How to Calculate Deck Stain for a Deck in 5 Simple Steps

Deck stain in one line

Deck stain is simple: deck area, times the coats, divided by the coverage, rounded up to whole gallons. A 16 ft by 12 ft deck is 192 square feet, two coats make 384 square feet of staining, and at 250 square feet per gallon the deck needs 2 gallons, against 1.54 gallons exact. The rest of this guide is about getting those three inputs honest: the deck size, the coats, and the coverage.

Step 1: Measure the deck, length and width

Measure the deck in feet, length and width, at the deck itself — not at a plan nearby, since decks are built to the ground, not to the drawing. The worked example is 16 ft long and 12 ft wide, a common backyard deck, and 16 times 12 is 192 square feet. If the deck is not a rectangle, split it into rectangles, measure each one, and add the areas: an L-shaped deck is two rectangles, and the stain follows the total area, not the longest side.

Step 2: Multiply by the coats

Multiply the deck area by the coats, since every coat is another full deck of stain. Two coats on the example deck make 192 times 2, or 384 square feet of staining, and that is the area the coverage divides — not the deck area alone. A deck stained once needs half the stain of the same deck stained twice, and a weathered deck that drinks a third coat needs half as much again, since every coat is a full deck, never a fraction of one.

Step 3: Divide by the coverage

Divide the area with coats by the coverage on the tin, in square feet per gallon. The example tin covers 250 square feet per gallon, and 384 divided by 250 is 1.536, or 1.54 gallons exact. Do not use the coverage off a different tin without checking: a stain that covers 350 square feet per gallon needs only 1.1 gallons exact on the same deck, and that gallon saved is real money, not a rounding trick.

Step 4: Round up to whole gallons

Round the exact gallons up to whole gallons, since stain is sold by the gallon. The example rounds from 1.54 to 2 gallons, and the spare gallon is not wasted — it is the stain the second coat soaks into dry boards and the stain left in the brush and roller. Round up at the end, not at the start: rounding the deck area first and then dividing double-counts the rounding and over-orders.

Step 5: Buy, stain and check the deck

Buy the stain — 2 gallons for the example — and check the deck before staining it: every board dry, the first coat brushed with the grain, the second coat laid once the first is dry, since a second coat laid wet lifts the first coat off the boards. Then count what gallons do not cover: boards, screws, railing and stairs. For the boards under the stain, the decking calculator counts the boards, so the deck and the stain are ordered together, and the stain is not bought for boards that were never counted.

Deck stain in numbers

Suppose the deck is 16 ft long and 12 ft wide, the coats are 2, and the coverage is 250 square feet per gallon. Deck area: 16 × 12 = 192 sq ft. Area with coats: 192 × 2 = 384 sq ft. Exact gallons: 384 ÷ 250 = 1.54 gal. Gallons to buy: 2, rounded up. Order: 2 gallons, plus boards, screws and railing bought separately.

Common mistakes

The most common mistake is dividing the deck area alone, forgetting the coats, which buys for one coat and runs dry halfway through the second. The second is using the coverage off a different tin, which under-buys on a stain that covers less. The third is measuring only the longest side and forgetting the width, which buys for a strip instead of a deck. The fourth is rounding up before dividing, which double-counts the rounding and over-orders. The fifth is forgetting the railing, which stains a deck and leaves the railing bare.

Worked example, start to finish

A backyard deck, 16 ft long and 12 ft wide, 2 coats, 250 sq ft per gallon. Deck area: 192 sq ft. Area with coats: 384 sq ft. Exact gallons: 1.54. Gallons to buy: 2. Result: buy 2 gallons, and buy boards, screws and railing separately. If the same deck got only 1 coat instead, the need would fall to 1 gallon, but the deck stained would be half as well protected.

What changes the answer most

The deck area changes the answer most: every square foot of deck is another fraction of a gallon, and a wrong measurement rebuilds the whole order. The coats are second: moving from 2 coats to 3 adds a full deck of stain, from 2 gallons to 3. The coverage is third, and it changes only the top of the order, never the deck itself — moving from 250 to 350 sq ft per gallon still buys 2 gallons, since 1.1 gallons still rounds up to 2. The deck shape is fourth, and it changes only the area, never the stain maths.

Frequently asked questions