Deck calculator: boards, joists and screws
A 12 x 16 ft deck with 5.5 in boards and 1/8 in gaps takes 26 rows of 16 ft boards, 29 boards with 10% extra. Joists at 16 in on center means 13 joists, and two screws per board at every joist adds up to 676 deck screws, close to CAMO's 350 per 100 sq ft.
Updated · By the Interior Redesign AI editorial team · Sources cited on the page
Deck boards to buy
29 × 16 ft
26 rows, 676 deck screws, 13 joists.
Joists
13 × 12 ft
at 16 in on center, both ends included
Deck screws
676
2 per board at every joist
| Deck area | 192 sq ft |
|---|---|
| Board rows (depth ÷ board width + gap) | 26 |
| Boards per row | 1 |
| Boards before extra | 26 |
| Boards incl. 10% extra | 29 |
| Rim board and ledger (front + house side) | 32 ft |
| Deck screws (2 × rows × joists) | 676 |
- OKA 12 ft joist span is within the 14 ft 0 in AWC DCA 6 Table 2 allows for 2x10 southern pine at 16 in on center (40 psf live load, No. 2 grade).
- OKComposite boards: 16 in on center fits Trex's 16 in residential span for 1 in boards laid square to the joists; diagonal layouts need closer spacing (Trex: 4 in less at 45°, half at 30°).
Material count only, not a structural design: footings, beams, ledger attachment and guards come from DCA 6 or your local code, and most decks need a permit. Screw count assumes 1 in or more from board ends and no extra screws at butt joints.
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Key numbers
- Fastening
- 2 #8 screws (or 2 8d threaded nails) per board at each joist (AWC DCA 6)
- Board gap
- About 1/8 in between wood boards (DCA 6); composite boards follow the maker's gap
- Joist spacing
- 12, 16 or 24 in on center in DCA 6 Table 2; Trex: 16 in for 1 in boards on residential decks
- Screw coverage
- CAMO: a 350-count box of Edge screws covers 100 sq ft at 16 in on center
- Minimum support
- Each piece of decking bears on at least 3 joists (DCA 6)
- Design load in DCA 6
- 40 psf live, 10 psf dead, No. 2 grade lumber, wet service
Worked example
A 12 x 16 ft deck off the back door
16 ft along the house, 12 ft out, 5.5 in wide boards in 16 ft lengths, 1/8 in gaps, 2x10 southern pine joists at 16 in on center.
- Rows: 144 in ÷ (5.5 + 0.125) in = 25.6, rounded up to 26 rows of one 16 ft board.
- Boards with 10% extra: 26 × 1.1 = 29.
- Joists: 192 in ÷ 16 in = 12 spaces, so 13 joists, each 12 ft. At 12 in on center it would be 17.
- Screws: 26 rows × 13 joists × 2 = 676. CAMO's rate of 350 per 100 sq ft gives 672 for 192 sq ft.
- Span: 12 ft is within the 14'0" DCA 6 allows for 2x10 southern pine at 16 in.
Buy 29 boards, 13 joists, two 16 ft rim/ledger boards and about 676 screws (two 350-count boxes).
How we calculate
Boards: the deck's depth (out from the house) divided by one board's face width plus the gap gives the number of rows; each row needs the deck's width in boards, so a row longer than one board needs two. We add 10% for end cuts and culled boards, an assumption you can change.
Joists: the deck's width divided by the spacing, rounded up, plus one, so both outer joists are counted. The rim board and ledger are the deck's width each. Screws are 2 per board at every joist crossing, the DCA 6 rule; the count leaves out extra screws at butt joints.
Span check: we compare the joist span (the deck's depth, assuming joists run from a ledger to a beam with no overhang) with the allowable span in AWC DCA 6 Table 2 for your lumber, size and spacing. It is a pointer to the table, not a structural design. DCA 6 also requires at least a 2x8 ledger, and 2x8 or deeper for any joist or rim that carries guard posts, so 2x6 joists suit only decks with no guard posts fastened to them. Footnote 7 notes the 18 ft entries are a prescriptive length cap, not the lumber's limit. Beams, posts, footings, ledger attachment and guards follow DCA 6 or your local code, and most decks need a permit.
