How Many Deck Boards Do I Need for a 16x20 Deck?
For a 16 × 20 foot deck with 16-ft boards running the 16-ft direction, divide the 20-ft span by each board’s installed pitch — 5.5 in of board plus a 1/8 in gap = 5.625 in: 240 ÷ 5.625 = 42.67 → 43 boards exact, or 48 with a 10% cutting allowance. The chart below covers other deck widths.
Key Takeaways
- 16-ft boards run the 16-ft direction; count spans the 20-ft width.
- Installed pitch: 5.5 in board + 0.125 in gap = 5.625 in.
- 240 ÷ 5.625 = 42.67 → 43 boards exact.
- Add 10% → order 48 boards.
- Stairs, benches, and picture-frame borders need extra boards.
How to Calculate Deck Boards for a 16x20 Deck
Unlike a quick estimate, this math includes the drainage gap between boards — over a 20-foot span, those eighths of an inch add up to nearly two full boards.
- 1Decide board direction: 16-ft boards run the 16-ft length.
- 2Find the span: 20 ft = 240 in of width to cover.
- 3Find installed pitch: 5.5 in board + 0.125 in gap = 5.625 in.
- 4Divide: 240 ÷ 5.625 = 42.67 → 43 boards.
- 5Add 10% waste: 43 × 1.10 = 47.3 → 48 boards.
The Deck Board Formula (With Gap)
Including the gap matters on wide decks. For diagonal patterns, multiply the result by about 1.15 instead of 1.10 — angled cuts waste more.
Boards = Span (in) ÷ (Board width + Gap, in)Worked Example: 16 ft × 20 ft Deck
Full math for a 16 × 20 deck with 16-ft 5/4×6 boards, plus a chart for other deck widths.
Deck: 16 ft × 20 ft; boards run the 16-ft direction.
Width to span: 20 ft = 240 in.
Installed pitch: 5.5 + 0.125 = 5.625 in.
Exact boards: 240 ÷ 5.625 = 42.67 → 43.
With 10% waste: 43 × 1.10 = 47.3 → 48 boards.
| Deck width spanned | Boards (exact) | With 10% waste |
|---|---|---|
| 12 ft | 26 | 29 |
| 16 ft | 35 | 39 |
| 20 ft | 43 | 48 |
Common Mistakes
- Ignoring the gap — over 20 ft it silently eats almost two boards.
- Buying 16-ft boards for a deck slightly over 16 ft and butt-jointing every row.
- Forgetting stair treads, which are usually two boards deep per step.
- Not planning the stagger pattern before the first board goes down.
- Ordering exact with no spare for splits, knots, and bad grain.
Practical Tips
- Pull the worst boards for stair risers and blocking where looks matter less.
- Use a spacer jig (a 16d nail works) to keep gaps consistent.
- Start from the house and work outward so rips land at the rim.
- Let boards acclimate on site for a few days before fastening.
- Estimate framing, footings, and fasteners with the NOVORY Deck Calculator.
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