Deck Board Calculator

How to use this deck board calculator

1. Measure the finished decking surface, including any overhang past the rim joist or fascia. For an L-shaped deck, split it into two rectangles and add a second section.
2. Choose which way the boards run. Boards always cross the joists, so boards running along the length mean joists running along the width. Diagonal boards sit at 45° to the joists.
3. Check the board width and gap. Most composite boards and 5/4×6 wood boards are 5.5 inches wide. Trex calls for a 3/16 inch gap, and the American Wood Council suggests about 1/8 inch for wood.
4. Set the joist spacing. Straight boards usually sit on joists 16 inches apart. When you pick diagonal, the calculator switches to 12 inches, which Trex requires for diagonal boards.
5. Untick any board length your store doesn’t carry, and choose hidden clips or face screws.

The result recommends one board length and lists the others, so you can compare scrap, joints, and cost at your prices.

How the calculator works

Rows

rows       = (deck width + gap) ÷ (board width + gap), rounded up
row length = deck length, or the long point of each diagonal row

The last row is usually ripped to fit. For a diagonal deck, each row is measured at its longest edge, so every piece is long enough for the angled cuts at both ends.

Choosing the board length

Trex says to trim 3/16 inch off each factory end, so the calculator counts every board 3/8 inch shorter than its label. A 16 ft board covers 15 ft 11 5/8 in. For each length you can buy, the calculator cuts every row into pieces and then fits the pieces into boards:

• If a trimmed board is at least as long as the row, the row is one piece. A long board can also give two short rows, so a 20 ft board covers two 9 ft rows, but not two 10 ft rows.
• If the row is longer than the board, the pieces meet over a joist. Trex and the AWC guide both say each piece must rest on at least three joists, so no piece is shorter than two joist spaces. Every other row moves its joint to a different joist so the joints don’t line up.
• All the pieces are then fitted into boards, longest first, and each offcut goes to the next piece that fits.

We recommend the length with the fewest butt joints. If two lengths tie, the one with the least scrap wins, and then the cheaper one. This assumes you reuse offcuts, so we add 5% extra for damaged boards and miscuts, or 10% on a diagonal deck, where fewer offcuts end up the right length. Those percentages are our estimates.

Fasteners

crossings   = rows × joists under each row, + 1 per butt joint
clips       = crossings × 1.05, in boxes of 90
face screws = crossings × 2 × 1.05

Trex rates a 90-clip box of its Hideaway fasteners for 50 square feet with joists 16 inches apart. Our joist count lands close to that. The 216 square foot deck in the example below has 390 crossings, or 4.3 boxes before the extra, and the Trex rating also gives 4.3 boxes. For face screws, Trex recommends two per joist and the AWC guide calls for two #8 screws or two 8d threaded nails.

A worked example

Take an 18 × 12 ft deck with the boards running along the 18 ft side, 5.5 inch boards, 3/16 inch gaps, joists 16 inches apart, and hidden clips.

• Rows: (144 + 0.1875) ÷ 5.6875 = 25.4, so 26 rows of 18 ft
• 20 ft boards: one board per row with no joints, 26 boards with 10% scrap, or 28 with 5% extra
• 16 ft boards: a trimmed board stops 3/8 inch short of the joist at 16 ft, so each joint moves back to the joist at 14 ft 8 in. The rows alternate between 14 ft 8 in + 3 ft 4 in and 6 ft 8 in + 11 ft 4 in pieces. They fit in 33 boards, 35 with extra, with 26 butt joints.
• 12 ft boards: 52 boards with 26 joints, and 25% ends up as scrap
• Clips: 26 rows × 15 joists = 390, plus 5% = 410, which is 5 boxes of 90

At our example prices, 28 boards of 20 ft and 5 boxes of clips come to $2,129. The 16 ft boards cost $14 less but add a joint to every row, so the 20 ft boards are the better buy here. Laid diagonally on joists 12 inches apart, the same deck takes 25 boards of 20 ft before the extra, or 28 with 10%.

Framing is a separate job

This calculator counts decking only. It doesn’t size joists, beams, posts, footings, or the ledger that ties a deck to the house. For those, the American Wood Council’s free DCA 6 deck construction guide lays out joist spans, beam sizes, footings, and ledger fastening for single-level wood decks based on the International Residential Code. Have your building department or an engineer check the framing before you build, and ask whether you need a permit. For footings, the concrete calculator counts the bags.

Tips for laying deck boards

• Square up each factory end with a fresh cut before the board goes down, as Trex recommends. The calculator already allows for those cuts.
• Plan where each joint falls before you cut. Every butt joint needs a joist under it.
• Leave end gaps based on the temperature. Trex calls for 1/8 inch above 40°F and 3/16 inch below, and 1/4 to 1/2 inch where boards meet a wall.
• Mix boards from different bundles as you go. Trex suggests it for an even blend of color tones.
• Paint the tops of the joists black if you don’t want them showing through the gaps, as Trex suggests.
• Ask an engineer or your building department before you put a hot tub or a large planter on a deck.