How the Deck Calculator works
For a rectangular deck, the calculator counts the deck boards to buy in the stock length you choose, the rows of boards and linear feet, the joists and their length at 12", 16" or 24" on center, and an approximate number of deck screws. Rim joists, beams, posts, footings and blocking are not included.
Rows of boards = (deck width across the boards + gap) ÷ (board face width + gap), rounded up. When a row is shorter than a board, one board can give several rows, allowing 1/2" per cut; longer rows are spliced over a joist. Joists = the side the boards run along ÷ joist spacing, rounded up, + 1 for the starting joist, and each joist is as long as the side the boards cross. Screws = 2 per board at every joist. Waste is added to boards and screws.
Example: a 12' × 10' deck with 5/4×6 boards (5 1/2" face) and 1/8" gaps, boards running along the 12' side, 12' boards, joists 16" on center and 10% waste. Rows = (120 + 1/8) ÷ (5 1/2 + 1/8) = 120.125 ÷ 5.625 = 21.36, so 22 rows. One 12' board per row is 22 boards, × 1.10 = 24.2, so 25 boards. Joists: 144 ÷ 16 = 9 bays, so 10 joists, each 10' long. Screws: 2 × 22 rows × 10 joists = 440, × 1.10 = 484.
Joist spacing is limited by the decking: IRC Table R507.7 allows 16" on center for 5/4 wood boards laid square to the joists and 12" on a diagonal, and composite boards follow the maker's instructions. The calculator does not size joists, beams or footings (IRC R507); check the span tables and your local code, which may differ, before you build.
Frequently asked questions
How many deck boards do I need?
Divide the deck's width across the boards by one board plus one gap (5 1/2" + 1/8" = 5 5/8" for 5/4×6) and round up to get the rows; a 10' wide deck needs 22 rows. If each row fits in one board, that is 22 boards; add about 10% for waste. Rows longer than your boards need butt joints over a joist, which uses a little more stock.
What is the standard joist spacing for a deck?
16" on center is the most common. IRC Table R507.7 allows up to 16" for 5/4 (1 1/4" thick) wood decking and 24" for 2× decking laid square to the joists, and 12" and 16" when laid on a diagonal. Composite and PVC boards follow the manufacturer's limit, often 16", or 12" on a diagonal; local codes may differ.
How many screws do I need for a deck?
Plan on 2 screws per board at every joist it crosses. For 5/4×6 boards on joists 16" on center that works out to roughly 350 screws per 100 sq ft. Hidden fastener systems use their own clips, so follow the maker's count for those.
How far can deck joists span?
It depends on the joist size, wood species and grade, and spacing. IRC Table R507.6 lists maximum deck joist spans and cantilevers; closer spacing allows a longer span. This calculator counts joists but does not check spans, so look up your size in the table and confirm with your local building department.
How much gap should I leave between deck boards?
1/8" is common for dry, kiln-dried wood. Wet pressure-treated boards are often laid tight or nearly tight because they shrink as they dry. Composite and PVC boards expand with heat, so use the side and end gaps in the manufacturer's instructions.