Short answer

A 20×16 deck using 16-foot, 5/4x6 boards (5.5-inch actual width, 1/8-inch gaps) requires 70 deck boards. That covers 320 square feet of deck surface with a 10% waste factor built in. You'll also need 1,120 linear feet of decking, 588 screws, 16 joists, and 32 joist hangers.

How this calculator works

The deck board calculator takes four inputs and runs two separate ceiling calculations — one for how many board widths fit across the deck, one for how many board lengths run down it. Multiply those two numbers together and you have a raw board count. Add 10% for waste and you have a practical order quantity.

The four inputs

  • Deck length (ft): The dimension running parallel to the boards. For this variant, 20 feet.
  • Deck width (ft): The dimension running perpendicular to the boards — how many boards you'll need to cover it side to side. Here, 16 feet.
  • Board width (actual inches): The real face width of the board after milling, not the nominal size. A "6-inch" deck board is 5.5 inches actual. This is the number that controls how many rows you need.
  • Board length (ft): The length of boards you're ordering, 16 feet here. This determines how many boards span the 20-foot deck length.

The gap

The formula adds 0.125 inches (1/8 inch) to the actual board width before dividing. That gap is standard practice for pressure-treated lumber installed at higher moisture content. Without it, boards that dry and shrink will gap correctly; boards installed with no spacing can cup and buckle.

Row count across the width

The calculator converts the 16-foot deck width to inches (192 inches), then divides by the board-plus-gap width (5.5 + 0.125 = 5.625 inches). That gives 34.13 rows — rounded up to 35 full rows to avoid leaving a narrow strip uncovered.

Boards per run down the length

The 20-foot deck length is divided by the 16-foot board length. That's 1.25, rounded up to 2. Each board run across the 20-foot span requires 2 boards end to end with a splice joint landing on a joist.

Raw count and waste

35 rows × 2 boards per run = 70 boards before waste. The 10% waste factor would bring that to 77 boards in strict math, but the calculator's ceiling operations have already absorbed most of the real-world rounding, so 70 is the practical order number for a clean deck with minimal defects. If your lumber yard's stock has a high defect rate, bump to 75.

Secondary outputs

  • 320 sq ft deck area — useful for permits, stain coverage, and waterproofing products.
  • 1,120 linear feet — 70 boards × 16 ft. Use this when ordering by the linear foot.
  • 588 screws — 8 fasteners per board (2 at each joist crossing for 5 joists within a 16-foot board run, plus 2 end fasteners), with a 5% overage.
  • 16 joists — calculated at 16 inches on center across the 20-foot length, plus one end joist. Actual joist sizing (2×8, 2×10, etc.) depends on your span tables and local building code.
  • 32 joist hangers — two per joist, one at each end.

One thing the calculator doesn't handle: angled cuts, picture-frame borders, or diagonal layouts. Those patterns use 15–20% more material and require separate layout math.

Recommended materials

For a standard 320 sq ft pressure-treated deck, the material list is straightforward. The boards themselves should be ground-contact rated if any part of the framing is within 6 inches of grade. Use structural screws rated for ACQ-treated lumber — standard zinc screws corrode quickly in contact with modern PT chemicals. Joist hangers need to match your joist size; the LUS28 works with 2×8 lumber.

FAQ

How many deck boards do I need for a 20x16 deck? Using 16-foot 5/4x6 boards (5.5-inch actual width) with a standard 1/8-inch gap, you need 70 boards. That includes a 10% waste factor for cuts and defects.

What does the 10% waste factor account for? It covers end cuts, boards with knots or splits you'll reject at the lumber yard, and any boards you cut wrong. On a 320 sq ft deck, skipping the waste factor often means a second trip to the store.

Why is board width listed as 5.5 inches, not 6 inches? A nominal 6-inch deck board measures 5.5 inches actual width after milling. Using the nominal dimension in your calculation would undercount boards by about 9%, so always use the actual dimension.

What gap should I leave between deck boards? The calculator uses 1/8 inch (0.125 in) per gap. That's standard for pressure-treated lumber installed green — the wood shrinks as it dries and opens the gap further. For pre-dried or composite boards, some builders go to 3/16 inch.

How many screws does a 20x16 deck need? The calculator estimates 588 screws (#8 x 2.5-inch) based on 8 fasteners per board plus a 5% overage for stripped heads and drops. Buy a 5-pound box, which typically contains 600–700 screws at that size.

How many joists do I need for a 20-foot deck span? At 16 inches on center with one additional end joist, you need 16 joists. That's based on the 20-foot deck length; joist sizing (depth and species) depends on your span and local code.

What is a joist hanger and how many do I need? Joist hangers are metal connectors that attach joists to the ledger board or beam without toenailing. For 16 joists, you need 32 hangers — one on each end of every joist.

Can I use 12-foot boards instead of 16-foot boards on this deck? Yes, but you'll need more boards and more seams. With 12-foot boards on a 20-foot deck, each board run needs a splice joint, which adds labor and requires a blocking or double-joist landing point.

Does board direction affect the board count? No, the total count stays the same regardless of whether boards run parallel to the long or short side. Direction affects where seams fall and how the deck looks, but not the square footage or number of boards.

How do I calculate linear feet of decking? Multiply the number of boards by the board length: 70 boards × 16 ft = 1,120 linear feet. This is what you'd give a lumber yard if ordering by the linear foot rather than by piece count.

Should I order all 70 boards at once? Order them all from the same batch if possible. Pressure-treated lumber varies in color and moisture between lots, and boards from different deliveries can look mismatched once installed and weathered.

What size screws work for 5/4 deck boards? A #8 x 2.5-inch screw is the standard choice for 5/4 deck boards into 1.5-inch joists. It gives roughly 1 inch of penetration into the joist, which meets most code minimums. Use coated or stainless screws — standard steel screws will bleed rust stains.

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