For a 16 × 20 ft deck with 5.5 in boards, 1/4 in gaps, and boards running the 20 ft direction, you need 34 board rows. If you buy 20 ft boards and add 10% contingency, the purchase estimate is 38 boards. Different board lengths produce different purchase counts, even when the deck size stays the same.
The useful question is not just how many square feet your deck covers. You need to know the actual board width, the space between adjacent boards, the board direction, and the stock lengths you can buy. Those inputs determine whether a row needs one board or multiple cut pieces.
Start with actual board width, not the size printed in the name
A board sold as a 1 × 6 is not necessarily 6 in wide. Many standard decking products have an actual width near 5.5 in, but dimensions vary by product. For example, Trex lists specific 5.5 in products in 12, 16, and 20 ft lengths; other widths and available lengths vary across collections.
Measure the actual coverage width from the manufacturer's product specification. Then use the board-to-board gap required for that specific product. Wood, composite, and PVC can have different requirements, and some gaps depend on temperature or installation method.
The deck board row formula
Board direction decides which deck dimension becomes the cover width. If boards run along a 20 ft deck length, the rows stack across the 16 ft deck width.
rows = ceil((cover width + board gap) ÷ (actual board width + board gap))The extra gap in the numerator is intentional. A layout with 34 boards has 33 spaces between them, not 34. The formula accounts for the missing gap beyond the last board and rounds up to the next complete row.
Worked example: a 16 × 20 ft deck
Convert the 16 ft cover width to 192 in. With 5.5 in boards and 1/4 in gaps:
ceil((192 + 0.25) ÷ (5.5 + 0.25)) = ceil(33.43) = 34 rows.
When every row can use one full-length 20 ft board, the layout needs 34 boards before contingency. A 10% allowance adds four boards, for a total purchase estimate of 38 boards.
The final row may need to be ripped narrower or the layout adjusted. Confirm edge details, border boards, and manufacturer spacing rules before purchasing.
How many deck boards do common deck sizes need?
This comparison uses 5.5 in actual board width, a 1/4 in gap, straight rectangular decking, and 10% contingency. Each row assumes stock boards matching the listed board-run length, so it does not include picture-frame borders, stairs, or awkward cuts.
| Deck size | Board run | Board rows | 10% extra | Boards to buy |
|---|---|---|---|---|
| 10 × 12 ft | 12 ft | 21 | 3 | 24 |
| 12 × 12 ft | 12 ft | 26 | 3 | 29 |
| 12 × 16 ft | 16 ft | 26 | 3 | 29 |
| 16 × 20 ft | 20 ft | 34 | 4 | 38 |
| 20 × 20 ft | 20 ft | 42 | 5 | 47 |
These are specific examples, not universal purchasing rules. Change the gap, board width, direction, shape, stock length, saw kerf, or trim and the result changes.
Board spacing can change the number of rows
On the same 16 ft cover width with 5.5 in boards, a 1/8 in gap produces 35 rows, a 1/4 in gap produces 34 rows, and a 3/8 in gap produces 33 rows. That is a two-row difference before contingency.
This does not mean you should choose the largest gap to reduce your material order. The correct spacing comes from the selected product's installation instructions. TimberTech identifies different spacing guidance for different decking lines, while Trex explains why product-specific gapping matters.
Why board rows are not the same as boards purchased
A 20 ft row can be covered by one 20 ft board, a 16 ft piece plus a 4 ft piece, or a 12 ft piece plus an 8 ft piece. All three layouts still contain the same number of rows, but they have different seams, reusable offcuts, and stock-board purchase totals.
For the same 16 × 20 ft example, DeckCalculator's current cut-planning assumptions produce approximately 38 boards at 20 ft, 51 boards at 16 ft, or 75 boards at 12 ft, each including 10% contingency. Those quantities come from practical repeat-piece packing; they cannot be recovered by dividing the deck's square footage by a board's nominal area.
Read the 12 ft vs. 16 ft vs. 20 ft stock-length comparison for the seam and overage breakdown.
What this estimate does not include
A straight board-count example does not automatically cover stairs, fascia, picture-frame borders, angled or herringbone patterns, curved edges, replacement boards, or structural framing. Add those separately. Confirm board-end support, maximum joist spans, fastening instructions, local building code requirements, and any product warranty conditions before building.
If you also need face screws, use the deck screw quantity guide. If you have actual local board prices, enter them in the Deck Cost Calculator.