Deck Calculator

How to estimate deck materials

A deck breaks into two material layers: the decking you walk on, and the joists underneath that carry it. This calculator sizes both, plus the screws that tie them together.

Decking boards

Count the rows first: the deck dimension running across the boards, divided by one board's width plus the gap between boards. A standard deck board covers about 5.5 inches and a 1/4 inch gap is typical, though composite gaps vary by brand. Multiply the rows by the board run length for total linear feet, then divide by the length of board you're buying to get the board count. A waste allowance covers cutoffs and the occasional bad board.

Joists

Joists run perpendicular to the decking, so choosing which way the boards run sets the joist direction. They're spaced 16 inches on center for most decking, or 12 inches for many composite boards and diagonal patterns. The joist count is the span they're spaced along, divided by the spacing, plus one for the starting joist.

What this does not size

Beams, posts, and footings depend on span tables, load, wood species, and soil, which are code-governed and beyond a simple quantity estimate. Size those with a span table or a licensed contractor, and use the concrete calculator for the footing volume once you know the hole dimensions.

What it will cost

Counts are one half of the picture and prices are the other. Our deck building cost guide covers installed cost per square foot for pressure treated, cedar, and composite, totals by deck size, and a component-by-component materials breakdown you can hold a contractor quote against.

Still choosing a board? The composite vs. wood decking comparison runs pressure treated, cedar, and composite over ten and twenty years on this same 12 by 16 footprint, including what the staining actually costs.

Building the stairs off the deck? Our stair code reference covers the riser, tread and variation limits, the guard height on the open side of a flight, and the 30 inch drop that decides whether a guard is required at all.

This calculator counts joists at the spacing you give it, and stops short of sizing them. Our deck joist span chart gives the maximum span for each size across three species groups, from the AWC table the code points at, along with the wet service and incising assumptions that make a deck joist span less than a floor joist of the same size.