How many deck boards do I need?
Count the rows of boards across the deck, then the boards in each row. Each board takes its own width plus one gap, and the last board needs no gap after it, so the rows are the deck's width plus one gap, divided by a board plus a gap, rounded up. The last board can be ripped (cut lengthwise) to fit.
Each row is made of whole boards, so a 20 ft deck with 16 ft boards needs two boards a row. Pick a board length that fits your deck to waste less: the calculator shows the linear feet you lay and the linear feet you buy.
Source Common Core State Standards · Grade 3, area of rectangles (3.MD.C.7) ↗NIST Handbook 44 (2026) · Appendix C, General Tables of Units of Measurement ↗
How to calculate deck boards
Here it is with your numbers:
- Rows across the deck: add one gap to the width and divide by a board plus a gap. (144 + 0.125) ÷ (5.5 + 0.125) = 25.6222, rounded up to 26 rows.
- Each 16 ft board covers a 16 ft row: 1 board in each row.
- Linear feet to buy: 26 × 16 = 416.00 ft, with no offcuts.
- Deck area: 16 × 12 = 192.00 sq ft.
It means you buy 26 deck boards, 416.00 linear feet in all.
Use the gap from your board's installation instructions. Structure (joists, beams, posts and footings) is not part of this calculator; follow your local building code.
Deck boards for common deck sizes
With 5.5 in boards, 1/8 in gaps and boards as long as the deck.
| Deck (ft) | Square feet | Boards |
|---|---|---|
| 8 × 8 | 64 | 18 × 8 ft |
| 10 × 10 | 100 | 22 × 10 ft |
| 12 × 10 | 120 | 22 × 12 ft |
| 12 × 12 | 144 | 26 × 12 ft |
| 16 × 10 | 160 | 22 × 16 ft |
| 16 × 12 | 192 | 26 × 16 ft |
| 16 × 16 | 256 | 35 × 16 ft |
FAQs
What size should I use for deck boards?
Wood deck boards are usually sold as 5/4 or 2 inches thick and 4 to 6 inches wide, which is actually 1 or 1-1/2 inches thick and 3-1/2 to 5-1/2 inches wide, in lengths of 8 to 16 feet. Type the actual width into the calculator.
How do you measure a deck?
Measure the length and width in feet and multiply them: a 16 × 12 ft deck is 192 square feet. For boards, note which side they run along (the length) and which side they are laid across (the width).
Source Common Core State Standards · Grade 3, area of rectangles (3.MD.C.7) ↗
How much does it cost to install a 12x12 deck?
It depends on your prices. A 12 × 12 deck is 144 square feet, so multiply your quote per square foot by 144. For the decking alone it takes 26 boards 12 feet long (5-1/2 inch boards, 1/8 inch gaps): multiply by your price per board.
Sources
- USDA Forest Service, Forest Products Laboratory: Wood Handbook (2021), Chapter 6: Commercial Lumber (radius-edged decking, Table 6–6)
- Common Core State Standards: Grade 3, area of rectangles (3.MD.C.7)
- NIST Handbook 44 (2026): Appendix C, General Tables of Units of Measurement