Deck board calculator

How deck boards are calculated

The boards run the long way, parallel to the deck length, and sit on joists that run across the width. Count the rows across the width first, then how many boards it takes to fill each row end to end.

The row formula comes from the fact that a deck with n rows has n boards and one gap fewer. Adding one gap to the width before dividing takes care of that missing gap. Because you round up, the rows almost always cover a little more than the deck is wide:

The framing and fasteners follow from the same layout. Every row crosses every joist, and each crossing gets its fasteners:

Worked example: 16 × 12 ft deck

The calculator opens on a 16 × 12 ft deck with 5-1/2" boards (5/4×6 or 2×6), 1/4" gaps, 16 ft boards, joists 16 in on center and 10% waste.

1. Rows: (144 + 0.25) ÷ (5.5 + 0.25) = 25.09, rounded up to 26 rows.
2. Boards per row: 16 ft ÷ 16 ft = 1, so each row is one full-length board with no butt joints.
3. Boards: 26 × 1 = 26; with 10% waste, 26 × 1.10 = 28.6, rounded up to 29 boards, or 464 lineal feet.
4. Covered width: 26 × 5.5 + 25 × 0.25 = 149.25 in (12' 5-1/4"), so the last board overhangs the 144 in framing by 5.25 in and gets ripped.
5. Joists: 192 ÷ 16 = 12, + 1 = 13 joists, each 12 ft long.
6. Fasteners: 26 rows × 13 joists = 338 crossings, so 676 screws or 338 hidden clips.

Choosing the gap and joist spacing

The gap changes the row count. Narrow gaps fit more rows into the same width, and the calculator lets you compare 1/8", 3/16" and 1/4".

• Wet pressure-treated wood arrives soaked and shrinks across its width as it dries. Lay it tight or with about 1/8"; the gap opens up over the first season.
• Composite and PVC boards do not shrink like wood but do expand and contract with heat. Most brands call for 3/16" side to side. Use the gap in the install guide for your exact board.
• Joist spacing of 16 in on center is standard for wood decking and most composite laid square to the joists. A diagonal layout makes each board span farther between joists, so composite on a diagonal usually needs 12 in on center. Check the board maker's span table before you frame.

Ordering and layout tips

• Pick a board length that matches the deck. A 16 ft deck takes 16 ft boards with no joints. If the deck is longer than your boards, each row needs butt joints, and the joints must land on a joist. Stagger them from row to row so they do not line up.
• Watch the last bay. The joist count is the length divided by the spacing, rounded up, plus one. When the length is not a multiple of the spacing, the last bay is short and an end joist closes it. A 10 ft deck at 16 in on center gives 9 joists: 7 full bays from the start, then a short last bay of 8" to the end joist.
• Plan the last row. Lay out the rows on the joists before fastening. If the last board would be a thin sliver, rip the first and last boards to equal widths, or tighten the gaps slightly across every row.
• Buy fasteners for the method you use. Face screws go 2 per board at each joist. Hidden clips go 1 per board per joist, plus starter and end clips from the clip maker.
• Split L-shaped and multi-level decks into rectangles. Enter each one with “+ Add another section”; they share the board, gap and joist settings. Each section has its own rows and its own joists, so the joists and fasteners are counted section by section, while the boards from every section are added up before the waste is applied and rounded once.
• Picture-frame borders, stairs and fascia are not in this count. Add them separately by their lineal feet.