How Many Boards for a 10x20 Deck?
Deck boards to buy
21 20 ft boards
200 square feet, 21 rows running the 20 foot way, joists 16 inches on center.
When the figures look right, the deck material list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
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 width10 ft
- Board face width5.5 in
- Gap between boards0.25 in
- Board length you buy20 ft
- Joist spacing on center16 in
Deck boards to buy21 20 ft boards
| Deck area | 200 sq ft |
|---|---|
| Board rows across the width | 21 |
| Linear feet of decking | 420 lf |
| Boards at 20 ft | 21 |
| Joists at 16 in on center | 16 |
The material list this produces
| Deck boards, 20 ft | 21 |
|---|---|
| Joists, 10 ft | 16 |
| Joist hangers, each | 32 |
| Deck screws, lb | 6 |
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 | 53 8 ft boards |
| 12 ft boards | 35 12 ft boards |
| 16 ft boards | 27 16 ft boards |
| 20 ft boards | 21 20 ft boards |
What else matters here
Twenty one rows at 20 feet is 420 linear feet, and in 20 foot stock that is 21 boards with every row a single piece and no butt joints anywhere on the deck.
The frame under it is the interesting part. Twenty feet of outer edge needs a beam, the beam needs posts at roughly 8 foot intervals, and the usual arrangement is three posts, which puts the beam spans at 10 feet and is past what a doubled 2x10 will carry in most species. Either add a fourth post or go to a tripled beam or a 2x12.
The other lever is the cantilever. Running the joists past the beam by a foot or two shortens the span they have to carry and lets the posts sit in from the edge, which often drops a post out of the layout entirely. Most codes allow a cantilever up to a quarter of the backspan, so a 10 foot joist can usually hang over by two and a half 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 10x20 deck?
Twenty one boards of 5/4 by 6 at 20 feet, one per row, with no butt joints.
How far apart should deck posts be?
It depends on the beam. Doubled 2x10s carry roughly 8 to 9 feet between posts, so a 20 foot edge usually wants three or four posts rather than two.
Can deck joists cantilever past the beam?
Usually up to a quarter of the backspan, so a 10 foot joist can hang over about two and a half feet. It shortens the carried span and often removes a post from the layout.
The deck calculator takes any dimensions you like and hands back the same list.