Deck Boards Calculator

What this calculator does

Deck boards are ordered by the piece, not by area, and the number of pieces depends on something most people leave out of the arithmetic entirely: the gap between boards. This calculator works from your deck dimensions, the actual face width of the board and the expansion gap to a board count, linear footage and cost per square foot.

The calculation

Each row of decking covers its own width plus one gap:

coverage per row (ft) = (face width in + gap in) ÷ 12
rows                  = deck width ÷ coverage per row, rounded up
linear feet           = rows × deck length
boards                = linear feet × (1 + waste) ÷ board length, rounded up

A 16 by 12 foot deck using 5.5-inch boards with a 1/4-inch gap:

• Coverage per row: (5.5 + 0.25) ÷ 12 = 0.479 ft
• Rows: 12 ÷ 0.479 = 26 rows
• Linear feet: 26 × 16 = 416 ft
• With 10% waste, in 16-foot boards: 29 boards

Nominal size is not actual size

This is where estimates go wrong before they start. US lumber is sold under a nominal name that has not matched its real dimensions since it was rough-sawn:

Nominal	Actual face width
2 × 6	5.5 in
5/4 × 6	5.5 in
2 × 4	3.5 in

Run the numbers with 6 inches instead of 5.5 and you get 24 rows instead of 26 — two rows short, which is roughly 32 linear feet of missing decking. Always enter the measured face width.

The gap is structural, not cosmetic

The spacing between boards lets water drain and lets the wood move. Which gap you choose depends on the moisture content of what you are buying:

• Wet pressure-treated lumber is heavy and damp on delivery and will shrink noticeably as it dries. Fasten it at about 1/8 inch and it opens up to roughly a quarter inch on its own.
• Kiln-dried or composite will not shrink, so leave the full 1/4 inch at installation.

Getting this backwards is a common and visible mistake. Wet lumber laid at a quarter inch dries to gaps wide enough to drop a phone through. Dry lumber laid tight has nowhere to expand and cups in the first humid spell.

What the board count does not include

The decking is the visible part and roughly half the cost. Budget separately for:

• Framing — joists, beams, posts, ledger board and hangers. Joist spacing is usually 16 inches on centre, tightening to 12 for diagonal layouts or some composites.
• Footings, which are concrete and sized by local frost depth. The concrete calculator handles the pour, and there is a guide on calculating concrete in cubic yards.
• Fasteners. Hidden clip systems consume a surprising quantity and are priced per square foot rather than per box.
• Railing and stairs, frequently the most expensive part per linear foot.
• Ground prep. Gravel under a low deck controls weeds and drainage — the gravel calculator sizes it by the ton.

Layout decisions that change the count

Diagonal decking looks good and costs more, because every board is cut at an angle at both ends and the offcuts rarely pair up. Add five to ten points of waste, and note that diagonal layouts usually require tighter joist spacing.

A picture frame border — a perimeter board running the other way — needs separate calculation and additional blocking underneath to fasten to.

Butt joints must land on a joist, so if your deck is longer than the boards you can buy, plan where the joints fall before ordering. Staggering them across rows looks deliberate; lining them up looks like a mistake.

For indoor equivalents, the flooring calculator handles plank goods sold by the carton, where waste works the same way but the gap does not exist.

For span tables, ledger attachment and railing load requirements, the American Wood Council’s DCA 6 deck guide is the prescriptive document most US building departments accept, and it is worth reading before framing anything you intend to have inspected.