and drying, and ask your supplier about new pressure-treated lumber.
Sources and review
- Ready Seal info and protocol, Ready Seal, 2023.
- Wolman RainCoat specification sheet, Wolman, 2017.
- Solid color acrylic stain data sheet, Penofin, no date shown, read November 2026.
- Waterproof exterior transparent wood sealer, Thompson's WaterSeal, no date shown, read November 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Gallons per coat
Gallons = Area ÷ Coverage
- Coverage
- Square feet per gallon, from your label
Worked through: 192 ÷ 150 = 1.28 gal
Fence area
Area = Length × Height × Sides
- Sides
- 1 or 2, as a fence you stain on both sides is twice the area
Worked through: 50 × 6 × 2 = 600 sq ft
The constants, and where they come from
STARTING_COVERAGE
Published range, narrowedStarting first-coat coverage, in square feet per gallon
Applies to: One oil stain on treated pine. Replace it with your own label's figure
Source: Ready Seal info and protocol, Ready Seal, published 2023, read 2026-11. This source is about 3 years old, and it is the one the figure rests on.
Also from: Wolman RainCoat specification sheet, Wolman, published 2017-12, read 2026-11.
Also from: Solid color acrylic stain data sheet, Penofin, no date on the page, read 2026-11.
Also from: Waterproof exterior transparent wood sealer, Thompson's WaterSeal, no date on the page, read 2026-11.
Coverage runs from 100 to 450 across products in our research, so no single default is right. Wolman prints 150 to 200 on rough wood and 300 to 350 on smooth, and Penofin 200 to 300 and about 450. No source gives a percentage for wood condition.
RAIL_FACES
ModelledFaces counted on a railing
Applies to: A rail treated as a solid panel
No published figure exists for this. It is derived, and the derivation is below rather than a citation to something that does not say it. Last checked in November 2026.
Our own model, since no source gives a figure for railings. Real spindles have more faces and less area, and the two roughly offset, but this is an estimate.
What the labels mean
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
- Modelled:
- derived rather than measured, and the derivation is shown.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the deck stain calculator
- 1
Measure the deck, and add rails, stairs and fence in the further section
- 2
Read the coverage for your wood on the can, rough or smooth
- 3
Enter a second coat's coverage if the label calls for one
- 4
Read the gallons
- 5
Buy to the next gallon, since stain batches can differ in color
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- Coverage is the figure from your own label, because it varies too widely for any default
- No factor is applied for wood condition, because no source publishes one. Use the label's rough or smooth figure
- A second coat has its own coverage, since it often goes further than the first
- Rails count as solid panels with two faces, which is our own estimate
Pro tips
- Do the splash test first: water that beads means the wood is not ready
- Stain in the shade, or early in the day, so it does not dry too fast
- Mix cans from different batches together so the color stays even