Deck Calculator
Enter your deck dimensions to estimate decking boards, joists, screws and material cost before you head to the lumberyard.
Formulas shown on this page · Last reviewed October 2026
Deck material formula
rows = deck width (in) ÷ (board width + gap)
boards = rows × deck length (ft) × (1 + waste) ÷ board length (ft)
joists = deck length (in) ÷ spacing + 1
screws = rows × joists × 2
The calculator assumes boards run along the length and joists span the width. Rows are rounded up because you cannot install a partial board, and the screw count is two fasteners at every board-and-joist crossing.
Boards and screws for common deck sizes
16 ft boards, 5.5 in wide, 1/8 in gap, 10% waste, joists 16 in on center.
| Deck size | Area | Boards | Joists | Screws |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 16 | 8 | 352 |
| 12 × 12 ft | 144 sq ft | 22 | 10 | 520 |
| 14 × 14 ft | 196 sq ft | 29 | 11 | 660 |
| 12 × 16 ft | 192 sq ft | 29 | 13 | 676 |
| 20 × 16 ft | 320 sq ft | 49 | 16 | 1,120 |
Practical tips
- Choose the board length that divides your deck length with the least offcut. A 16 ft deck uses 16 ft boards with almost no cutting.
- Joist count is the minimum. Add a rim joist on each end, doubled joists at stairs and under any planter or hot tub.
- Beams, posts, footings, railings and joist hangers are not included here, so budget extra for them.
- Pressure-treated, cedar and composite boards all have different gap requirements. Check the manufacturer sheet before fixing the gap.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
A 12×16 deck is 192 sq ft. With 5.5 in boards, a 1/8 in gap and 16 ft boards running the long side, you need 26 rows. With 10% waste that comes to about 29 boards.
How far apart should deck joists be?
Use 16 in on center for standard 5/4 or 2x6 wood decking. Drop to 12 in on center if the boards run diagonally or you use some composite decking that requires closer support.
How many deck screws do I need per 100 sq ft?
Roughly 350. At 16 in joist spacing, a 10×10 deck has 22 board rows crossing 8 joists, and two screws per crossing gives 352 screws.
How much waste should I add for a deck?
Add about 10% for straight boards and 15% for diagonal or herringbone patterns. Add more if many boards have knots, splits or crown problems you will cut around.