Quick answer: deck board rows = deck width (in) ÷ (board width + gap), rounded up. Joists = deck length (in) ÷ spacing + 1. A 16 × 12 ft deck with 5½ in boards needs 26 rows of 16 ft boards and 13 joists at 16 in on center.
Parts of a deck
- Footings: concrete below the frost line that carry the weight into the ground.
- Posts: usually 6×6 treated lumber, standing on the footings in metal post bases.
- Beam: two or three boards fastened together, sitting on the posts and carrying the joists.
- Ledger: a board bolted to the house that carries the house end of the joists, on attached decks.
- Joists: the boards that run out from the house at 12–24 in spacing; the decking sits on them.
- Rim (band) joist: the board across the ends of the joists.
- Decking: the boards you walk on, running across the joists.
How many deck boards you need
- Count the rows. Deck width in inches ÷ (board width + gap), rounded up. A 12 ft deck (144 in) with 5½ in boards and ³⁄₁₆ in gaps: 144 ÷ 5.69 = 25.3, so 26 rows.
- Choose a length. Boards come in 8, 10, 12, 16 and 20 ft. A board that matches the deck length (or exactly half of a long board) means no joints and no waste.
- Count boards per row. If the deck is longer than your boards, the joints must land on a joist and are staggered from row to row, so some length is lost to trimming.
- Add 5–10% for split, bowed or badly marked boards and for mistakes.
Rows = ⌈ W × 12 ÷ (b + g) ⌉ · Boards = rows × boards per row
The calculator tries every board length and recommends the one that buys the fewest linear feet. Ties go to the length that needs the fewest cuts.
Deck board chart
Boards for common attached deck sizes, using 5½ in boards with ³⁄₁₆ in gaps, 16 in joist spacing and 5% extra.
| Deck | Area | Rows | Best board | Joists (16 in O.C.) | Screws |
|---|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 22 | 24 × 10 ft | 9 | ≈ 396 |
| 12 × 12 ft | 144 sq ft | 26 | 28 × 12 ft | 10 | ≈ 520 |
| 16 × 12 ft | 192 sq ft | 26 | 28 × 16 ft | 13 | ≈ 676 |
| 20 × 12 ft | 240 sq ft | 26 | 28 × 20 ft | 16 | ≈ 832 |
| 16 × 16 ft | 256 sq ft | 34 | 36 × 16 ft | 13 | ≈ 884 |
| 20 × 16 ft | 320 sq ft | 34 | 36 × 20 ft | 16 | ≈ 1,088 |
Joists: how many and what size
How many: deck length in inches ÷ joist spacing, rounded up, + 1. That count includes the two end joists.
What size: the joist must span from the ledger (or one beam) to the beam. The size check uses the 40 psf live load rows of 2021 IRC Table R507.6 (No. 2 grade, wet service, 10 psf dead load); the values below were checked against that table on 10 Nov 2026. It is a planning guide, not a structural design: it does not cover the snow-load rows (50–70 psf ground snow), other species groups, cantilever limits or local amendments. Many states and cities adopt a different edition or amend the table, and your building department has the final word.
| Joist | Southern pine 12 in | 16 in | 24 in | DF, hem-fir, SPF 12 in | 16 in | 24 in |
|---|---|---|---|---|---|---|
| 2×6 | 9′ 11″ | 9′ 0″ | 7′ 7″ | 9′ 6″ | 8′ 4″ | 6′ 10″ |
| 2×8 | 13′ 1″ | 11′ 10″ | 9′ 8″ | 12′ 6″ | 11′ 1″ | 9′ 1″ |
| 2×10 | 16′ 2″ | 14′ 0″ | 11′ 5″ | 15′ 8″ | 13′ 7″ | 11′ 1″ |
| 2×12 | 18′ 0″ | 16′ 6″ | 13′ 6″ | 18′ 0″ | 15′ 9″ | 12′ 10″ |
Joists can run past the beam (cantilever), which lets you set the posts in from the edge. Table R507.6 limits the cantilever by joist size, species and back span, and some combinations allow none. This calculator does not check the cantilever limit, so look up your joist in the table or ask your building department.
