Deck Calculator
This deck calculator works out how many deck boards and joists you need for a rectangular deck. Enter the size, board width, gap and joist spacing, and it rounds up to whole boards with a waste allowance.
Result
29 deck boards (16 ft)
| Deck area | 192 ft² |
|---|---|
| Rows of boards | 26 |
| Boards per row | 1 |
| Total decking | 464 ft |
| Joists | 13 × 12 ft |
Results update as you type.
How Many Deck Boards Do I Need?
Decking is counted in rows. Work out how many boards fit side by side across the deck, multiply by how many boards each row takes end to end, then add some waste. Joists are counted along the length at your chosen spacing. The calculator does both. Here’s the math.
How to Calculate Deck Boards and Joists by Hand
1. Rows of boards
Rows = (deck width (ft) × 12) ÷ (board width (in) + gap (in)), rounded up
Use the board’s actual width. A nominal “6 inch” deck board is 5.5 inches wide.
2. Boards per row
Boards per row = deck length ÷ board length, rounded up
3. Total boards
Boards = rows × boards per row × (1 + waste), rounded up
4. Joists
Joists = (deck length (ft) × 12) ÷ joist spacing (in) + 1, rounded up
Each joist is as long as the deck is wide. The +1 accounts for the joist at the far end.
Worked example: a 20 × 10 ft deck with 12 ft boards
Boards run the 20 foot length, they’re 5.5 inches wide with a 1/8 inch gap, joists are 12 inches on center, and you’re adding 10% waste:
- Rows: 120 ÷ 5.625 = 21.3, rounded up to 22 rows
- Boards per row: 20 ÷ 12 = 1.67, rounded up to 2
- Boards: 22 × 2 × 1.10 = 48.4, rounded up to 49 boards of 12 ft
- Joists: 240 ÷ 12 + 1 = 21 joists, each 10 ft long
Notice that 12 foot boards on a 20 foot deck leave 4 feet of offcut per row. Switching to 10 foot or 20 foot boards would cut that waste a lot. Try it in the calculator.
Tips & Common Mistakes
Pick board lengths to suit the deck
Board length matters more than almost anything for decking waste. A 16 foot deck uses 16 foot boards perfectly, while 12 footers leave big scraps. If you need butt joints, plan them to land on a joist (or doubled joists) and stagger them across rows.
Gaps change by material
Composite boards expand and contract with temperature, and wood shrinks as it dries. Use the gap from your decking’s installation guide. The calculator lets you set any gap from 0 to 1 inch.
Framing isn’t all joists
The joist count covers the joists across the deck, including both end joists. You also need two rim joists along the long sides, beams, posts and a ledger if the deck attaches to the house. Size them from a span table such as the AWC’s DCA 6 guide. Set posts with our concrete calculator and put gravel under the deck for drainage and weed control.
Picture frames and stairs add boards
A border of boards around the edge (“picture framing”) and stair treads aren’t part of the main field. Add them separately. Stair treads alone often take a dozen or more boards.
Hardware adds up
Hidden fasteners or deck screws, joist hangers, structural screws and flashing tape all scale with deck size. Count joists × 2 for joist hangers on a ledger-attached deck, and buy fasteners by the coverage listed on the box.
Frequently Asked Questions
How many deck boards do I need for a 12x16 deck?
With boards running the 16 foot length, 5.5 inch boards and a 1/8 inch gap give 26 rows across the 12 foot width. That is 26 boards 16 feet long, or 29 with 10% waste. With 1/4 inch gaps it is still 26 rows, because each row rounds up.
What size gap should I leave between deck boards?
Follow the decking manufacturer's instructions. Trex, for example, specifies a 1/4 inch side-to-side gap for its hidden fastener system. With wood decking, gap spacing depends on how wet the boards are when installed. Kiln-dried boards are typically gapped about 1/8 inch, and wet pressure-treated boards are often butted tight because they shrink as they dry.
What joist spacing do I need for a deck?
Sixteen inches on center is standard for most residential decks with 1 inch-thick composite or 5/4 wood decking. Diagonal decking and some composite products need 12 inch spacing. Use 24 inches only where the decking is rated for it. The manufacturer's span chart and your local code have the final say.
Which direction should deck boards run?
Deck boards run perpendicular to the joists. On a deck attached to a house, that usually means parallel to the house wall, with joists running out from the ledger. Diagonal boards look striking but need closer joist spacing and more waste.
Should I buy deck boards as long as the deck?
If you can, yes. Boards that span the full length avoid butt joints, which look busier and need extra blocking or a doubled joist underneath. If your deck is longer than the boards, stagger the joints so they don't line up row to row.
Does this calculator include beams, posts and the ledger?
No. It covers the decking surface and the joists. Beam size, post size, footing depth and ledger attachment depend on spans and loads set by your building code, and most areas need a permit for an attached deck.
Related Calculators
Sources & Assumptions
- Rows = deck width ÷ (board width + gap), rounded up; boards = rows × boards per row × (1 + waste), rounded up; joists = deck length ÷ joist spacing + 1.
- Trex hidden fastener installation instructions — 6.4 mm (1/4 in) width-to-width gap
- American Wood Council, DCA 6 Prescriptive Residential Wood Deck Construction Guide — joist spans and framing details.
- Rim joists along the two long sides, beams, posts, ledger, stairs and railings are not included.
- Lengths are rounded only for display. Board and joist counts always round up.
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