How many deck boards for a 20×20 deck?
Quick Answer
A 20×20 foot deck (400 sq ft) requires approximately 85–90 boards of 5/4×6 decking at 20-foot lengths, or about 57 boards at 16-foot lengths (with a butt joint mid-span). With 10% waste, order 95–100 boards at 20-foot length. This covers decking boards only.
Quick Reference Table
| Board size | Deck boards for 20×20 deck |
|---|---|
| 5/4×6×20 (no seam) | 43 boards + 10% = 47 |
| 5/4×6×16 (with seam) | 57 boards + 10% = 63 |
| 5/4×6×12 | 76 boards + 10% = 84 |
| 2×6×20 | 43 boards + 10% = 47 |
How to Calculate It Yourself
- 1
Area: 20 × 20 = 400 sq ft.
- 2
Effective board width (5/4×6 with 1/8" gap): 5.625".
- 3
For 20 ft span: 240" ÷ 5.625" = 42.7 → 43 boards.
- 4
Add 10%: 43 × 1.10 = 47 boards (running the full 20-ft length).
- 5
If using shorter boards: double the count and add 15% for seams.
Pro Tip
For a 20×20 deck, use 20-foot boards to avoid mid-span butt joints. Joints weaken the deck surface and are a tripping hazard. If 20-ft boards are too expensive, stagger joints so they never align on adjacent boards.
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