Stain coverage is the number most likely to be wrong on the can, and it is wrong in a predictable direction. The figure printed on the label is measured on smooth, sealed, previously finished wood. Almost no deck is that.
What the wood does to the number
Smooth, new, previously sealed lumber will take stain at close to the label rate — call it 250 to 350 square feet per gallon.
Rough-sawn lumber has several times the actual surface area of a planed board, because every saw mark is a small valley. Coverage drops to 150 or even 100 square feet per gallon.
Old, grey, weathered decking is the worst case. The surface fibres have broken down and act like a sponge. A first coat on a deck that has been bare for three years can disappear at 100 square feet per gallon and still look thin.
If your deck is rough or weathered, change the coverage figure in the calculator rather than trusting the default.
One coat or two
This depends on the stain type, and getting it backwards wastes either money or a weekend.
Semi-transparent and penetrating stains are generally one coat. They soak in rather than building a film, and a second coat applied over a saturated first coat has nowhere to go — it sits on top, stays tacky, and peels within a season.
Solid colour stains behave like thin paint and normally want two coats. So do most film-forming sealers.
The rule with penetrating stain is to apply until the wood stops accepting it and then wipe off what is left, not to apply a set number of coats.
Railings are the hidden area
Deck floor area is easy to measure and easy to underestimate the job from. A forty foot railing run with balusters every four inches has a surprising amount of surface — four sides on every baluster, plus the rails, plus the posts.
As a rough allowance, add twenty to forty percent of the floor area for a railing, more if the balusters are close together or square rather than round.
And be aware that railings are all brush and rag work. They take several times as long per square foot as the floor, which you roll or pad.
Before any of it goes on
New pressure-treated lumber is usually too wet to accept stain. The common test is to sprinkle water on it — if it beads, the wood is not ready. If it soaks in within a minute or so, it will take stain.
That can mean waiting several months on new construction. Staining wet treated lumber is the most common reason a deck finish fails in its first year.
Old decks need cleaning and often brightening before stain. Whatever the previous finish was, if it is peeling anywhere it has to come off everywhere, because new stain over a failing film fails with it.
Weather matters more than with paint
Stain wants moderate temperatures and no direct sun on the surface as you work. Hot boards flash the solvent off before the stain penetrates, and you get a thin, blotchy finish.
Rain within twenty-four hours of application will ruin most of a day's work. Check the forecast properly, not optimistically — this is the failure people repeat annually.