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.
What spacing should deck joists be?
Sixteen inches on center covers most wood decking laid perpendicular. Composite decking and any diagonal lay usually require 12 inches on center, and the decking manufacturer's instructions control.
How many screws per square foot of decking?
Two screws per board per joist. That works out around 350 screws per 100 square feet with 16 inch spacing, which is roughly 3 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.