Enter your deck size and Trex board length to get the exact number of composite boards - set up for Trex board widths, joist spacing and edge styles. Works in feet/inches or metres/millimetres.
Deck area
24.0 sq ft
Total board run
6.0 ft
Joists
5
The calculator divides your deck area by the area one Trex board covers, adds a wastage allowance for cuts, and rounds up to whole boards:
boards = ceil( deck area ÷ (board length × board width) × wastage )
It is set up for Trex dimensions - 5.5-inch-wide boards in 12, 16 and 20 ft lengths - and lets you set the joist spacing (16 in perpendicular, 12 in diagonal) and the edge style (flush, overhang or a mitred picture-frame border). Building an L-shaped or multi-level deck? Add a section for each and the boards total into one order. All math runs in exact metric internally, so switching units never changes the answer.
| Profile | Thickness × width | Lengths | Use |
|---|---|---|---|
| Grooved edge | 0.94 in × 5.5 in | 12 / 16 / 20 ft | Field boards, hidden fasteners |
| Square edge | 0.94 in × 5.5 in | 12 / 16 / 20 ft | First/last board, stairs, picture frame |
A 20 ft × 12 ft deck with 12 ft boards run straight:
Divide the deck area by the area one Trex board covers, then add wastage for cuts. Trex boards are 5.5 inches wide and come in 12, 16 and 20 ft lengths, so a 20 ft × 12 ft deck is about 240 sq ft, or roughly 53 boards of 12 ft including a 10% allowance. Enter your deck size and board length above for an exact count.
Trex composite deck boards are about 0.94 inches thick and 5.5 inches wide (the same nominal width as a 2×6), in lengths of 12, 16 and 20 ft - some lines also offer 8 ft. They come in square-edge and grooved-edge profiles; the grooved edge takes Trex Hideaway hidden fasteners. The calculator defaults to the 5.5-inch width.
Trex requires joists no more than 16 inches on centre for boards laid perpendicular to the joists, and 12 inches on centre for a 45-degree diagonal layout or for stair treads. Closer spacing gives a firmer, less bouncy deck. Set the joist spacing in the calculator to match - it affects the substructure, not the board count.
Allow about 10% for boards run straight (parallel) across the deck, and 15% for a diagonal or picture-frame layout, where every board meets the border at an angle and needs a cut. Picture-frame borders and breaker boards add cuts, so use the higher figure. The calculator lets you pick the allowance that matches your layout.
Use grooved-edge boards across the field of the deck with Trex Hideaway hidden fasteners for a clean, screw-free surface, and square-edge boards for the first and last boards, stair treads, and any picture-frame border (which are face-screwed or plugged). Plan a mix: mostly grooved, with a few square-edge for the perimeter.
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