Short answer
A 16x8 deck (128 sq ft) requires 18 deck boards using standard 5/4x6 pressure-treated lumber at 5.5-inch actual width in 16-foot lengths. That count includes a 10% waste factor. You'll also need 152 deck screws, 13 joists at 16 inches on center, and 26 joist hangers.
How this calculator works
The deck board calculator takes four inputs: deck length, deck width, the actual face width of your boards, and the length of boards you plan to order. It outputs the number of boards to buy, plus secondary counts for screws, joists, and hangers. Here's how each piece of the formula works.
Rows across the width
The calculator divides the deck width (converted to inches) by the board's actual face width plus a 1/8-inch gap. That gap is not optional — it's the standard drainage and expansion spacing between deck boards. For an 8-foot-wide deck with 5.5-inch boards:
- 8 ft × 12 = 96 inches of width
- 96 ÷ (5.5 + 0.125) = 96 ÷ 5.625 = 17.07 rows
The result rounds up to 17 board rows across the width.
Boards per row (length direction)
Next, the calculator divides the deck length by the board-length you're ordering. For a 16-foot deck buying 16-foot boards: 16 ÷ 16 = 1 board per row. That rounds up to 1, meaning each row runs end-to-end with no seam. If the deck were 18 feet and you ordered 16-foot boards, you'd need 2 boards per row.
Total boards with waste
17 rows × 1 board per row = 17 boards net. Apply the 10% waste factor: 17 × 1.10 = 18.7, which rounds up to 18 boards. The waste factor accounts for end trim cuts, boards rejected at the yard for splits or excessive knots, and any mistakes during installation.
Screws
Each board gets 8 screws — 2 fasteners per joist crossing, across 4 joists on a 16-foot board. The calculator multiplies 18 boards × 8 screws × 1.05 overage and rounds up to 152 screws.
Joists
Joist spacing of 16 inches on center is the residential standard. For a 16-foot deck length: (16 ft × 12 in/ft) ÷ 16 in = 12 spaces, plus 1 end joist = 13 joists. The end joist is a rim joist at the far end of the deck, not part of the repeated spacing — it's easy to forget and a common shortage at the lumber yard.
Joist hangers
Each joist gets 2 hangers (one at each end where it connects to the ledger or beam). 13 joists × 2 = 26 joist hangers. If your rim joists are attached differently — toe-nailed, for example — adjust this count.
What the outputs represent
The 18-board count is your order quantity, not your net material need. The secondary outputs (linear feet of decking at 288 ft, screw count, joist count) are purchasing quantities for a single trip to the lumber yard. They assume standard 16-inch joist spacing and the board dimensions you entered. If you're using hidden fasteners, composite decking with different spacing, or a non-standard joist layout, you'll need to adjust those numbers manually.
One thing to check before ordering
Verify the actual face width printed on the board's end tag at the store. Different mills and species can vary by 1/16 to 1/8 inch. A nominal 5/4x6 is usually 5.5 inches, but if your specific boards are 5.375 inches, you'd fit an extra row and potentially save a board. Run the numbers with the measured width before you load the cart.
Recommended materials
For a standard 16x8 pressure-treated deck, you need boards rated for ground-contact or above-ground use depending on how the framing sits. Most residential above-ground decks use above-ground-rated (UC3B or UC4A) lumber, which is lighter and easier to work with than ground-contact stock. Buy joists from the same treatment grade as your region requires, and use structural screws rated for ACQ or MCA-treated lumber — standard zinc-plated screws will corrode.
- 5/4x6 pressure-treated deck board (16 ft)
- GRK Fasteners Cabinet Screw 2-1/2 inch (5 lb)
- 2x8 pressure-treated joist (16 ft)
- Simpson Strong-Tie LUS28 joist hanger (case)
FAQ
How many deck boards do I need for a 16x8 deck? You need 18 boards when using standard 5/4x6 pressure-treated lumber (5.5-inch actual width) in 16-foot lengths. That accounts for a 10% waste factor built into the calculation.
What does the 10% waste factor cover? It covers end cuts, boards with knots or splits you'll reject at the lumber yard, and any boards you split during installation. On a small deck like a 16x8, that's roughly 1–2 boards of cushion.
Why does the formula add 0.125 inches to board width? That 1/8-inch gap is the standard spacing between deck boards. It allows water to drain and gives the wood room to expand in wet weather. Without it, boards nailed tight will buckle after the first rain.
Can I use 12-foot boards instead of 16-foot boards on a 16x8 deck? Yes, but you'll need to add blocking at mid-span so every board end lands on a joist or blocking member. You'd use more boards (approximately 26–28 at 12 ft) and have more seams, which can trap moisture over time.
How many deck screws do I need for 18 boards? The calculator calls for 152 screws, based on 8 screws per board with a 5% overage. A 5-pound box of #8 x 2.5-inch screws typically contains around 350–400 screws, so one box covers this deck comfortably.
How many joists does a 16x8 deck need? At 16 inches on center, a 16-foot run needs 13 joists (12 spaces plus 1 end joist). That gives you a joist roughly every 16 inches across the full length of the deck.
What size joist should I use for a 16x8 deck? 2x8 joists at 16 inches on center are the standard for a 16-foot span. Check your local code before buying — some jurisdictions require 2x10 depending on species and loading requirements.
How many joist hangers do I need? Two hangers per joist (one on each end) equals 26 hangers total for 13 joists. Buy a case rather than individual hangers — it's cheaper and you'll want a few spares.
What is the actual width of a 5/4x6 deck board? The nominal size is 5/4x6, but the actual dimensions are 1 inch thick by 5.5 inches wide. Always use actual dimensions when calculating board count, not nominal sizes.
Should I buy all 18 boards or order extras? The 18-board count already includes a 10% waste buffer. If you're new to decking, ordering 1–2 extra boards is cheap insurance against splitting or a bad cut. Return what you don't use.
How much does the decking material cost for a 16x8 deck? Material costs vary by region and lumber market conditions. Get a current quote from your local supplier using the 18-board count — lumber prices shift frequently and published estimates go stale fast.
Can composite decking use the same board count? If the composite board has the same actual face width (5.5 inches) and you're buying 16-foot lengths, the count holds at 18. But composite products vary — check the manufacturer's actual dimensions and adjust accordingly.