How to use this calculator
- Enter the deck length, which is the direction the boards will run, and the deck width.
- Choose the length of the boards you plan to buy. Longer boards mean fewer joints.
- Choose the board width and the gap you will leave between boards.
- Pick the joist spacing. Sixteen inches on center is common, but check your decking maker.
- Keep 10% for cuts and defective boards.
How the numbers are calculated
The number of board rows is the deck width in inches divided by the board width plus the gap, rounded up. Each row needs the deck length divided by the board length, rounded up. Rows multiplied by boards per row, plus waste, gives the total.
Joists run across the width and are spaced along the length, so their count is the length in inches divided by the spacing, plus one. Screws are estimated at two for every board crossing a joist.
Worked example
A deck 16 ft long and 12 ft wide is 192 sq ft. With 5½ inch boards and a ⅛ inch gap, you need 26 rows. Using 16 ft boards, that is 26 boards, and 29 with 10% extra. Joists at 16 inch spacing number 13, and you need about 676 screws.
Tips before you buy
- Composite decking often needs closer joist spacing than wood, and has specific gap rules. Follow the manufacturer.
- Pressure-treated boards that are still wet often shrink as they dry, so they are commonly laid closer together than dry boards.
- Stagger board joints across rows and make butt joints land over a joist.
- Pre-drill near the ends of boards to reduce splitting.
- Many areas require a building permit for decks. Check local rules before you buy materials.
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board width plus gap to find rows, then multiply by boards per row and add waste. The calculator does this for you.
What joist spacing should I use?
Sixteen inches on center is common for wood decks. Composite and diagonal layouts often need 12 inches. Always check the decking manufacturer instructions.
How many screws per board?
Two per joist the board crosses. A board spanning many joists therefore takes many screws, which is why the total is high.
Does this include the frame and footings?
No. It covers the surface boards and the joist count. Posts, beams, footings, railing and stairs need separate planning.
Related calculators
These results are estimates for planning purposes. Material sizes, densities and local requirements vary, so confirm quantities with your supplier or a qualified contractor before ordering.
Last updated October 11, 2026.