Deck Calculator

A deck calculator turns your deck’s length and depth into a full framing and surface takeoff: decking boards, joists, rim joist footage, posts, and screws. Rows of decking = deck depth in inches ÷ (board width + gap). Joists = length in inches ÷ spacing + 1. Screws = rows × joists × 2. A 16 × 12 ft deck with 16 ft boards and 16 in joist spacing needs 29 boards (with 10% waste), 13 joists, 3 posts, and 676 screws.

Enter the deck length along the house, the depth out from the house, your board width and length, joist spacing, board gap, and waste allowance above. The calculator assumes a rectangular deck attached to the house with a ledger, joists running out from the house, and decking laid parallel to the house.

How the deck calculator works

Picture the deck from above. The joists run out from the house like the teeth of a comb, spaced along the deck’s length. The decking boards run parallel to the house, crossing every joist.

• Area = length × depth
• Board rows = (depth × 12) ÷ (board width + gap), rounded up
• Boards per row = length ÷ board length, rounded up
• Boards = rows × boards per row × (1 + waste %), rounded up
• Joists = (length × 12) ÷ joist spacing, rounded up, + 1. This includes the two end joists, and a deck that is not an exact multiple of the spacing still gets a joist at the far end.
• Joist length = deck depth
• Rim joist = 2 × length: one ledger board against the house and one rim (band) board across the outer ends of the joists
• Posts = length ÷ 8, rounded up, + 1. This assumes one beam along the outer edge with posts no more than 8 ft apart.
• Screws = rows × joists × 2: two screws per board at every joist it crosses

Worked examples

All three use the calculator’s outputs, so you can enter the same numbers and match them.

Example 1: 16 × 12 ft deck, 5.5 in boards, 16 ft lengths, 16 in on center, 3/16 in gap, 10% waste

• Area: 16 × 12 = 192 sq ft
• Rows: 144 ÷ (5.5 + 0.1875) = 25.3, rounded up to 26
• Boards: 26 rows × 1 board per row × 1.10 = 28.6, rounded up to 29 boards × 16 ft
• Joists: 192 ÷ 16 = 12, + 1 = 13 joists × 12 ft
• Rim joist: 2 × 16 = 32 ft
• Posts: 16 ÷ 8 = 2, + 1 = 3
• Screws: 26 × 13 × 2 = 676

Change only the joist spacing to 12 in and you get 17 joists and 884 screws. Tighter spacing adds framing and fasteners but makes the floor feel stiffer.

Example 2: 24 × 12 ft deck with 12 ft boards (butt joints)

• Area: 288 sq ft
• Rows: 26 (same 12 ft depth)
• Boards per row: 24 ÷ 12 = 2
• Boards: 26 × 2 × 1.10 = 57.2, rounded up to 58 boards × 12 ft
• Joists: 288 ÷ 16 = 18, + 1 = 19 joists × 12 ft
• Rim joist: 48 ft; Posts: 24 ÷ 8 = 3, + 1 = 4
• Screws: 26 × 19 × 2 = 988

Each row has a butt joint, and every joint must land on a joist. Stagger joints from row to row, and add a doubled joist or blocking under them so both board ends get two screws.

Example 3: 12 × 10 ft deck with 2x4 boards (3.5 in), 12 ft lengths, 1/8 in gap

• Area: 120 sq ft
• Rows: 120 ÷ (3.5 + 0.125) = 33.1, rounded up to 34
• Boards: 34 × 1 × 1.10 = 37.4, rounded up to 38 boards × 12 ft
• Joists: 144 ÷ 16 = 9, + 1 = 10 joists × 10 ft
• Posts: 12 ÷ 8 = 1.5, rounded up to 2, + 1 = 3
• Screws: 34 × 10 × 2 = 680, about 567 per 100 sq ft

Narrow boards need roughly 1.5 times as many rows as 5.5 in boards, so they use more screws for the same deck.

Deck material by size

Defaults: 5.5 in boards with a 3/16 in gap, boards as long as the deck (12 ft boards for the 10 ft deck), 16 in on center, 10% waste.

