How stain is estimated
gallons = surface × coats ÷ coverage × (1 + extra)
- Surface is the deck area, plus stair treads and risers, plus anything else you add.
- Coverage comes from the product label — commonly 150–300 sq ft per gallon per coat.
- The result is rounded up to whole quarts, then shown as gallons and quarts.
Coverage ranges come from Angi and DeckStainHelp; your product's label always wins.
Example: a 20 × 12 ft deck with three steps
240 sq ft of deck plus three 4 ft steps (about 18.5 sq ft of treads and risers) is 258.5 sq ft. Two coats at 200 sq ft per gallon plus 10% extra works out to 2.8 gallons — buy 3 gallons.
Before you stain
- Clean the deck and let it dry fully; most products list a minimum dry time and temperature range.
- Test on an offcut or hidden board — colour varies with the wood.
- If you have composite or PVC decking, check with the manufacturer before applying any stain or coating.
Frequently asked questions
How much area does a gallon of deck stain cover?
Typically 150–300 sq ft per gallon for the first coat, depending on the product and the wood. Rough, weathered or new wood absorbs more, so coverage drops; a second coat usually goes further. The number on the can is the one to use.
Do I need two coats of deck stain?
Many semi-transparent and solid stains recommend two coats, while some penetrating oils are applied as one. Follow the product’s instructions — the calculator lets you choose 1, 2 or 3 coats.
Should I include railings in the stain estimate?
Yes, if you’re staining them. Railings and balusters have a lot of surface for their footprint, so measure or estimate their area and add it under “Other surfaces”.
Why add extra stain?
Spills, drips, uneven absorption and the stain left in the tray all add up. 5–10% extra is a reasonable buffer.