Free deck board and fastener calculator. Enter deck dimensions, board size, gap, and joist spacing to get the exact number of boards, screws or hidden clips, and total material cost.
Built and maintained by Paul Clark, Redmoon Software
Use when quoting materials for a new deck build, including the fastener count that most board-only calculators skip.
Most decking calculators give board count only. This one adds fastener totals so the hardware run isn't a separate trip.
A 20.0×12.0 ft deck needs 52 boards across 26 rows, fastened with screws at every joist crossing.
Rows across the deck width = (deck width in inches) ÷ (board width + gap), rounded up so the last row still fits.
Boards per row = deck length ÷ available board length, rounded up to whole boards since every board must end on a joist.
Joist crossings = joist count × rows, and each crossing needs 2 screws or 1 hidden clip; total cost sums linear footage of decking plus every fastener.
The default 3/16-inch gap lets water drain through the deck surface instead of pooling, and gives the boards room to expand and contract with humidity and temperature. Too tight a gap can cause cupping or buckling; too wide wastes boards and can catch heels or debris.
Screws are cheaper per unit, faster to install without special tools, and easy to replace a single board later, but leave visible fastener heads. Hidden clips give a clean, fastener-free surface and slightly reduce trip hazards, but cost more per unit and this calculator assumes only one clip per crossing versus two screws.
Whenever a board doesn't span the full deck length, its end must land on a joist so it's properly supported — that's why this calculator rounds boards-per-row up to whole boards rather than assuming boards can be trimmed to fit perfectly. Planning your board lengths against your joist layout in advance minimizes awkward short offcuts.
Input
16×10 ft deck, 5.5" boards, 3/16" gap, 16 ft boards, 16" joist spacing, screws, $2.00/ft boards, $0.10/screw.
Output
22 boards, 572 screws, $761.20.
Rows = ceil((10×12)/(5.5+0.1875)) = 22. Boards per row = ceil(16/16) = 1, so 22 boards total using 352 linear ft. Joists = ceil((16×12)/16)+1 = 13. Fasteners = 22 rows × 13 joists × 2 screws = 572. Cost = 352×$2.00 + 572×$0.10 = $704 + $57.20 = $761.20.
Board count is computed on coverage width — the actual board width plus the gap between boards — not on nominal size. A "6 inch" deck board measures about 5.5 inches, and with a 3/16 inch gap covers roughly 5.7. Using the nominal figure under-counts boards by around 5%, which on a large deck is several boards.
The gap is not optional. It handles drainage and seasonal movement, and timber laid tight will cup or buckle.
Running boards diagonally is common for appearance and adds roughly 15% waste from the angled end cuts, as well as requiring closer joist spacing because the unsupported span between joists increases at an angle.
Butt joints are the other planning decision. Boards shorter than the deck must meet over a joist, which means either buying lengths that span the full run or planning joint positions deliberately — random joints look untidy and unsupported ones fail.
A calculator tells you the exact boards a deck needs. The lumberyard order is a different number — here is how waste factor, seam placement, and board length turn one into the other.
Why the 3/16-inch board gap matters, how joist spacing determines your fastener count, and screws vs. hidden clips — with a full worked example for a 20x12 deck.
Capes, skirts, armor → yards / meters needed.
Maker & workshopScale a custom paint recipe to your army size.
Maker & workshopNote divisions in ms / Hz for any tempo.
Maker & workshopReal feet → 28mm, 32mm, HO, 1:35 model size.