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Deck Material Calculator
Get the number of decking boards to buy for your deck size, board direction and the lengths you can get — with offcuts, cost, an optional joist count and a layout preview.
Compare board lengths
Same deck and direction, with 10% allowance. Board lengths that divide your rows evenly waste the least.
| Board length | Boards | Bought | Offcuts & spare | Joints |
|---|---|---|---|---|
| 2.4 m | 55 | 132 m | 12 m | Yes |
| 3 m | 55 | 165 m | 45 m | Yes |
| 3.6 m | 38 | 136.8 m | 16.8 m | Yes |
| 4.2 m | 32 | 134.4 m | 14.4 m | Yes |
| 4.8 m | 28 | 134.4 m | 14.4 m | No |
| 5.4 m | 28 | 151.2 m | 31.2 m | No |
Assumes a plain rectangular deck with straight rows, butt joints that land on joists, and no picture-frame border. Saw kerf isn't deducted. Not a structural design.
How boards are counted
- Rows = (distance across the boards + gap) ÷ (board width + gap), rounded up. A narrower last row is ripped to fit.
- If a board is longer than a row, each board gives as many whole rows as fit; the rest is offcut.
- If a row is longer than a board, it takes whole boards plus an end piece; end pieces are cut from shared boards.
- Your allowance is added and every count is rounded up.
Worked example
A 16 × 10 ft deck, 5.5 in boards, no gap, 16 ft boards. Running along the 16 ft side: 120 ÷ 5.5 = 21.8 → 22 boards. Running along the 10 ft side: 192 ÷ 5.5 = 34.9 → 35 rows, each from its own 16 ft board → 35 boards.
Frequently asked questions
How many deck boards do I need?+
Work out the number of rows — the distance the boards cover divided by board width plus gap, rounded up — then how many rows you can cut from each board length. A 12 × 12 ft deck with 5.5 in boards, no gap and 12 ft boards needs 27 rows, so 27 boards before any allowance.
Why does board direction matter?+
Boards running along a 16 ft side can each be one 16 ft board; running across the 10 ft side, a 16 ft board gives only one 10 ft piece and 6 ft of offcut. The calculator shows both directions so you can compare.
What is the actual width of a 6-inch deck board?+
Usually about 5.5 inches — timber is sold by nominal size and planed smaller. Composite boards vary by brand. Measure a board or check the maker's data, and enter the actual face width.
How big should the gap between boards be?+
Follow the board maker's instructions. Timber is commonly gapped about 3–6 mm (1/8–1/4 in) and composite boards often use clips that set the gap.
Does the joist estimate meet building codes?+
No. It only counts joists at the spacing you choose. Joist size, span, spacing and fixings depend on the decking, loads and local building rules.