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Deck Stain Calculator Guide
Gallons for a deck, counting the railing properly — because the railing is usually the part that runs you out of stain.
Gallons = total area × coats ÷ coverage per gallon
A 16×20ft deck is 320 sq ft. Add 52ft of railing at ~4.5 sq ft per foot and it's 554 sq ft — nearly double.
The Railing Is Not a Detail
This is what catches people. A 16×20 deck is 320 sq ft of floor, and the railing around it adds another 234 — the railing is 42% of the job.
It's easy to see why it gets missed: a railing looks like a thin line. But it has two faces, a top cap, and a baluster every four inches with four sides each. Roughly 4.5 sq ft per linear foot is a working figure, and it's why one gallon "for a small deck" never finishes.
Coverage on the Can Is for Ideal Wood
The 250 sq ft per gallon on the label assumes smooth, sound, moderately absorbent timber. Old, weathered, rough-sawn or very dry decking can drink half again as much.
The first coat on thirsty wood always uses more than the second. If you buy exactly the calculated amount, you'll come up short on coat one — so the wood condition setting above is doing real work.
Two Thin Coats, Not One Thick One
Stain penetrates; it doesn't build a film like paint. Flooding it on doesn't get you a thicker layer — it gets you a sticky surface that never cures and stays tacky underfoot for weeks.
Apply, let it soak in, and wipe back anything still sitting on top after 15 minutes. Excess stain isn't protection, it's a mistake you'll be sanding off.
The Weather Window Is Narrow
The wood must be dry — genuinely dry, not "it stopped raining yesterday". Wet wood won't absorb, so the stain sits on top and peels.
Avoid direct sun, which flashes the surface before it can penetrate, and avoid staining if rain is due within about 24 hours. A cool, dry, overcast day is the ideal and it's why deck staining is a spring or autumn job in most climates.
Do You Need the Underside?
Usually not, and it's a genuinely miserable job. The exception is a deck close to damp ground with poor airflow, where the underside stays wet and rots first.
If you can't get a hand under it comfortably, that's a sign airflow is bad enough to be worth the effort.
Related: deck cost calculator, decking calculator, epoxy floor calculator.
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