How Many Boards for a 20x20 Deck?
Deck boards to buy
42 20 ft boards
400 square feet, 42 rows running the 20 foot way, joists 16 inches on center.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Deck length20 ft
- Deck width20 ft
- Board face width5.5 in
- Gap between boards0.25 in
- Board length you buy20 ft
- Joist spacing on center16 in
Deck boards to buy42 20 ft boards
| Deck area | 400 sq ft |
|---|---|
| Board rows across the width | 42 |
| Linear feet of decking | 840 lf |
| Boards at 20 ft | 42 |
| Joists at 16 in on center | 16 |
The material list this produces
| Deck boards, 20 ft | 42 |
|---|---|
| Joists, 20 ft | 16 |
| Joist hangers, each | 32 |
| Deck screws, lb | 12 |
Board length compared
Same deck, four board lengths.
| Board length | Deck boards to buy |
|---|---|
| 8 ft boards | 105 8 ft boards |
| 12 ft boards | 70 12 ft boards |
| 16 ft boards | 53 16 ft boards |
| 20 ft boards | 42 20 ft boards |
What else matters here
Forty two rows at 20 feet, 840 linear feet, forty two boards each running the full width with no joints. Twelve pounds of screws, figured at 3 pounds per 100 square feet.
What this calculator assumes
- A nominal 2x6 board is 5.5 inches of actual face. Nominal sizes are not real sizes, which is where most deck orders go wrong.
- Joist count is the span across the deck width at the spacing given, plus one for the last joist.
- Waste is not added. Add 5 to 10 percent if the deck is not a plain rectangle.
Questions people ask
How many boards for a 20x20 deck?
Forty two boards of 5/4 by 6 at 20 feet, one per row, with no butt joints.
The deck calculator takes any dimensions you like and hands back the same list.