Ledger, rim and beams
The ledger is usually the same size as the joists and must be fastened to the house framing (not to siding or brick veneer) with lag screws, through-bolts or approved structural screws, with flashing above it to keep water out. The rim joist runs across the outer ends of the joists. The beam is commonly two or three 2× boards of the same depth as the joists or deeper, fastened together; longer beams are joined over a post.
How far a beam can span between posts depends on its size, the joist span and the species. As a rough guide, many residential beams have posts every 6–8 ft. The AWC DCA6 guide has beam span tables you can check your design against.
Posts, footings and concrete
Posts: posts per beam = beam length ÷ post spacing, rounded up, + 1. A 16 ft beam with posts every 6 ft needs 4. Most deck guides now call for 6×6 posts, sitting in metal post bases on top of the footings rather than buried in the ground.
Footings: one per post, below the frost line and sized for the load and your soil. The calculator works out the concrete for round footings and the number of 80 lb and 60 lb bags. For other footing shapes, use the concrete calculator.
| Footing | Concrete each | 80 lb bags each | 60 lb bags each |
|---|---|---|---|
| 10 in × 36 in deep | 1.64 cu ft | 3 | 4 |
| 12 in × 36 in deep | 2.36 cu ft | 4 | 6 |
| 12 in × 48 in deep | 3.14 cu ft | 6 | 7 |
| 16 in × 36 in deep | 4.19 cu ft | 7 | 10 |
| 18 in × 48 in deep | 7.07 cu ft | 12 | 16 |
Screws, clips and hardware
- Deck screws: 2 at every joist for every board: rows × joists × 2. Use coated or stainless screws rated for treated lumber.
- Hidden clips: about 1 clip at every joist for every board, plus starter and end clips. Check the clip maker’s coverage chart.
- Joist hangers: one per joist at the ledger, with the nails or screws the hanger maker specifies.
- Hurricane ties: one per joist at each beam it sits on.
- Ledger fasteners: often two every 16 in, staggered; the number depends on the joist span and the fastener.
- Post bases and post caps: one of each per post.
Railings
Under the IRC, decks more than 30 in above the ground need guards, typically 36 in high, with openings small enough that a 4 in sphere can’t pass through. With 2×2 balusters (1½ in wide), a gap of about 3½ in gives roughly 2.4 balusters per foot. Railing posts are usually no more than 6 ft apart.
The calculator measures railing along the open sides (three sides on an attached deck, four on a freestanding one), takes off the stair opening, and counts posts, balusters and top and bottom rails.
Wood vs composite decking
| Decking | Typical joist spacing | Gap | Notes |
|---|---|---|---|
| Pressure-treated pine | 16 in | Tight to ⅛ in (shrinks as it dries) | Least expensive; needs sealing |
| Cedar or redwood | 16 in | ⅛ in | Naturally weather-resistant; softer |
| Composite | 16 in straight, 12 in diagonal | ³⁄₁₆–¼ in | Low maintenance; check end gaps for temperature |
| PVC | 16 in straight, 12 in diagonal | ³⁄₁₆–¼ in | Light, moves with heat |
| Hardwood (ipe and similar) | 16 in | ⅛–³⁄₁₆ in | Very hard and durable; predrill |
Worked example
An attached deck 16 ft along the house and 12 ft deep, 3 ft off the ground, 5½ in composite boards with ³⁄₁₆ in gaps, joists 16 in on center, Southern pine.
Decking: 144 ÷ 5.69 = 26 rows. 16 ft boards cover a row in one piece, so 26 boards + 5% = 28 boards of 16 ft.
Joists: 192 ÷ 16 = 12, + 1 = 13 joists. Span 12 ft: 2×8 is good to 11 ft 10 in, so 2×10 (14 ft). Joists just under 12 ft long (the ledger takes 1½ in), bought as 12 ft.
