Worked example
A 16 by 12 foot deck is 192 square feet. Two coats at 200 square feet per gallon need 192 × 2 ÷ 200 = 1.92 gallons. Add 10% and you are at 2.11 gallons, so you buy 3 one-gallon cans. That is the number this page opens on.
What the number leaves out
Rails, stair treads, risers, and fascia are not in the rectangle. A guard around a 12×16, stained on both faces, can drink as much as a second small deck. Measure those square feet and run them through the same formula, or keep the spare gallon.
New pressure-treated lumber is often too wet to stain. The can will say so. Waiting does not change the gallon count. It changes whether the first coat is wasted.
Read the longer notes in pressure-treated versus composite. Composite is often a soap-and-water surface, not a stain job. Do not buy stain for a cap-stock board unless the manufacturer says to.
Common questions
Does deck stain coverage include the rails?
No. This count is the flat deck rectangle only: length times width, times the number of coats, divided by the square feet per gallon on the can. Rails, stairs, fascia, and the edges of the boards take more finish. Add them by hand or buy the extra gallon the waste percent is already pushing you toward.
Is the coverage number for one coat or for the whole job?
Use the square feet per gallon printed for one coat. Two coats divide that coverage in half, which is why the calculator multiplies area by the number of coats before it divides. Rough, dry, or thirsty wood often covers less than the smooth-wood number on the can. If the label gives a range, use the low end.
Why buy whole gallons?
The calculator rounds up to the next gallon because that is how most cans are sold. A job that needs 2.11 gallons buys 3. Quarts exist; if you would rather buy a gallon plus quarts, use the gallon-and-fraction line and split it yourself. Color can vary between batches, so one extra can from the same lot is often the safer buy.