Deck Board Calculator

How the boards are laid out

Each row runs the full board run. Where a row needs more than one board, the pieces meet at a butt joint centred on a joist, so both ends are fastened. The calculator tries every way to split a row at the joists and keeps the ones that follow these rules:

Rows are the fewest boards whose widths and gaps cover the deck; the last row is ripped to fit and counted as a full board. Joists start at one end at the spacing you choose, with a joist at each end and a short last bay. Offcuts long enough for another piece are reused before a new board is bought, so the plan's waste is what's really left over.

Three plans: the cheapest mix of the ticked lengths (by cost when every length has a price, otherwise by linear feet), the plan with the fewest joints, and the best plan from a single length, which is what most deck calculators assume.

Worked example: a 12 × 22 ft deck

Boards run along the 22 ft side: Wood 5/4×6 (5-1/2″ wide, 1/8″ gaps), joists 16″ on center, stock12, 16, 20 ft, no prices. That's 26 rows, the last ripped to 3-3/8″, over 18 joists. No ticked length covers 22′, so every plan has joints.

By linear feet all three plans come out the same here: 31 × 20 ft, 26 joints over 26 rows, with the short pieces cut from offcuts.

Prices change the answer. With example prices of $18.00 for a 12 ft board, $32.00 for a 16 ft board, $50.00 for a 20 ft board (made up for illustration; enter your yard's), the cheapest plan is 56 × 12 ft for $1,008.00, against $1,550.00 for the 31 × 20 ft that's shortest in linear feet:

Fasteners for the cheapest plan: 988 nails or screws (2 per board at each joist). Board feet: 387.5 on the nominal 5/4×6 size.

Deck boards for common deck sizes

5/4×6 wood, 1/8″ gaps, joists 16″ on center, stock 12/16/20 ft, boards along the longer side. The cheapest plan by linear feet:

Joist spacing for deck boards

Wood decking follows IRC Table R507.7 (2024; the 2021 table is the same). A board on two joists is a single span; on three or more, a multiple span.

Every board in these plans bears on at least three joists, so the multiple-span column applies: 5/4 up to 16″ on center, 2× up to 24″. Composite and PVC boards follow the maker's instructions (IRC R507.2). TimberTech's Advanced PVC guide, for example, allows 16″ on center on homes, a 1/8″–1/4″ gap, and no more than 1/2″ overhang past the framing (installation guide). Diagonal decking isn't in this calculator yet.

How many screws per deck board

Wood decking gets 2 fasteners per board at every joist: two 8d deformed-shank nails or two No. 8 wood screws (IRC R507.7; DCA 6 says the same). The calculator counts 2 for each board at each joist it crosses, so a joist under a joint gets 4. Hidden-fastener systems differ, so for composite it gives the joist crossings to count clips from, plus the face screws TimberTech asks for at board ends: at least 2 within 1/2″ of each end, at joints and at the deck's edges.

5/4 vs 2×6 decking

5/4×6 radius-edge decking is 1″ × 5-1/2″ dressed (SFPA); a 2×6 is1-1/2″ × 5-1/2″ dry (PS 20-25, Table 3). Both cover the same width per row. Board feet use the nominal size (PS 20-25, §2.2): 0.625 board feet per linear foot of 5/4×6 and 1 of 2×6. The thicker 2× boards can span joists up to 24″ apart; 5/4 needs 16″ or closer.

Pricing by the board foot? The linear feet to board feet calculator converts the yard's price, and the board foot calculator tallies a whole lumber list. Wood boards are spaced about 1/8″ apart (DCA 6).

Not covered yet: diagonal or herringbone layouts, picture-frame borders, stairs, railings and framing sizes. Joints sit only on joists; blocking between joists isn't modelled.

Stock lengths are planned at exactly their nominal length. Offcuts are matched to pieces after the row layout is chosen, trying each ticked length as the main board; a hand-tuned plan can occasionally save a board more.