How Much Deck Stain Do I Need? Chart by Deck Size

Deck area × coats ÷ the coverage on the can. A 12 × 12 ft deck with two coats at 150 sq ft per gallon needs about 1.9 gallons. Railings and steps add more.

Rows = width ÷ (board width + gap). Screws = rows × joists × 2. Dashed lines are joists.

Deck stain by deck size

Deck floor only, two coats at 150 sq ft per gallon
Deck	Stain	Cans to buy
10 × 10 ft	1.3 gal	1 × 1 gallon + 2 × quart
12 × 12 ft	1.9 gal	2 × 1 gallon
12 × 16 ft	2.6 gal	3 × 1 gallon
16 × 20 ft	4.3 gal	1 × 5 gallon
20 × 20 ft	5.3 gal	1 × 5 gallon + 2 × quart

Metric: deck floor only, two coats at 3.7 m² per litre
Deck	Stain	UK tins	Australian tins
3 × 3 m	4.9 L	1 × 5 L	1 × 4 L + 1 × 1 L
4 × 3 m	6.5 L	1 × 5 L + 1 × 2.5 L	1 × 10 L
5 × 4 m	10.8 L	1 × 10 L + 1 × 1 L	1 × 10 L + 1 × 1 L
6 × 5 m	16.2 L	1 × 10 L + 1 × 5 L + 1 × 2.5 L	1 × 15 L + 2 × 1 L

Why 150 sq ft per gallon?

Behr lists 150–250 sq ft per gallon for its semi-transparent deck stain on new smooth wood, and 125–150 on weathered or rough wood, applied in two coats. We start at 150 because most decks being re-stained are weathered. Use the figure on your own can in the deck stain calculator.

Railings and steps

Railings: length × height × 2 for both faces is a quick estimate. Balusters take more than they look.
Steps: tread plus riser, times the number of steps.
Fascia and rim boards: their length × height.

Common mistakes

Using paint coverage. Stain soaks into wood and covers far less than paint.
Staining wet or dirty wood. Clean and let it dry, as the can says.
Too much on the first coat. Many stains want thin coats; puddles stay sticky.

Sources

Behr Premium Semi-Transparent deck stain: coverage (spec summary)