How the calculator works
It adds the deck boards, the railing (both sides, with the open space between balusters removed) and about 4.5 sq ft for each stair step, tread plus riser. It divides by the coverage for your wood’s condition and adds 10% for waste.
Coverage for deck stain
Smooth, newer wood typically takes 250 sq ft per gallon. Weathered, rough or very dry wood absorbs much more, around 150 to 200. Do a small test area: if the first section uses more than expected, adjust before you buy the rest.
One coat or two?
Many semi-transparent stains are designed for one coat or two thin coats applied wet-on-wet; solid stains usually need two. Thick coats of penetrating stain do not soak in and can stay sticky or peel, so follow the label rather than adding extra.
Timing
Stain in dry weather, out of direct midday sun, when no rain is expected for at least a day. New pressure-treated wood usually needs to dry for several weeks to a few months before it will accept stain: water sprinkled on the boards should soak in, not bead.
Results are estimates. Coverage depends on the product, the tools and the wall; the rate printed on your can wins.