Estimates, sources
Planning estimates and published guidelines, not quotes, designs or code rulings. Figures were checked on each source on the date shown. Local prices, your home and your jurisdiction's adopted code decide the real answer.
- DCA 6: Prescriptive Residential Wood Deck Construction Guide (2015 IRC edition) (American Wood Council, as of Oct 26, 2026): attach decking to each joist with 2 #8 screws (or 2 8d threaded nails), boards about 1/8 in apart, each piece on at least 3 joists; Table 2 joist spans at 12/16/24 in o.c., e.g. southern pine 2x8 13'1"/11'10"/9'8", 2x10 16'2"/14'0"/11'5" (40 psf live, 10 psf dead, No. 2, wet service)
- CAMO 2025 Product Catalog and Edge installation guides (CAMO (National Nail Corp.), as of Oct 26, 2026): catalog p.18: 1-7/8 in × #7 Edge screws, 350 count covers 100 sq ft, 700 covers 200 sq ft, 1,750 covers 500 sq ft. The 16 in o.c. basis, two fasteners per board at each joist and the 1 in end distance come from CAMO's installation guides via search excerpts and are not printed in the catalog
- 2024 Decking Installation Guide (Trex, as of Oct 26, 2026): span chart for 1 in boards laid perpendicular: 16 in o.c. residential, 16 in o.c. commercial at 100 psf and 12 in at 200 psf; 2x6 boards 24 in residential; diagonal: 4 in less at 45°, half the span at 30°
Maximum deck joist spans (AWC DCA 6, Table 2)
Southern pine, No. 2 grade, 40 psf live load and 10 psf dead load. Douglas fir, hem-fir, spruce-pine-fir, redwood and cedar span less; the calculator has those too.
| Joist | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 2x6 | 9'11" | 9'0" | 7'7" |
| 2x8 | 13'1" | 11'10" | 9'8" |
| 2x10 | 16'2" | 14'0" | 11'5" |
| 2x12 | 18'0" | 16'6" | 13'6" |
Composite decking and joist spacing
Composite and PVC boards are more flexible than 5/4 or 2x wood decking, so the maker's manual sets the joist spacing. Trex's 2024 installation guide spans 1 in boards 16 in on center on residential decks (12 in for heavy commercial loads, 24 in for 2x6 boards), 4 in less when laid at 45° and half the span at 30°. Check your board's guide before you frame.
How to use this tool
Measure the deck
Width along the house and depth out from it; the joists run in the depth direction.
Describe the boards
Face width, gap and the length you will buy.
Set the framing
Joist spacing, joist size and lumber species.
Read the list
Boards, joists, rim and ledger, screws, and the DCA 6 span check.
Questions
How many deck boards do I need for a 12x16 deck?
With 5.5 in boards and 1/8 in gaps running along the 16 ft side, 26 rows of 16 ft boards, about 29 with 10% extra.
How many screws do I need per square foot of deck?
About 3.5 per square foot at 16 in on center: two screws per board at every joist. CAMO rates a 350-count box at 100 sq ft. At 12 in on center you need about a third more.
How far apart should deck joists be?
16 in on center is the common choice and the residential spacing in Trex's guide; 12 in is used for diagonal decking, commercial decks and some composite boards. DCA 6 lists spans for 12, 16 and 24 in.
How far can a 2x8 deck joist span?
In southern pine at 16 in on center, 11'10" under DCA 6 Table 2; in Douglas fir, hem-fir or SPF, 11'1".
How much gap should I leave between deck boards?
DCA 6 says about 1/8 in for wood decking. Composite and PVC boards set their own gaps for heat expansion, so follow the board maker's guide.
Does this calculator design my deck?
No. It counts materials and checks joist span against DCA 6 Table 2. Beams, posts, footings, the ledger connection and guards need DCA 6, your local code and usually a permit.
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