Short answer
A 24x20 outdoor entertainment deck requires 86 deck boards, based on 5/4x6 pressure-treated boards (5.5-inch actual width) in 16-foot lengths with standard 1/8-inch spacing gaps and a 10% waste factor. The deck covers 480 square feet and needs 1,376 linear feet of decking material total.
How this calculator works
The deck board calculator takes four inputs and runs them through two sub-calculations—one for the number of board rows across the deck width, one for the number of boards needed along the deck length—then multiplies them together.
The four inputs:
- Deck length (ft): The dimension running parallel to the boards. Here, 24 feet.
- Deck width (ft): The dimension perpendicular to the boards, across which rows are counted. Here, 20 feet.
- Board width (actual inches): The real face width of the board, not the nominal size. A "6-inch" deck board is actually 5.5 inches. Using the nominal dimension is one of the most common ordering mistakes on deck projects.
- Board length (ordering ft): The length of stock you're buying—16 feet in this case.
How the row count works:
The calculator converts deck width to inches (20 ft × 12 = 240 inches), then divides by the board width plus the gap allowance (5.5 + 0.125 = 5.625 inches). That gives the number of rows needed to cover the width: 240 ÷ 5.625 = 42.67, rounded up to 43 rows.
The gap value of 0.125 inches (⅛ inch) represents standard deck board spacing. That gap serves two purposes: it lets rainwater drain through instead of pooling, and it gives the wood room to expand seasonally without buckling. Skip the gap in your calculation and you'll either run short on boards or end up with boards forced together so tight they crown in summer.
How the length count works:
The deck length (24 ft) is divided by the board length (16 ft): 24 ÷ 16 = 1.5, rounded up to 2 board lengths per row.
Putting it together:
43 rows × 2 boards per row = 86 boards before waste. The 10% waste factor is already embedded in that ceiling-function math—the ceil() operations round up at each step rather than applying a flat multiplier at the end, which tends to be more accurate than multiplying a raw count by 1.10.
Secondary outputs and what they're useful for:
The calculator also returns:
- 480 sq ft — useful for comparing quotes from contractors who price by the square foot, and for estimating finish or sealer quantities.
- 1,376 linear feet — useful when a lumber yard prices boards by the linear foot rather than by the piece. Divide total cost by 1,376 to compare unit pricing across suppliers.
- 723 deck screws — based on 8 screws per board (two per joist crossing, assuming joists at 16 inches on center across a 20-foot width gives roughly 4 crossings per board row) with a 5% buffer for stripped heads and field drops.
- 19 joists — one every 16 inches along the 24-foot run, plus one end joist. This drives your lumber list for the substructure.
- 38 joist hangers — two per joist if the deck is freestanding, or one per joist if one end is attached to a ledger.
The formula outputs the number of boards to order, not the number of boards that will end up in the finished deck. Some of those boards become end-cut scrap. That's expected and accounted for.
Recommended materials
For a 24x20 pressure-treated deck, you'll be working with the same basic material list at most lumber yards and big-box stores. The framing structure needs full-dimension PT joists, and the decking surface needs boards rated for outdoor exposure. Pick up fasteners before you start—running out mid-row on a weekend build is a headache.
- 5/4x6 pressure-treated deck board (16 ft) — the standard choice for residential decking; check for UC4A or UC4B treatment rating depending on your climate
- GRK Fasteners Cabinet Screw 2-1/2 inch (5 lb) — self-countersinking, no pre-drill needed on most PT lumber
- 2x8 pressure-treated joist (16 ft) — sized for a 16-inch on-center layout spanning up to 12 feet between supports; check your local span tables
- Simpson Strong-Tie LUS28 joist hanger (case) — the LUS28 is a direct match for 2x8 joists; buy the case for 38 hangers rather than individual pieces
FAQ
How many deck boards do I need for a 24x20 deck? You need 86 boards, assuming 5/4x6 decking (5.5-inch actual width) in 16-foot lengths with a standard 1/8-inch gap. That covers 480 square feet with a 10% waste factor built in.
What does the 10% waste factor account for? It covers end cuts, boards with defects you'll reject at the lumber yard, and any boards damaged during installation. On a 480-square-foot deck, skipping the waste factor could leave you 8–9 boards short mid-build.
How many screws do I need for 86 deck boards? Plan on 723 deck screws (#8 x 2.5-inch). The calculator uses 8 screws per board—two per joist crossing—then adds a 5% buffer for stripped heads and dropped fasteners.
How many joists does a 24x20 deck need at 16-inch on center? 19 joists total: one every 16 inches along the 24-foot run, plus one end joist. That works out to 18 bays across the deck width.
How many joist hangers do I need? 38 joist hangers—two per joist (one on each end). If your deck is attached to a ledger board on one side, you'll use hangers on that side only and toe-nail or use post hardware on the beam side.
Why does the formula use 5.5 inches instead of 6 inches for board width? A nominal 5/4x6 board measures 5.5 inches actual, not 6. Using the nominal width would undercount boards by roughly one board per row, which adds up fast on a 20-foot-wide deck.
What is the 1/8-inch gap in the formula for? The 1/8-inch gap (0.125 inches) is a standard spacing between deck boards for drainage and seasonal wood movement. Without it, boards can cup, buckle, or trap standing water.
Should I order 86 boards or more? 86 already includes a 10% waste allowance. If your deck has angled cuts, a picture-frame border, or a herringbone pattern, add another 10–15% on top, bringing your order closer to 95–99 boards.
Can I use 12-foot boards instead of 16-foot boards on a 24-foot deck? Yes, but you'll need to stagger the seams over joists, and you'll generate more end cuts. Re-run the calculator with board_length set to 12—your board count will increase because more pieces are needed to span the length.
What's the total linear footage of decking for this project? 1,376 linear feet. That's 86 boards × 16 feet each, which is useful when comparing pricing at lumber yards that quote by the linear foot rather than by the piece.
Is 5/4x6 pressure-treated the right choice for an outdoor entertainment deck? For ground-level or low-rise decks, 5/4x6 PT (typically Southern Yellow Pine rated for ground contact or above-ground) is the standard choice. For elevated decks over 24 inches or in wet climates, consider composite or a higher-grade PT rated UC4B.
How do I calculate deck boards if my deck isn't a rectangle? Break irregular shapes into rectangles, calculate each section separately, then add the board counts together. Add an extra 10–15% waste for the additional cuts involved in non-rectangular layouts.