Deck calculator
This deck calculator estimates the deck boards, joists and screws for a rectangular deck from its size, your board width and length, the gap between boards and the joist spacing. It counts materials; it is not a structural design.
- Boards and linear feet with the gap allowed for
- Joists at 12, 16 or 24 inches on center
- Screw count and a joist spacing check
Last updated
How to use the deck calculator
- Enter the deck length along the direction the boards run, and the depth along the joists (usually out from the house).
- Enter the board’s actual width. A nominal 6 inch board, 5/4 × 6 or 2 × 6, is 5.5 inches wide. Pick the gap you will leave between boards.
- Choose the board length you will buy, the decking type and the joist spacing.
- Read the number of boards, joists and screws. Add rim joists, ledger, beams, posts and footings from your plan or your building department’s guide.
How the deck materials are counted
Boards run along the deck’s length and sit on joists that run the other way. Each board plus its gap takes up the board width + gap, so the number of rows is:
rows = (deck depth in inches + gap) ÷ (board width + gap), rounded up
boards = rows × deck length × (1 + waste) ÷ board length, rounded up
joists = deck length in inches ÷ joist spacing, rounded up, + 1
Screws assume two face screws where each board crosses each joist: rows × joists × 2. Hidden fastener systems use one clip per crossing instead, so follow the maker’s count if you use them.
The waste allowance covers cuts and boards you reject. Diagonal or patterned layouts need more.
Worked example
A 16 ft × 12 ft deck with 5.5 inch boards, a 3/16 inch gap, 16 ft boards, joists 16 inches on center and 10% waste.
- Rows: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35, so 26 rows.
- Linear feet: 26 × 16 = 416 ft. With 10% extra: 457.6 ft.
- Boards: 457.6 ÷ 16 = 28.6, so 29 boards 16 ft long.
- Joists: 192 ÷ 16 = 12 spaces, so 13 joists 12 ft long.
- Screws: 26 rows × 13 joists × 2 = 676 screws.
Maximum joist spacing for wood decking
From Table R507.7 of the residential code (as printed in the 2025 California Residential Code, which follows the IRC). Spacing is on center, in inches. Composite and plastic decking follow the manufacturer’s span instead.
| Wood decking | Perpendicular, single span | Perpendicular, multiple span | Diagonal, single span | Diagonal, multiple span |
|---|---|---|---|---|
| 1¼ in thick (5/4) | 12 | 16 | 8 | 12 |
| 2 in thick | 24 | 24 | 18 | 24 |
A board resting on two joists is a single span; on three or more it is a multiple span. Diagonal means up to 45 degrees. Your local code edition and inspector have the final say.
What this calculator does not cover
- Structure. Joist size, beam size, post spacing and footings depend on spans, loads, wood species and local code. Use your building department’s deck guide or the American Wood Council’s prescriptive deck guide (DCA 6).
- Framing extras. Rim joists, the ledger, blocking, hangers and flashing are not counted.
- Stairs and railings. Count these separately.
Common questions
How many deck boards do I need for a 12x16 deck?
With 5.5 inch boards and a 3/16 inch gap, a 12 ft deep deck needs 26 rows. Running 16 ft boards along the 16 ft side, that is 26 boards, or 29 with 10% extra for waste.
How do I calculate deck boards per square foot?
With 5.5 inch boards and a 3/16 inch gap, each board covers 5.6875 inches of width, so you need about 2.1 linear feet of board per square foot (12 ÷ 5.6875). A 200 sq ft deck needs about 422 linear feet before waste.
What joist spacing do I need for a deck?
For 1¼ inch (5/4) wood decking, IRC Table R507.7 allows joists up to 16 inches apart when boards run square to the joists, and 12 inches on a diagonal. 2 inch thick wood decking can span up to 24 inches. For composite, follow the maker’s guide.
How many screws do I need for a deck?
With two screws at every joist, multiply board rows × joists × 2. A 16 × 12 ft deck with 26 rows and 13 joists takes about 676 screws. Buy a little extra for mistakes.
How much gap should I leave between deck boards?
It depends on the board. Many wood and composite boards are gapped between 1/8 and 1/4 inch side to side; check the installation guide for your boards, because wet lumber shrinks and composite expands with heat.
Sources
- 2025 California Residential Code, Section R507 Exterior Decks (Table R507.7). Maximum joist spacing for wood decking: 1¼ in wood 12/16 in perpendicular, 8/12 in diagonal; 2 in wood 24/24 and 18/24. Last checked 28 October 2026
See the methodology page for how YardageHQ builds and checks its calculators.