Deck Calculator for Decking Boards

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.

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.

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.

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.

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.

With 5.5 in boards, 3/16 in gaps and 16 ft boards running along the 16 ft side, the 12 ft width takes 26 rows, so 26 boards. With a 10% allowance, buy 29. Board width, gap and direction all change the count, so enter your own.

Around 5–10% for a straight layout covers cutting mistakes and boards with defects. Allow more for diagonal boards, picture-frame borders or decks with many corners. The calculator adds your allowance and rounds up to whole boards.

One that equals the row length or divides into it evenly. Running boards across a 12 ft width, 12 ft boards need 34 boards with 0 ft of offcut, while 16 ft boards also need 34 but leave 136 ft of offcut. 8 ft boards need 51 with 0 ft offcut, but every row has a butt joint. The table under the calculator compares common lengths for your deck.

The final row is ripped (cut lengthways) to fit. In the 12 × 16 example the last row is only 1.81 in wide; the calculator shows the ripped width so you can see it coming. If a thin strip would look odd or be hard to fix, try a slightly different gap or rip the first and last rows to equal widths.

The calculator divides the board run by the joist spacing, rounds up and adds one for the end. A 16 ft run at 16 in centers needs 13 joists of 12 ft. This is a quantity count only; joist size, span and spacing must follow your board maker's instructions and local building rules.

No. It assumes a plain rectangle with straight rows and butt joints that land on joists. For diagonal boards, borders or odd shapes, use it for the main field and raise the allowance.

No. Everything is calculated in your browser and nothing is uploaded or stored.