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. Longer boards mean fewer end joints and more offcut risk.
| 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, which surprises people the first time they buy them by weight.
Four hundred square feet is unambiguously permitted work everywhere that permits decks at all, and the threshold in many places is far lower: 200 square feet, or 30 inches above grade, or any deck attached to the house. Building without one is not a paperwork problem, it is a problem at resale and an argument with an insurer after a failure.
The permit also buys something. An inspector looking at footings, the ledger connection and the guard assembly is checking the three things that actually fail, and the plan review will catch an undersized beam before it is buried under 840 feet of decking rather than after.
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.
Do I need a permit for a 20x20 deck?
Almost certainly. Common triggers are area over 200 square feet, height over 30 inches, or any attachment to the house, and a 20 by 20 clears all three in most jurisdictions.
How many pounds of deck screws for 400 square feet?
About 12 pounds at roughly 3 pounds per 100 square feet, for boards at 16 inch joist spacing with two screws per joist.
The deck calculator takes any dimensions you like and hands back the same list.