How Many Boards for a 12x20 Deck?
Deck boards to buy
33 16 ft boards
240 square feet, 520 linear feet of decking, 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 width12 ft
- Board face width5.5 in
- Gap between boards0.25 in
- Board length you buy16 ft
- Joist spacing on center16 in
Deck boards to buy33 16 ft boards
| Deck area | 240 sq ft |
|---|---|
| Board rows across the width | 26 |
| Linear feet of decking | 520 lf |
| Boards at 16 ft | 33 |
| Joists at 16 in on center | 16 |
The material list this produces
| Deck boards, 16 ft | 33 |
|---|---|
| Joists, 12 ft | 16 |
| Joist hangers, each | 32 |
| Deck screws, lb | 8 |
Board length compared
Same deck, four board lengths. Longer boards mean fewer end joints and more offcut risk.
| Board length | Deck boards to buy |
|---|---|
| 8 ft boards | 65 8 ft boards |
| 12 ft boards | 44 12 ft boards |
| 16 ft boards | 33 16 ft boards |
| 20 ft boards | 26 20 ft boards |
What else matters here
Twenty feet is past the length most yards stock in decking, so plan the joints rather than discovering them. The clean way is to stagger the butt joints across the deck in a deliberate pattern, with a doubled joist or a pair of joists under every joint so both board ends land on solid wood.
The alternative is a picture frame border with the field boards running between, which hides every cut end under the border. It costs a few more boards and a lot more cutting, and it is the difference between a deck that looks built and one that looks assembled.
Sixteen joists at 16 inch centers, cut to 12 feet. At 240 square feet you are also into footing territory where the local code wants a drawing, so check whether the permit needs a plan before you buy lumber.
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 deck boards for a 12x20 deck?
About 33 boards of 5/4 by 6 at 16 feet, covering 520 linear feet across 26 rows.
How do I join deck boards end to end?
Over a doubled joist or a pair of joists so each board end has its own bearing and its own fasteners. Never butt two boards over a single one and a half inch joist.
Do I need a permit for a 12x20 deck?
In most US jurisdictions yes, especially once the deck is attached to the house or more than 30 inches off the ground. Ask the building department before ordering.
The deck calculator takes any dimensions you like and hands back the same list.