How to calculate the square footage of a deck
A deck's square footage is simply its surface area — length times width, measured in feet. For a straightforward rectangular deck, measure the two sides, multiply them, and you're done: a 16-foot by 12-foot deck is 192 square feet. This single number drives everything downstream, from how much decking to order to how much stain you'll need to seal it, so it's worth measuring the actual deck rather than working from a plan that may have changed during the build.
Most decks aren't perfect rectangles, though. If your deck wraps around a corner of the house or steps down to a lower level, break it into separate rectangles at each corner, calculate the area of each one, and add them together. This is the same approach you'd use to measure flooring in an irregular room — clean rectangles are easy to measure accurately, while trying to eyeball an L-shape as a whole leads to mistakes. The calculator above handles the multiplication and waste factor once you enter each section.
From square footage to decking boards
Deck boards are sold by the linear foot, not the square foot, so you need to convert. A common 5.5-inch-wide board with a standard 1/8-inch gap between boards covers about 0.47 square feet for every linear foot of board. That means the linear feet of decking you need is roughly your total area divided by 0.47. For a 192-square-foot deck, that's about 409 linear feet before waste, or around 449 linear feet once you add 10%.
Always buy extra. A 10% waste factor covers the off-cuts at board ends and the pieces you'll inevitably ruin. If you're running boards diagonally or framing the perimeter with a picture-frame border, bump that to 15%, because angled cuts waste more material. Order boards in lengths that minimize seams across your joist spacing to keep waste down.
Estimating stain, sealer, and the rest of the project
Once you know the deck's square footage, estimating finish is easy: most deck stains and sealers cover 150 to 200 square feet per gallon on a smooth surface, though rough or weathered wood soaks up more. A 192-square-foot deck therefore takes roughly one to two gallons for a single coat — round up if the boards are older or you're applying two coats.
Remember that railings and stairs aren't included in the deck surface area, but they still need finishing and material of their own. Measure the stairs and railing separately and add them to your stain estimate so you don't come up short partway through the job.