Deck Calculator
Estimate deck boards, joists, linear feet of decking and screws for a rectangular deck from its size, board width and joist spacing.
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How to Calculate
Rows of decking = deck width in inches ÷ (board width + gap), rounded up. Each row needs as many boards as it takes to cover the deck length, so a 16 ft deck with 16 ft boards takes one board per row and a 20 ft deck takes two.
Joists run under the boards at right angles. Count = deck length in inches ÷ joist spacing, plus one for the end. Use 16 in on center for wood and most composite, 12 in for diagonal decking.
Screws assume 2 per board at every joist it crosses. Hidden fastener clips work out to about one clip per joist crossing. Buy about 5% extra of both.
See our methodology for how every calculator on this site is built.
Worked Example: A 14 × 20 ft Deck
A ground-level deck 20 ft long and 14 ft deep. Boards run along the 20 ft side, so the joists run front to back across the 14 ft depth. Decking is 5/4×6 (5.5 in actual width) in 20 ft lengths with a 1/8 in gap, on joists at 16 in on center.
- Width covered per row: 5.5 + 0.125 = 5.625 in
- Rows: (14 × 12) ÷ 5.625 = 168 ÷ 5.625 = 29.9, so 30 rows
- Boards per row: 20 ÷ 20 = 1
- Deck boards: 30 × 1 = 30 boards
- Linear feet: 30 × 20 = 600 ft
- Joists: (20 × 12) ÷ 16 = 15, plus one end joist = 16 joists, 14 ft each
- Screws: 30 rows × 16 joists × 2 = 960 screws
Matching board length to deck length saves both money and butt joints. If you used 16 ft boards on this deck, every row would need a second board, so 60 boards with 12 ft of each second board going to waste. The last row will likely get ripped to width, so pick out a straight board for it.
Boards and Screws per 100 ft²
| Board | Gap | Linear ft per 100 ft² | Screws per 100 ft² (16 in OC) |
|---|---|---|---|
| 5/4×6 or 2×6 (5.5 in) | 1/8 in | 213 | ~320 |
| 2×4 (3.5 in) | 1/8 in | 331 | ~500 |
| 5/4×6 (5.5 in) | 1/8 in, joists 12 in OC | 213 | ~430 |
Mistakes to Avoid
Joist count without rim and blocking
The calculator counts field joists plus the end. You also need two rim joists (or a ledger and a rim), and blocking between joists at midspan on longer spans. Add those separately.
Spacing composite on 16 in centers by default
Many composite boards are rated for 16 in straight, 12 in diagonal. Some cheaper composites need 12 in even run straight. Check the install guide before you frame.
Wet pressure-treated lumber
Fresh treated boards are soaked and will shrink in width. Butt them tight and they open to about 1/8 in on their own. Gap wet boards and you get 1/4 in gaps by fall.
Wrong fasteners
Treated lumber eats plain steel. Use coated deck screws or stainless, and joist hangers rated for ground contact lumber (G185 or ZMAX).
Practical Tips
- Crown every joist the same way, hump up, before you hang it.
- Stagger butt joints so no two sit on the same joist in adjacent rows.
- Pre-drill board ends within an inch of the cut to stop splitting.
- Set the deck at least 1 in below the door threshold and slope it 1/8 in per foot away from the house.
Rough Cost
Pressure-treated 5/4×6 decking runs roughly $1.50 to $3 per linear foot, cedar $3 to $6 and composite $3 to $7. A 5 lb box of deck screws is around $30 to $45. A built deck usually costs $25 to $50 per square foot for treated lumber and $40 to $80 for composite.
Sources
Common questions
Frequently Asked Questions
How many deck boards do I need for a 12 × 16 deck?
With 5.5 in boards and a 1/8 in gap running along the 16 ft side, 26 rows of 16 ft boards. Add a couple of spares for culls.
How far apart should deck joists be?
16 in on center for most wood and composite decking. Some composite brands require 12 in for diagonal patterns or on stairs.
How big a gap should be between deck boards?
About 1/8 in for kiln-dried lumber and composite. Wet pressure-treated boards can go tight, since they shrink as they dry.
How many screws per square foot of deck?
Roughly 3.2 screws per ft² with 5.5 in boards on 16 in centers. A 5 lb box of 2.5 in deck screws holds about 350 to 400.
Should deck boards run the long way?
Usually, because it means fewer butt joints. On wide decks, a single picture-frame border or breaker board hides seams where boards meet.