Choose the deck dimensions, the actual board width and the gap required for your product. The tool counts full-length board rows and adds your waste allowance.
The joist setting is used to estimate screws only. Board spans, framing, footings, fixings and gaps still need to follow the manufacturer’s instructions and the requirements for your site.
How to use this calculator
- Decide the board direction first — boards should usually run away from the house or along the longer dimension for fewer joints.
- Measure the deck frame, not the old surface you are replacing.
- Use the actual face width of your board; a '6 inch' deck board is really 5.5 inches wide.
- Calculate, then plan butt joints if your deck is longer than the longest board you can buy — stagger them across different joists.
How deck boards are calculated
Rows = round up ((deck width in inches + one gap) ÷ (actual board width + gap)). Linear feet = rows × deck length, plus waste. Joist crossings use your selected spacing, with two through-screws per crossing. This counts materials; it does not check structural spans or a complete fixing specification.
A 16 ft × 12 ft deck with 5.5-inch boards and a 1/8-inch gap needs (144 + 0.125) ÷ (5.5 + 0.125) = 25.62, rounded up to 26 rows. With full 16 ft boards and 10 percent waste, that is 29 boards to buy. At 16-inch joist spacing, the tool counts 13 crossings per row and two screws at each crossing: 26 × 13 × 2 = 676 screws. Confirm the fixing system for your actual boards and layout.
A few things worth checking
- Using the nominal board width instead of the measured width.
- Choosing a gap without checking the board manufacturer and installation conditions.
- Forgetting extra framing and fixings at butt joints, borders, stairs and edges.
Your questions, answered
Which way should deck boards run?
Compare board lengths, joints and the layout you want. Confirm the framing and drainage details before deciding.
What gap should I leave?
Use the manufacturer’s instructions for the exact material, temperature and installation conditions. Wet timber can shrink as it dries, which can increase gaps.
Does this tool design my deck frame?
No. The joist-spacing field is only for the screw estimate. Framing, footings and connections need a project-specific design.
Plan Your Deck Boards, Gaps and Fasteners
A clearer materials list starts with the real board size and the instructions for the product you have chosen.