Decking Calculator — Deck Boards & Joists Needed

Ordering deck boards from a room's length and width alone is a common way to come up short: lumber is sold by its nominal size, but a nominal 2×6 or 5/4×6 board is actually milled to 5.5 inches wide once it's dried and dressed (planed smooth) at the mill — the same rounding-down convention behind every piece of dimension lumber in a US-market lumber yard. This calculator uses those real dressed widths, not the nominal label, so the board count matches what actually shows up on the pallet.

Enter the deck's length and width, pick a board size from the built-in actual-dimension table, and set the expansion gap you're leaving between boards — typically 1/8 to 3/16 inch so the decking has room to swell without buckling. The calculator adds that gap to the board's actual width to get an effective coverage width per board, divides the span the boards need to cover by that effective width, and rounds up to a whole board, because a lumber yard doesn't sell partial boards. Multiplying by the run length gives the total linear footage, and a waste percentage (10% by default) covers the offcuts from butt joints, angled ends and mistakes.

Joist count uses a different, equally real piece of framing math: joists run perpendicular to the decking boards and are spaced at standard 16-inch or 24-inch on-center intervals. Dividing the run length by that spacing tells you how many gaps there are between joists — but the number of joists is always one more than the number of gaps, because a joist has to sit at both ends of the run as well as at every interval in between. That's the identical '+1' logic used to count fence posts along a fence line, and skipping it is one of the most common framing miscounts.

Switch the board-run direction if you're laying boards the short way instead of the classic lengthwise run — the calculator swaps which dimension gets covered by boards and which gets spaced by joists accordingly. If you're also pouring footings for the deck's support posts, the concrete calculator on this site works out bag counts for post holes using the same kind of real construction-math approach. Everything runs locally in your browser — nothing you enter is uploaded or stored.