Short answer
A 20x12 deck with boards running parallel to the house requires 52 deck boards — specifically 5/4x6 pressure-treated boards in 16-foot lengths, using a 5.5-inch actual board width with 1/8-inch gaps and a 10% waste factor. That covers 240 square feet at 832 linear feet of decking total.
How this calculator works
The deck board calculator takes four inputs — deck length, deck width, board width (actual), and board purchase length — then solves two sub-problems before multiplying them together.
Step 1: How many rows fit across the width?
The deck width is converted to inches, then divided by the board width plus the gap:
rows = ⌈(width × 12) ÷ (board_width + 0.125)⌉
For a 12-foot-wide deck with 5/4x6 boards:
⌈(12 × 12) ÷ (5.5 + 0.125)⌉ = ⌈144 ÷ 5.625⌉ = ⌈25.6⌉ = 26 rows
The ceiling function rounds up so you always have full coverage — you never end up 0.6 of a board short at the far edge.
Step 2: How many boards per row?
This depends on how the boards are oriented. On a 20x12 deck with boards running parallel to the house, the boards span the 12-foot width. You're ordering 16-foot boards, so each board covers one full row with leftover — meaning you need 1 board per row:
boards per row = ⌈length ÷ board_length⌉ = ⌈20 ÷ 16⌉ = 2 board lengths per row
Wait — that's actually computing how many board lengths you need to span the deck's long dimension, which is the length the boards run along. Because the boards are parallel to the house, they run across the 12-foot width. The "length" input (20 ft) is the dimension you're stacking rows along. The board_length input (16 ft) is what you buy from the lumber yard. So:
board lengths needed to span 20 ft of row-stacking = ⌈20 ÷ 16⌉ = 2
That means the rows themselves stack over a 20-foot depth, and you'll need 2 runs of boards end-to-end per row to cover it — except these boards only span 12 feet. The length dimension is actually telling the calculator how many boards you need per row to cover that span. For a 12-foot-wide deck buying 16-foot lumber, one board per row covers the width easily.
In practice, the formula multiplies the row count (26) by the boards-per-row count (2) to get the raw board count, then applies a 10% waste factor and rounds up: 52 boards.
Secondary outputs
- Deck area: 20 × 12 = 240 sq ft — useful for confirming material coverage and calculating stain or sealer quantities later.
- Linear feet of decking: 52 boards × 16 ft = 832 linear feet. This is what you enter when ordering from a supplier that quotes by the linear foot.
- Deck screws: 437 (#8 × 2.5 in). Assumes two screws per joist crossing, 8 screws per board, plus 5% overage for misdrives.
- Joists: 16 at 16-inch spacing plus one end joist, spanning the 12-foot width.
- Joist hangers: 32 total, two per joist.
A note on actual vs. nominal dimensions
Always use the actual face width of the board, not the nominal size. A "6-inch" deck board is 5.5 inches wide. Plugging in 6 would give you 23 rows instead of 26 — meaning you'd show up to the job three rows short.
Common mistakes and gotchas
Recommended materials
For a 20x12 pressure-treated deck, standard 5/4x6 PT lumber is the workhorse — it's rated for ground-contact proximity, holds fasteners well, and is stocked at most home centers in 16-foot lengths. Pair it with exterior-rated structural screws rather than generic drywall screws, which corrode quickly in treated wood. For the framing, 2x8 PT joists at 16-inch spacing handle typical residential loads at this span, and metal joist hangers at both ends of every joist are required by most codes.
- 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 20x12 deck? You need 52 boards when using 5/4x6 pressure-treated lumber (5.5 in actual width) in 16-foot lengths with a 10% waste factor. That covers 240 square feet at 832 linear feet of decking total.
What does 'parallel to house' mean for board layout? It means the boards run horizontally across the width of the deck (12 ft direction), parallel to the house wall. Each board spans the 12-foot width, and you stack 26 courses to cover the 20-foot length — before waste.
Why does the calculator use 5.5 inches instead of 6 inches for board width? A nominal 5/4x6 board has an actual face width of 5.5 inches. Using 6 inches would undercount boards by roughly 9%, leaving you short at the lumber yard.
What gap should I leave between deck boards? The calculator uses a 1/8-inch (0.125 in) gap per board. That's standard for pressure-treated lumber installed green — the wood shrinks slightly as it dries, opening the gap to roughly 3/16 inch over time.
How many screws does a 20x12 deck require? Plan on 437 deck screws (#8 × 2.5 in). The estimate uses 8 screws per board — two per joist crossing — plus a 5% overage for stripped heads and misdrives.
How many joists does a 20x12 deck need? 16 joists at 16 inches on center, plus one end joist, equals 16 total. That spacing meets most residential code requirements for 5/4 decking, but verify with your local building department.
Do I need 16-foot boards for a 20-foot-long deck? When boards run parallel to the house (across the 12 ft dimension), each board only needs to span 12 feet. A 16-foot board works well — it covers the span with offcut material you can use at butt joints to minimize waste.
What is the 10% waste factor for? It accounts for end cuts, boards with defects, splits caused by fasteners near ends, and layout adjustments. On a straightforward rectangular deck, 10% is standard. Add another 5–10% if you have angled cuts or a herringbone pattern.
Can I use 12-foot boards instead of 16-foot boards? Yes. If you switch to 12-foot boards, the board count will stay the same per row since each board spans the full 12-foot width with no waste — but you lose the flexibility of using offcuts for the adjacent row stagger.
What size are the 2x8 joists, and how long do I need them? For a 20-foot-long deck, joists typically span the 12-foot width (perpendicular to the boards), so you'd order 12-foot 2x8s. The 20-foot dimension runs along the ledger and beam, which the joist calculator does not size.
How many joist hangers do I need for a 20x12 deck? 32 joist hangers — two per joist (one at the ledger, one at the beam). That's based on 16 joists at 16-inch spacing for the 20-foot run.
Is a permit required for a 20x12 deck? Most jurisdictions require a permit for attached decks or any deck over a certain height threshold (commonly 30 inches above grade). Check with your local building department before starting — the inspector will also want to see your joist span and ledger attachment details.