How to Calculate Deck Board Quantities
The key math: boards run parallel to one dimension (usually the length). The number of rows = deck width in inches ÷ (board width + gap). Then each row requires enough board lengths to span the deck length.
rows = (deck width × 12) ÷ (board width + gap)
boards per row = ceil(deck length ÷ board length)
total boards = rows × boards per row × (1 + waste)Common Deck Board Widths (Actual)
| Nominal size | Actual width | With 3/16 in gap | Boards/100 sq ft |
|---|---|---|---|
| 5/4×6 | 5.5 in | 5.6875 in pitch | ~22 at 16 ft lengths |
| 2×4 | 3.5 in | 3.6875 in pitch | ~33 at 16 ft lengths |
| Composite 1×6 | 5.5 in | 5.6875 in pitch | ~22 at 16 ft lengths |
Joist Spacing and Span Limits
| Joist size (SYP #2) | 16 in spacing max span | 12 in spacing max span |
|---|---|---|
| 2×6 | 9 ft | 10.5 ft |
| 2×8 | 12.5 ft | 14.5 ft |
| 2×10 | 15.5 ft | 18 ft |
| 2×12 | 19 ft | 22 ft |
Source: American Wood Council DCA6 — Prescriptive Residential Wood Deck Construction Guide. Spans assume 10 psf dead load, 40 psf live load. Always verify with local code and a qualified inspector.
Deck permits: Most jurisdictions require a building permit for decks above 30 inches off grade or attached to the house. Unpermitted decks can be a liability issue during home sale. Check with your local building department before building.