Beam and posts: a double 16 ft beam with posts every 6 ft: 16 ÷ 6 = 2.67 → 3, + 1 = 4 posts and 4 footings. At 12 in × 36 in, that is 9.4 cu ft of concrete, 16 bags of 80 lb.
Railing: 16 + 12 + 12 − 4 ft stairs = 36 lin ft: about 87 balusters and 72 lin ft of top and bottom rail.
Notes for deck builders
Permit first. Most areas need a permit and inspections for footings and framing. The inspector’s span and footing rules override any calculator.
Picture-frame borders and breaker boards need extra boards and blocking. Add them as a separate line or raise the waste.
Diagonal decking needs 12 in joist spacing for most composites and about 15% more boards.
Ground contact. Use lumber rated for ground contact for posts, beams and joists near the ground, and flash the tops of joists and beams if you want them to last.
Tips
- Crown up. Install every joist with the crown (the slight bow) facing up.
- Stagger joints on long decks so they don’t line up row after row.
- Plan the whole project: press Add to my shopping list, then add the concrete and any cut list from the lumber calculator.
Frequently asked questions
How many deck boards do I need?
Divide the deck width in inches by the board width plus the gap to get the number of rows, then work out how many boards each row takes. A 16 × 12 ft deck with 5½ in boards and a 3/16 in gap needs 144 ÷ 5.69 = 26 rows; with 16 ft boards that is 26 boards, plus about 5% for culls and mistakes.
How many joists do I need for a deck?
Divide the length of the deck along the ledger (in inches) by the joist spacing, round up and add one. A 16 ft deck at 16 in on center needs 192 ÷ 16 = 12, + 1 = 13 joists, including the two end joists.
What size joists do I need?
It depends on the span, spacing and wood species. As a guide from the IRC deck tables (No. 2 grade, 40 psf live load), Southern pine 2×8 joists at 16 in on center span about 11 ft 10 in, 2×10s about 14 ft and 2×12s about 16 ft 6 in. Douglas fir, hem-fir and SPF span a little less. The calculator picks the smallest size that works and warns you if a size is too small.
How far apart should deck joists be?
Most wood decking is installed on joists 16 in on center. Many composite boards also allow 16 in for boards laid straight, but 12 in when the boards run at an angle or for stairs. Always check the decking maker’s instructions.
How many deck screws do I need?
Two screws at every joist for every board. Multiply the number of board rows by the number of joists, then by 2. A 16 × 12 ft deck with 26 rows and 13 joists needs about 676 screws; buy a little extra.
How many posts and footings does a deck need?
Posts are usually spaced 6–8 ft apart along each beam, with one at each end. A 16 ft beam with posts every 6 ft needs 4 posts and 4 footings. A ledger-attached deck has one beam; a freestanding deck usually has two.
How deep should deck footings be?
Footings must reach below the frost line in your area and sit on undisturbed soil. In mild climates that may be 12 in; in cold climates it can be 36–48 in or more. Your local building department sets the depth, and many require an inspection before you pour.
What gap should I leave between deck boards?
Wood decking is often gapped about ⅛ in, or butted tight if the boards are wet and will shrink. Composite and PVC boards usually need ³⁄₁₆–¼ in side to side and a gap at the ends that depends on temperature. Follow the maker’s guide.
Do I need a railing on my deck?
The IRC requires guards on decks more than 30 in above the ground, usually 36 in high, with openings small enough that a 4 in sphere cannot pass through. Many people add a railing on lower decks too.
How much does it cost to build a deck?
Material prices vary a lot by region and by decking type: pressure-treated wood is usually the least expensive, cedar and composite cost more, and hardwoods and PVC the most. Enter your local prices per foot of decking and framing and per bag of concrete in the calculator to price your own deck.
Is this deck calculator free to use online?
Yes. The CalcWall Deck Calculator is completely free and works online in any browser, on a phone, tablet or computer. There is nothing to download and no account to create, and the math runs right in your browser, so results appear as you type.