Decking Calculator
Work out how many deck boards a deck surface needs from its dimensions, board width, gap, and board length.
Estimates are for planning. Confirm quantities against your measured site and product packaging before ordering, and follow local building codes.
How to measure the deck
Measure the framing, not your ambitions. Deck length is the dimension your boards will run along; deck width is the dimension the rows of boards stack across. On most decks the boards run parallel to the house, so length is the side along the house and width is how far the deck sticks out.
For an L-shaped or wraparound deck, split it into rectangles and run the calculator once per rectangle. Each section usually has its own board direction anyway, with a divider board where they meet. For a deck with a 45 degree corner clipped off, just calculate the full rectangle; the “waste” from the clipped corner is real waste, since those angled cut-offs are rarely reusable.
If you’re re-decking an existing frame, also note the joist spacing while you’re measuring. 16 inches on center handles 5/4 wood and most composites run perpendicular; diagonal decking and some composite lines require 12 inch spacing, and finding that out after the boards arrive is an expensive surprise.
The formula and what it assumes
The calculator works in rows. Effective board width is the board’s actual width plus the gap, in inches: 5.5 plus 0.25 gives 5.75 inches per row for the standard setup. Rows is the deck width divided by that effective width. Total linear feet is rows times deck length, multiplied by 1 plus your waste percentage. Board count is total linear feet divided by the board length you’re buying, rounded up.
A 12 by 16 foot deck with boards running the 16 foot direction: 12 feet is 144 inches, divided by 5.75 gives about 25 rows. 25 rows times 16 feet is 400 linear feet, plus 10% waste is 440. Buy 16 foot boards and that’s 27.5, so 28 boards.
Assumptions worth stating plainly. The default 5.5 inch width matches standard 5/4x6 and 2x6 lumber and most composite profiles; if you’re using 3.5 inch (nominal 4 inch) boards, change the input. The gap default of 1/4 inch assumes dry material; wet pressure-treated lumber goes down nearly tight and shrinks to a gap on its own. The waste default assumes parallel boards on a rectangle. Diagonal layouts, borders, and inlays need 15% or more. And this is surface boards only: no joists, beams, posts, ledger, hangers, hardware, stairs, or railing. Those typically cost as much as the decking itself, sometimes more.
Buying tips
Pressure-treated 5/4x6 deck boards run about $1 to $2 per linear foot at the big box stores, in 8, 10, 12, and 16 foot lengths. Cedar lands around $2 to $4 per linear foot depending on region (cheaper in the West). Composite spans a wide range, roughly $2.50 to $7 per linear foot, sold mostly in 12, 16, and 20 foot lengths, with grooved-edge boards (for hidden fasteners) and square-edge boards priced about the same.
Buy the longest boards that fit your deck without a seam. A 16 foot deck covered with 16 foot boards has no butt joints, which look better and last longer than any seam. If the deck is longer than stock lengths, plan staggered joints landing on doubled joists, and order lengths that minimize cut-off waste rather than just the cheapest per foot.
On fasteners: exterior-grade screws (coated or stainless, 2.5 to 3 inch) are the cheap, strong, fully serviceable choice, about 350 screws per 100 square feet at two per joist crossing. Hidden fastener clips give composite a clean face and automatically set the side gap, but cost roughly 2 to 4 times more per square foot and make replacing a single damaged board harder. Most pros run hidden fasteners in the field of a composite deck and face-screw the border and stair treads, where clips can’t reach and feet hit hardest.
Sight down every wood board before it goes in the cart and reject crooks and twists; a bowed board can be sprung straight during install, a twisted one can’t. With composite, buy all of it from one lot if you can. Color varies slightly between production runs, and a mid-deck shade change is forever.
Frequently asked questions
Why is a deck board 5.5 inches wide, not 6?
Deck boards are sold by nominal size. A 5/4x6 or 2x6 board actually measures 5.5 inches across after milling, and that 5.5 is what the layout math uses. Most composite boards copy the same 5.5 inch actual width so they're interchangeable with wood framing layouts.
What gap should I leave between deck boards?
1/4 inch is the common target for drainage and debris. Kiln-dried wood and composite get gapped at install (composite makers specify 1/8 to 1/4 inch side gaps). Wet pressure-treated lumber is the exception: install it nearly tight, because it shrinks about 1/4 inch as it dries and creates its own gap.
How much waste should I add for deck boards?
10% covers a simple rectangular deck with boards run parallel. Bump it to 15% or more for diagonal decking, picture-frame borders, or decks whose width doesn't divide evenly into stock board lengths, since every cut-off end is waste.
Does this calculator include joists, posts, and fasteners?
No, it counts surface decking only. Framing (ledger, posts, beams, joists at 16 inches on center, or 12 for diagonal or some composites), hangers, and fasteners are a separate list. Figure roughly 350 screws or one box of hidden fastener clips per 100 square feet of deck.