How to calculate how many deck boards you need
Deck boards run in rows across the deck, separated by a small gap for drainage and expansion. The method is: work out how much width each board covers (board width + gap), divide the deck width by that to get the number of rows, then figure out how many boards each row needs based on the board length. Add waste for cuts and you're done — this calculator also estimates the screws.
The formula, step by step
1. Coverage per board = board width + gap.
2. Rows = deck width ÷ coverage per board (rounded up).
3. Total decking length = rows × deck length.
4. Boards = total length ÷ board length, × (1 + waste %), rounded up.
Why the gap matters
A 1/8 to 1/4 inch gap between boards lets water drain and gives the wood or composite room to expand in heat. It also affects your board count: a wider gap means each board covers more width, so you need slightly fewer rows. Always use the gap your board manufacturer recommends — composite decking usually wants a bit more than wood.
Common deck board sizes
| Nominal size | Actual width | Notes |
|---|---|---|
| 5/4 × 6 in | ~5.5 in | Most common decking |
| 2 × 6 in | ~5.5 in | Thicker, longer spans |
| 2 × 4 in | ~3.5 in | Narrow boards |
Don't forget the framing
This calculator covers the deck boards and screws — the visible surface. It does not size the joists, beams, posts, or footings underneath, which depend on your span and local code. Plan those separately, and add about 350 screws per 100 sq ft for fastening the boards down (two per board at each joist).