Deck size	Area	Boards	Joists	Posts	Screws
10 × 10 ft	100 sq ft	25 × 12 ft	9 × 10 ft	3	396
12 × 12 ft	144 sq ft	29 × 12 ft	10 × 12 ft	3	520
16 × 12 ft	192 sq ft	29 × 16 ft	13 × 12 ft	3	676
20 × 12 ft	240 sq ft	29 × 20 ft	16 × 12 ft	4	832
16 × 16 ft	256 sq ft	38 × 16 ft	13 × 16 ft	3	884
20 × 20 ft	400 sq ft	48 × 20 ft	16 × 20 ft	4	1,376

The first column is length along the house × depth out from the house. A 12 ft deep deck needs the same 26 rows no matter how long it is; only the board length changes. The 10 × 10 ft deck has 120 ÷ 16 = 7.5, which rounds up to 8 gaps, so 9 joists.

Board coverage

Each row of decking covers its width plus one gap:

Board	Gap	Coverage per row	Rows per foot of depth	Linear ft of board per sq ft
5.5 in (5/4x6 or 2x6)	3/16 in	5.6875 in	2.11	2.11
5.5 in	1/8 in	5.625 in	2.13	2.13
3.5 in (2x4)	3/16 in	3.6875 in	3.25	3.25

So 100 sq ft of 5.5 in decking with a 3/16 in gap takes about 211 linear feet of board before waste. Gap size matters: composite boards often specify a gap based on the temperature at install and the board length, so follow the manufacturer’s chart.

Screws per square foot

The rule of thumb used by fastener makers is about 350 deck screws per 100 sq ft for 5.5 in boards on joists 16 in on center. The formula above lands right on it: the 16 × 12 deck uses 676 screws on 192 sq ft, which is 352 per 100 sq ft.

• 12 in joist spacing: about one-third more screws
• 3.5 in boards: about 55% more screws
• Hidden-fastener clips: count clips per board-joist crossing instead, typically about one clip per board at each joist, so roughly half the screw count (check the maker’s coverage chart)

Buy screws rated for your lumber: treated lumber needs coated or stainless fasteners, and composite boards usually need the maker’s color-matched screws.

Joist spacing and span

Joist spacing is set by the decking; joist size is set by the span.

• 16 in on center: standard for 5/4 wood and most composite boards laid square to the joists.
• 12 in on center: typical for diagonal layouts and for many composite or PVC boards laid on a diagonal or used on stairs.
• 24 in on center: generally only for 2x (1.5 in thick) wood decking laid square to the joists.

How far a joist can span from ledger to beam depends on the lumber size, species, grade, spacing, and your snow load. The International Residential Code has deck joist, beam, and footing tables in section R507, and the American Wood Council’s DCA 6 guide presents the same information for deck builders. Look up your exact lumber there, or use the deck joist calculator and joist spacing calculator as a starting point.

How to measure for a deck

1. Length: measure along the house wall where the ledger will go.
2. Depth: measure straight out from the house to the outer edge of the deck.
3. Overhang: if the boards will overhang the rim by an inch or so, add it to the depth.
4. L-shaped or wraparound decks: split them into rectangles, run each one, and add the results. Subtract one shared joist where two sections meet.
5. Stairs: count separately with the stair stringer calculator.

Common mistakes

• Mixing up length and depth. Joists run out from the house; boards run along it. Swapping the two changes the joist and row counts.
• Ignoring the gap in the math. Each gap adds coverage, so leaving it out overcounts: 144 ÷ 5.5 = 26.2, or 27 rows, instead of 26. The bigger risk is installing boards with no gap at all, which traps water and debris.
• Choosing board lengths that waste a lot. A 14 ft deck built from 12 ft boards needs two boards per row. 16 ft boards cover it in one, with less cutting.
• Not landing butt joints on joists. Every board end needs solid framing under it.
• Skipping footings and beam sizing. Footing size and depth depend on soil and frost depth; see the footing size calculator and beam span calculator.

From material list to budget

Once you have quantities, the deck cost calculator prices materials and labor, and the deck stain calculator covers finishing a wood deck. If you only need a board count from total square footage, the decking board calculator and composite decking calculator are quicker. Run this deck calculator first so the board, joist, and screw counts all come from the same dimensions.

Most attached decks need a building permit. Ledger attachment, joist and beam sizing, footing depth, guard height, and stair rules are set by your local code. Confirm your plan with your building department before you buy lumber.