How the deck calculator counts boards
Picture the deck from above with the house along the top. The joists run out from the ledger on the house to the rim at the far edge, and the deck boards run across them, parallel to the house. So the number of board rows depends on the projection, and the number of boards in each row depends on the length.
rows = round up (projection ÷ (board width + gap))
boards per row = round up (deck length ÷ board length)
boards to buy = rows × boards per row × (1 + waste)
joists = round up (deck length ÷ spacing) + 1
screws = rows × joists × 2
Take a 16 × 12 foot deck with 5 1/2″ boards, 3/16″ gaps and joists at 16″ on center. Each row takes 5 11/16″, so 144″ of projection needs 26 rows. With 16-foot boards that's one board per row, 26 boards, or 29 with 10% waste. That's 416 linear feet of decking. The 192″ length is 12 bays of 16″, which means 13 joists counting both ends, 338 places where a board crosses a joist, and 676 screws.
One detail the math exposes: 25 full rows and their gaps take up 142 3/16″, so the 26th row only needs 1 13/16″. A sliver that thin looks bad and splits. Either let the decking run a little past the rim, open or close the gap slightly, or rip the first and last rows to matching widths. The calculator shows the last row's width so you can decide before you start.
Butt joints, board lengths and waste
If the deck is longer than your boards, rows need butt joints, and every joint has to land on a joist. You also don't want joints lined up across neighboring rows. The drawing shows a simple stagger. That's why it's worth pricing out a longer board: a 20-foot board on an 18-foot deck wastes 2 feet a row, but a 16 and a short piece wastes more labor and leaves a joint in every row.
For waste, 5% is enough on a simple rectangle with lengths that match. Use 10% if you're cutting around posts or the boards come with a few bad ones, and 15% for diagonal or picture-frame decking.
Wood, composite and hidden fasteners
- Pressure-treated wood is often installed wet and shrinks. Many builders butt the boards tight and let the drying open the gap. If yours are kiln-dried after treatment, gap them from the start.
- Composite and PVC boards expand and contract along their length with temperature. The end gaps and side gaps are set by the manufacturer, and so is the maximum joist spacing. Grooved-edge boards are meant for hidden clips.
- Hidden clips sit in the board edges, one per joist crossing. The first and last rows are usually face-screwed or fixed with a starter clip, so add a handful of screws even on a clip deck.
What this doesn't size
This calculator counts joists. It doesn't size them. How far a 2×8 or 2×10 can span depends on the species, grade, spacing and load, and getting it wrong makes a bouncy deck at best. Size joists and beams from the American Wood Council's Prescriptive Residential Wood Deck Construction Guide (DCA 6) or the span tables in your local code, and pull a permit. A joist span chart is planned for this site, built only from those cited tables.
Building steps off the deck? The stair calculator works out the risers from the deck surface to grade, and the stringer calculator lays out the cuts.