Planning a deck: boards, gaps, and fasteners that actually add up

Standing in the decking aisle with a tape measure and a phone calculator, most people size a deck the same way: floor area divided by board width, round up, done. That number is close enough to order lumber, and wrong enough to leave you short on fasteners, guessing at gap spacing, and unsure whether the box of hidden clips at twice the price of screws is actually worth it. A deck isn't one calculation — it's three that depend on each other: how many boards, how wide the gap between them, and how many fasteners every one of those boards needs at every joist it crosses.

The 3/16-inch gap isn't a style choice

Nearly every decking spec sheet calls for a gap of roughly 3/16 inch (0.1875") between boards, and it's tempting to treat that as arbitrary — tighter looks cleaner, right? It isn't arbitrary, for three reasons:

• Drainage. A deck with zero gap holds standing water after every rain, which accelerates rot, mildew, and — on wood decking — the freeze-thaw damage that splits boards from the inside.
• Wood movement. Pressure-treated lumber is installed wet and shrinks as it dries, sometimes closing a chunk of whatever gap you started with. Install boards tight together and they can end up warped or cupped against each other once the wood dries and finds it has nowhere to move. The 3/16" gap gives the boards room to shrink without leaving a canyon-sized gap or fusing back together.
• Debris. A working gap lets leaves, dirt, and snowmelt fall through instead of collecting in a trough on top of the deck.

Composite decking sometimes calls for a slightly different gap than wood — always check your specific product's spec sheet — but 3/16" is the standard reference point the industry designs around, and it's the default worth starting from if you don't have a manufacturer spec in hand.

How the gap turns into a board count

Once you know the gap, the number of rows across the deck's width follows directly: take the width in inches and divide it by (board width + gap), then round up — because a partial row at the end still needs a full board, just ripped narrower. That row count, multiplied by however many boards it takes end-to-end to span the deck's length, gives you the total board count.

Joist spacing sets your fastener count, not your board count

This is the piece people miss entirely: fasteners aren't counted per board, they're counted per joist crossing. Every board runs across every joist underneath it, and needs to be fastened at each one. The number of joists is determined by the deck's length and the joist spacing (commonly 16" on center, sometimes 12" for extra stiffness or heavier board spans) — divide deck length by joist spacing, round up, and add one more joist for the far edge that the division alone doesn't capture.

Total fasteners is then rows × joists × fasteners-per-crossing. That last multiplier is where screws and hidden clips diverge.

Screws vs. hidden clips

• Screws typically take two fasteners per crossing — one on each side of the board — visible from above, cheap per unit, and forgiving if one strips or needs replacing later.
• Hidden clips take one fastener-equivalent per crossing since the clip locks into the groove on the board's edge instead of driving through the face, leaving no visible screw heads for a cleaner look. Clips typically cost noticeably more per unit than a deck screw, so halving the fastener count doesn't automatically mean halving the fastener budget — price both out before assuming clips are the cheaper route.

A worked example

Take a 20×12 foot deck: boards 5.5" wide, gap 3/16" (0.1875"), 16-foot board stock, 16" joist spacing.

• Rows: (12 × 12) ÷ (5.5 + 0.1875) = 144 ÷ 5.6875 ≈ 25.3 → round up to 26 rows.
• Boards per row: 20 ÷ 16 = 1.25 → round up to 2 boards end-to-end per row.
• Total boards: 26 × 2 = 52 boards, or 832 linear feet of decking.
• Joists: (20 × 12) ÷ 16 = 15, plus one for the far edge = 16 joists.

With screws at 2 per crossing: 26 rows × 16 joists × 2 = 832 screws. At $2.25/linear ft for boards and $0.15 per screw, that's $1,872 in decking plus $124.80 in fasteners — $1,996.80 total.

Switch to hidden clips at 1 per crossing: 26 × 16 × 1 = 416 clips. Even at the same $0.15 per-unit price used here just to isolate the count difference, that's $62.40 in fasteners instead of $124.80 — but real clip hardware usually runs several times the price of a deck screw, so the actual savings (or lack of them) depends entirely on the price of the specific clip system you're pricing against a box of exterior screws.

Plan the whole deck before you order anything

Boards, gap, and fasteners aren't three separate shopping trips — they're one calculation that starts with the gap you choose and cascades through board count, row count, and every joist crossing underneath. Our deck board and fastener calculator takes your deck's dimensions, board size, gap, board length, and joist spacing, and returns the exact board count, row layout, and fastener total for either screws or hidden clips — along with a full material cost — so you know exactly what to load into the truck before the first board goes down.