Decking Calculator

Count full-length deck board rows using actual board width and a fixed quarter-inch gap, then add board waste and estimate spaced joists. Calculate it clearly with Figur.

Estimate full-length board rows and joists for a rectangle

Rows = round up(deck width × 12 ÷ (board width + 0.25)), with deck width in feet and actual board width in inches. The formula includes a fixed quarter-inch gap in each row's module. Boards = round up(rows × (1 + waste ÷ 100)). Joists = round up(deck length × 12 ÷ joist spacing) + 1 closing joist. Deck area = length × width. Waste applies to boards, not joists.

Method and assumptions

Rows = round up(width × 12 ÷ (board width + 0.25)). Boards = round up(rows × (1 + waste ÷ 100)); joists = round up(length × 12 ÷ joist spacing) + 1.

  • Each counted board spans the full deck length. This is a row count, not a stock-length cutting list; it does not split rows into shorter boards.
  • The 0.25-inch gap is fixed. Use actual exposed board width, not nominal lumber width.
  • Enter whole-inch joist spacing from 8 to 24. Joists are spaced along deck length and span its width.
  • Diagonal patterns, picture-frame borders, stock joints, blocking, beams, posts, footings, connectors and structural/code checks are excluded.

Planning takeoff, not a binding bid, structural design or engineering advice. Confirm field measurements, product coverage, installation requirements and supplier quantities before buying.

Sources and references

The result uses your inputs and the tool's documented assumptions; it does not fetch current prices, rates or products.

Worked example

A 16 × 12 ft deck uses 5.5-inch boards, 16-inch joist spacing and 10% board waste.

144 ÷ (5.5 + 0.25) = 25.043 rows, rounded up to 26. 26 × 1.10 = 28.6, rounded up to 29 boards, each 16 ft long. Joists: 192 ÷ 16 + 1 = 13. Deck area: 192 sq ft.

Frequently asked questions

Does it choose available stock lengths?

No. A board means one full deck-length row. If stock is shorter than your deck, prepare a cutting and joint-support plan separately.

Can I change the board gap?

No. The row formula uses board width plus a fixed 0.25 inch. Confirm the manufacturer's installation gap before relying on the count.

Does waste increase joists too?

No. Waste increases only board rows. Joists use spacing plus one closing joist, with no waste allowance.

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