Short answer
A 16 ft × 12 ft deck needs 26 rows of 5.5 inch boards with a 1/4 inch gap — 416 linear feet, or 29 boards of 16 ft once a 10% waste allowance is added. The count comes from one rule: each board covers its own width plus the gap beside it. Skip that rule and the deck comes up short. Work through the mistakes below with your own size, or try the Deck Board Calculator for an instant result.
The formula
Coverage per board = board width + gap, in inches. Rows = deck width in inches ÷ coverage per board, rounded up. Linear feet = rows × board length in feet. Boards to buy = rows × 1.10 (10% waste), rounded up — using boards as long as the row, so one board per row. Joists at 16 inches on center = (run in inches ÷ 16) + 1. Screws = rows × joists × 2, at two screws per board per joist.
Worked example: 16 ft × 12 ft deck, boards run the 16 ft length
Step 1: coverage per board is 5.5 + 0.25 = 5.75 inches. Step 2: rows across the 12 ft width are 144 ÷ 5.75 = 25.04, rounded up to 26 rows — the last row is ripped narrower to fit. Step 3: linear feet are 26 × 16 = 416 ft. Step 4: with 10% waste, 26 × 1.10 = 28.6, rounded up to 29 boards of 16 ft. Step 5: joists at 16 inches on center across the 16 ft run are (192 ÷ 16) + 1 = 13 joists. Step 6: screws at two per board per joist are 26 × 13 × 2 = 676 screws — order a box, not a bag.
The 6 mistakes
1. Counting the nominal width, not the actual width. A nominal 6 inch board is dressed to 5.5 inches. Counting 6 inches across a 12 ft width gives 24 rows; the real count is 26. Two rows short, before you start.
2. Forgetting the gap entirely. A 1/4 inch gap across 26 boards adds 6.5 inches of width — more than a whole board. Ignore it and the last row does not fit, or the deck ends up wider than the frame.
3. Ordering the bare row count, with no waste. Cuts, end pieces, a ripped last row and one defective board all eat stock. A straight rectangular deck needs 10% extra; angles, curves, stairs or a picture-frame border need 15% to 20%.
4. Running the boards the wrong way for your stock length. On this deck, rows along the 16 ft side need 26 rows from 16 ft stock. Rows along the 12 ft side need 34 rows across the 16 ft width — 34 boards of 16 ft stock, each cut down, against 29 the other way. Check direction against the lengths you can actually buy.
5. Spacing joists for lumber when you are laying composite, or for square when you are laying diagonal. 16 inches on center suits most lumber run perpendicular to the joists. Diagonal decking and many composite boards need 12 inches on center, because composite spans are often shorter than lumber. Check the manufacturer's span table, not a rule of thumb.
6. Forgetting fasteners, and the frame under the surface. Two screws per board per joist runs into four figures on even a modest deck — 676 on the example above — and hidden-clip systems have their own coverage rating per square foot. Boards are only the surface: joists, beams, posts and footings are a separate count, and footings are the genuinely structural part. In most places a deck attached to the house or raised above a certain height needs a permit, so check with your local building department before you build.
Deck boards for common deck sizes
This table uses 5.5 inch boards with a 1/4 inch gap, boards run along the longer side, and a 10% waste allowance. Use it to sense-check a quote, then run your own size through the formula above.
| Deck size | Rows | Linear feet | Boards to buy (with 10% waste) |
|---|---|---|---|
| 12 ft × 12 ft | 26 | 312 ft | 29 boards of 12 ft |
| 16 ft × 12 ft | 26 | 416 ft | 29 boards of 16 ft |
| 20 ft × 16 ft | 34 | 680 ft | 38 boards of 20 ft |
Method and limits
This guide counts boards for a flat, rectangular deck: rows from the actual board width plus the gap, rounded up, plus a 10% waste allowance for straight layouts. It assumes one board per row (stock at least as long as the row), two screws per board per joist, and joists at 16 inches on center for lumber run perpendicular. Diagonal layouts, composite spans, picture-frame borders, stairs, railings and footings change the count, so treat the total as a planning estimate and confirm the final list with your supplier and your local code. For the full deck, including joists, see the Deck Calculator; to keep the surface looking new, see the Deck Stain Calculator.
Try it with your own numbers
Open the Deck Board Calculator, enter your deck size, board width and gap, and it gives the board count. More construction tools are in Construction calculators, and more guides are on the BuildCalc Blog. See How Our Calculators Are Checked for how the numbers behind these guides are verified.
Results are estimates and general information only. Always check the manufacturer's span and gap instructions, and confirm final quantities with your supplier before you order.