Deck Boards Calculator
Deck boards, screws, and area from deck size and board width.
What it does
Decking lays boards side by side across the width with small drainage gaps, so the count is width divided by board plus gap, rounded up. Screws follow at two per joist crossing, and joists land every sixteen inches along the length.
The tool above takes deck size, board width, and gap in imperial or metric. It returns board count, screw count, and deck area.
Gaps do structural work beyond looks. Boards swell with rain and shrink with sun, and tight joints trap water that rots edges years early. Hidden fasteners set their own spacing per clip maker.
Picture framing the border eats a few extra boards past the field count. Stairs, benches, and skirting each want their own takeoff on top.
How to use
- Switch the unit system first: imperial for feet, metric for meters.
- Enter deck length along the boards and width across them.
- Enter actual board width plus the gap.
- Buy boards plus a few spares for crooked stock.
Formula
- board + gap
- Width each board consumes along the deck width.
- 16 in / 40 cm
- Standard joist spacing along the length.
- x 2
- Two screws per board per joist crossing.
Diagonal patterns lengthen cuts and waste near fifteen percent more boards. Straight runs use this math directly.
Worked example
Example: 16 x 12 ft deck, 5.5 in boards, quarter inch gaps
Deck 16 by 12, boards 5.5 in wide with 0.25 in gaps.
- Boards: 144 / (5.5 + 0.25) = 144 / 5.75 = 25.04, round up to 26.
- Joists: 192 / 16 = 12, plus one end joist = 13.
- Screws: 26 x 13 x 2 = 676.
- Area: 16 x 12 = 192 ft2.
Reference table
| Board | Per 12 ft | Screws (13 joists) |
|---|---|---|
| 2x4 (3.5 in) | 39 | 1014 |
| 2x6 (5.5 in) | 26 | 676 |
| 5/4x6 (5.5 in) | 26 | 676 |
| 2x8 (7.25 in) | 20 | 520 |
Limits
- Straight runs assumed. Patterns and borders add boards.
- Framing lumber and posts need separate takeoffs.
- Hidden clips replace screw counts.
- Stairs, rails, and skirting excluded.
Common mistakes
- Butting short boards mid span instead of buying length.
- Skipping gaps so water rots edges.
- One screw per crossing, which cups boards.
- Ignoring crown direction on joists and boards.
- Forgetting stairs in the board order.
Try it yourself
Quick checks. Answers sit tucked under each question.
10 ft wide deck, 5.5 in boards, 0.25 gap. Boards?
120 / 5.75 = 20.87, so 21 boards.
Screws for 21 boards over 9 joists?
21 x 9 x 2 = 378 screws.
Questions
What gap goes between deck boards?
About a quarter inch for wood, per clip maker for composites. Drainage and swelling decide.
How many screws per deck board?
Two per joist crossing. A 16 ft deck on 16 in joists needs about 26 per board.
Should boards span the full length?
Yes when stock allows. Butt joints over joists work but show and move.
How much extra decking should I buy?
Five to ten percent for straight runs, fifteen for diagonals and borders.
Do composite boards gap differently?
Yes, follow the clip system. Composites move with temperature more than wood.
Which way do joists run?
Perpendicular to the decking boards, spaced 16 inches for wood, often 12 for composite.