Deck Material Calculator: Boards, Joists and Screws
Deck orders go wrong for one reason: nominal sizes are not real sizes. A 2x6 is 5.5 inches of actual face, so the board count is driven by 5.5 plus the gap, not by 6.
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.
Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- Deck length16 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 buy26 16 ft boards
| Deck area | 192 sq ft |
|---|---|
| Board rows across the width | 26 |
| Linear feet of decking | 416 lf |
| Boards at 16 ft | 26 |
| Joists at 16 in on center | 13 |
The material list this produces
| Deck boards, 16 ft | 26 |
|---|---|
| Joists, 12 ft | 13 |
| Joist hangers, each | 26 |
| Deck screws, lb | 6 |
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.
Nominal versus actual, and why it costs you boards
Lumber is named before it is dried and planed. A 2x6 leaves the mill at 2 by 6 and arrives at the yard at 1.5 by 5.5. A 2x4 is 1.5 by 3.5. A 5/4 deck board is about 1 inch thick and 5.5 wide.
On a 12 foot wide deck, figuring on 6 inch boards gives 24 rows. Figuring on the real 5.5 plus a quarter inch gap gives 25. One row short, times 16 feet, is a board you drive back for.
Gapping, and what the material does after you leave
Pressure treated lumber usually arrives wet and shrinks as it dries, so it is laid tight or nearly tight and opens up to a gap on its own. Kiln dried and composite boards do not shrink, so they are laid with a deliberate gap, commonly 3/16 to 1/4 inch, for drainage and expansion.
Composite expands and contracts with temperature along its length, which is why manufacturers specify end gaps at butt joints that vary with the install temperature. Follow the maker's table, not habit.
Joists, spans and the part this does not do
Joist count here is the span across the width at your chosen spacing, plus one. Sixteen inches on center suits most decking. Composite and diagonal lays usually want 12.
What this does not do is size the joists or the beam. Span tables depend on species, grade, spacing and load, and they are in your code book for a reason. Get the span right first, then come back here for the count.
Questions people ask
How many deck boards do I need for a 12x16 deck?
With 5.5 inch boards and a quarter inch gap, each row covers 5.75 inches, so a 12 foot width needs 26 rows. At 16 feet long that is 26 boards if you buy 16 footers.
How many screws per square foot of decking?
This calculator figures deck screws by weight, at 3 pounds per 100 square feet of deck, rounded up to the whole pound. A 12x16 deck of 192 square feet comes out at 6 pounds.
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Common sizes, worked out
The same calculator with the numbers already filled in, one page per size, each ending in its own material list.
More sizes, answered
Each row is this calculator run with those numbers when the page was built. Type the same numbers in above to see every line of the working.
| Example | What it assumes | Deck boards to buy |
|---|---|---|
| 4x4 deck | 16 square feet, 5/4 by 6 decking at a quarter inch gap, joists 16 inches on center. | 5 8 ft boards |
| 4x6 deck | 24 square feet, boards running the 6 foot way, joists 16 inches on center. | 5 12 ft boards |
| 5x5 deck | 25 square feet, 11 rows of decking, joists 16 inches on center. | 7 8 ft boards |
| 4x8 deck | 32 square feet, boards running the 8 foot way, joists 16 inches on center. | 9 8 ft boards |
| 6x6 deck | 36 square feet, 13 rows of decking, joists 16 inches on center. | 7 12 ft boards |
| 6x8 deck | 48 square feet, 13 rows running the 8 foot way, joists 16 inches on center. | 13 8 ft boards |
| 8x8 deck | 64 square feet, 17 rows of decking, joists 16 inches on center. | 9 16 ft boards |
| 8x10 deck | 80 square feet, 17 rows running the 10 foot way, joists 16 inches on center. | 9 20 ft boards |
| 10x10 deck | 100 square feet, 21 rows of decking, joists 16 inches on center. | 11 20 ft boards |
| 10x12 deck | 120 square feet, 21 rows running the 12 foot way, joists 16 inches on center. | 21 12 ft boards |
| 12x12 deck | 144 square feet, 5/4 by 6 decking at a quarter inch gap, joists 16 inches on center. | 20 16 ft boards |
| 12x16 deck | 192 square feet, boards running the 16 foot direction, joists 16 inches on center. | 26 16 ft boards |
| 10x20 deck | 200 square feet, 21 rows running the 20 foot way, joists 16 inches on center. | 21 20 ft boards |
| 12x24 deck | 288 square feet, 26 rows running the 24 foot way, joists 16 inches on center. | 52 12 ft boards |
| 16x20 deck | 320 square feet, 680 linear feet of decking, joists 16 inches on center. | 43 16 ft boards |
| 16x24 deck | 384 square feet, 816 linear feet of decking, joists 16 inches on center. | 51 16 ft boards |
| 20x20 deck | 400 square feet, 42 rows running the 20 foot way, joists 16 inches on center. | 42 20 ft boards |
| 20x30 deck | 600 square feet, 42 rows running the 30 foot way, joists 16 inches on center. | 63 20 ft boards |