Deck Calculator
Work out the lumber for a deck platform: enter the size, pick the board width and joist spacing, and get the number of boards to buy, the linear feet of decking and joists, and roughly how many screws the job takes.
Material estimate only — joist size, beam spans, footings and railings have to be designed to your local code, and a low deck with no permit is still a low deck that can fail. About 70 deck screws come in a pound.
Formula and calculation
Worked example
A 16 ft by 12 ft deck with 2 x 6 boards at a 1/8 in gap takes 26 rows across the width, which is 457.6 linear feet of decking with 10% waste — 39 boards of 12 ft. The joists at 16 in on center come to 13 pieces, or 156 linear feet, and the decking takes about 676 screws.
What this calculator does not size
Joist size, beam spans, footing depth, ledger attachment and railing height are structural decisions, and they depend on your snow load, your soil and your local code. A 2 x 8 joist spans further than a 2 x 6, and the span tables that cover it are in the building code, not in a materials estimate. Use this page for the shopping list, and have the framing itself checked by someone who is licensed where you live, especially for a raised deck or one that carries a hot tub.
Buying the decking
Buy all the decking in one order from the same mill run, because pressure-treated and composite boards vary in color between batches and a later addition will not match. Let the boards sit on the deck frame for a few days before you fasten them if they were stored in the rain: wet boards shrink, and a deck laid tight with wet lumber is a deck with gaps you did not plan. Joints should land on a joist, and stagger them across the deck rather than lining them up in one row.
What the results mean
- Boards to buy
- How many boards of the length you picked to buy, rounded up to whole boards and including the waste allowance.
- Board rows
- How many rows of decking run across the width, once the gaps between boards are counted. It decides how many screws and how many linear feet you need.
- Decking
- The total linear feet of decking to buy: every row times the deck length, plus the waste allowance.
- Joists
- How many joists to cut. One joist sits at each end and the rest are spaced on center along the deck length at the spacing you chose.
- Joist lumber
- The total linear feet of joist lumber, before you add the rim and any blocking.
- Screws
- Roughly how many deck screws the surface takes: every board crossing gets two or three of them.
- Deck area
- The platform area in square feet, for comparing against stain, sealer or composite pricing by the square foot.
Frequently asked questions
How many deck boards do I need for a 16 x 12 deck?
With 2 x 6 boards and a 1/8 in gap it takes 26 rows across the 12 ft width, which is 416 linear feet before waste and about 458 linear feet with 10%. That is 39 boards of 12 ft. Change the board width or the gap above and the count follows.
How far apart should deck joists be?
16 in on center is the standard for 5/4 and 2 x 6 decking. Go to 12 in for thinner or composite boards so the surface does not flex underfoot, and 24 in only for heavy 2 x 8 or thicker decking. The joist size and the span tables in your building code still decide how far a joist can reach between supports.
How many screws do I need for a deck?
Count the board crossings: every board passes over every joist, and each crossing takes two screws. A 16 ft by 12 ft deck has 26 rows over 13 joists, which is 338 crossings and about 676 screws. On average about 70 deck screws come in a pound, so this deck takes roughly 10 lb.
Should I leave a gap between deck boards?
Yes, for wood decking. A gap of 1/8 to 1/4 in lets rain drain, lets the boards swell without buckling and lets debris fall through. Butting boards tight means the first wet week pushes them up against each other and the surface buckles. Composite boards have their own required gap — usually a specific clip, so follow the maker's instructions.
How much waste should I allow for a deck?
Ten percent is right for a rectangular deck with boards running one way. Fifteen percent is safer when the deck is angled, has a diagonal layout, or has posts and stairs cutting through the decking. Keep a few spare boards instead of returning them — matching boards years later is hard.