Deck Board Calculator
Work out how many deck boards you need. Enter deck size, board width and length, and the gap you want to get board count, rows across and linear feet. How it works
Use the Deck Board Calculator
About the Deck Board Calculator
A 12 ft wide deck takes 26 rows of 5.5 in boards (a 2x6 or 5/4x6) with a 1/8 in gap: 25 whole boards and a last board ripped to 3.38 in. With 8 ft boards on a 12 ft run that is 2 pieces a row, 52 boards or 58 with 10% waste. The deck board calculator fits as many whole board-plus-gap widths across the deck as it can and adds one ripped row only when the space left is wider than one gap.
Boards across by deck width
| Deck width | 5.5 in boards, 1/8 in gap | Last board | 5.5 in boards, 1/4 in gap | Last board | 3.5 in boards, 1/8 in gap | Last board |
|---|---|---|---|---|---|---|
| 8 ft | 18 | 0.38 in | 17 | 4 in | 27 | 1.75 in |
| 10 ft | 22 | 1.88 in | 21 | 5 in | 34 | 0.38 in |
| 11.5 ft | 25 | 3 in | 24 | none | 39 | 0.25 in |
| 12 ft | 26 | 3.38 in | 25 | none | 40 | 2.63 in |
| 14 ft | 30 | 4.88 in | 30 | 1.25 in | 47 | 1.25 in |
| 16 ft | 35 | 0.75 in | 34 | 2.25 in | 53 | none |
| 20 ft | 43 | 3.75 in | 42 | 4.25 in | 67 | 0.75 in |
Last board = the width the final row is ripped to; none means the whole boards reach the edge within one gap. A last board under 1 in is flagged as a sliver that is hard to cut and fasten.
Deck boards needed for common deck sizes
| Deck size (along boards × across) | Board length | Boards | Linear feet | With 10% waste | Boards with 10% waste |
|---|---|---|---|---|---|
| 10 × 10 ft | 10 ft | 22 | 220.0 ft | 242.00 ft | 25 |
| 12 × 10 ft | 12 ft | 22 | 264.0 ft | 290.40 ft | 25 |
| 12 × 12 ft | 12 ft | 26 | 312.0 ft | 343.20 ft | 29 |
| 12 × 16 ft | 12 ft | 35 | 420.0 ft | 462.00 ft | 39 |
| 16 × 12 ft | 16 ft | 26 | 416.0 ft | 457.60 ft | 29 |
| 12 × 20 ft | 12 ft | 43 | 516.0 ft | 567.60 ft | 48 |
| 16 × 16 ft | 16 ft | 35 | 560.0 ft | 616.00 ft | 39 |
| 16 × 20 ft | 16 ft | 43 | 688.0 ft | 756.80 ft | 48 |
| 20 × 20 ft | 20 ft | 43 | 860.0 ft | 946.00 ft | 48 |
5.5 in boards with a 1/8 in gap. The first dimension is the direction the boards run, and the boards are as long as the deck, so there are no butt joints.
Methodology
Whole boards across = floor((deck width in inches + gap) ÷ (board width + gap)). If the width left after those boards and their gaps is more than one gap, one more row is laid and ripped to that width, so the boards across are the whole boards plus that row. Pieces per row = deck length ÷ board length, rounded up. Total boards = boards across × pieces per row, and linear feet = total boards × board length × (1 + waste ÷ 100). Boards with waste are rounded up. Joists run across the deck under the boards: joists = ceil(deck length ÷ joist spacing) + 1, one at each spacing from one end to the other with both end joists counted and a shorter last bay when the length does not divide evenly, so a 12 ft run takes 13 at 12 in, 10 at 16 in, 9 at 20 in and 7 at 24 in, and a 13 ft run 14, 11, 9 and 8. Metric sizes are converted exactly (1 in = 25.4 mm, 1 ft = 0.3048 m) and laid out the same way: a 3.6 × 3.6 m deck with 140 mm boards 2.4 m long and a 3 mm gap takes 26 rows, 52 boards and 58 with 10% waste, and its last board is ripped to 25 mm and flagged as a sliver.
How to use this tool
- Enter the deck length in the direction the boards run and the deck width across them. The width sets the number of rows.
- Pick the board width as the calculator lists it: 3.5 in for a 2x4 or 5/4x4, 5.5 in for a 2x6 or 5/4x6, 7.25 in for a 2x8 and 11.25 in for a 2x12. For composite boards, take the width from the product sheet.
- Pick a board length at least as long as the deck to avoid butt joints. Shorter boards multiply the count: 8 ft boards on a 12 ft run take 2 pieces a row.
- Optionally pick a joist spacing (12, 16, 20 or 24 in on center, or 300 to 600 mm in metric) to count the joists under the boards.
Frequently asked questions
- How many deck boards do I need?
- Rows across × pieces per row. With 5.5 in boards and a 1/8 in gap the rows are 22 for a 10 ft wide deck, 26 for 12 ft, 35 for 16 ft and 43 for 20 ft. Boards as long as the deck give one piece a row: a 16 × 16 ft deck takes 35 boards, 39 with 10% waste.
- How far apart should decking boards be?
- The calculator offers gaps of 1/16, 1/8, 3/16 and 1/4 in. The board maker’s installation guide sets the gap for its boards, so use the one it gives.
- What is the spacing for 2x6 deck boards?
- A 2x6 is entered as 5.5 in, so with a 1/8 in gap the boards repeat every 5 5/8 in, center to center. With a 3/16 in gap they repeat every 5 11/16 in, and with 1/4 in every 5 3/4 in.
- Does a wider gap change how many boards I need?
- Sometimes. On a 12 ft wide deck with 5.5 in boards it is 26 rows at 1/16, 1/8 and 3/16 in, and 25 at 1/4 in, where 25 whole boards reach the edge within one gap and no ripped board is needed.
- What does “last board ripped to” mean?
- The width the final row has to be cut to so the boards end at the deck edge. On an 8 ft wide deck with 5.5 in boards and a 1/8 in gap it is 0.38 in, which the calculator flags as a sliver; a narrower first board, another gap or another board width avoids it.
- How many joists do I need for a deck?
- Divide the deck length (along the boards) by the joist spacing, round up and add one, so both end joists are counted and the last bay is shorter when the length does not divide evenly. A 12 ft deck takes 13 joists at 12 in on center, 10 at 16 in, 9 at 20 in and 7 at 24 in; in metric, a 3.6 m deck takes 13 at 300 mm, 10 at 400 mm, 9 at 450 mm and 7 at 600 mm. Blocking, hangers and a ledger are extra, and the spacing your boards allow is set by the board maker and your local building code.
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