Deck Calculator
Enter the deck length and width, the board width and gap, a stock board length and the joist spacing to see how many boards, joists and fasteners the surface needs. Choose the joist direction and a straight or diagonal pattern, which sets the suggested waste. This is a material estimate, not a structural design.
Last updated November 4, 2026
A nominal 6 in board is about 5.5 in
An editable assumption; check your code and decking
Diagonal only raises the suggested waste to 15%. Rows and boards are counted as for a straight layout.
Typical: 10 straight, 15 diagonal
Result
58 boards of 10 ft
Includes the waste allowance. Boards are fitted row by row with no offcut reuse.
| Deck area | 192 sq ft |
|---|---|
| Rows of boards | 26 |
| Pieces per row (16 ft run) | 2 |
| Boards, no waste | 52 |
| Boards with 10% waste | 58 |
| Total board length with waste | 457.6 ft |
| Joists (12 ft long) | 13 |
| Fasteners with waste | 744 |
This is a material estimate, not a structural design. Joist size, spans, footings, ledger attachment and guards must follow your local building code and a qualified professional.
Show the work
Formula: rows = ceil((joist length + gap) ÷ (board width + gap)); boards = rows x ceil(board run ÷ board length); joists = ceil(board run ÷ spacing) + 1
- Area: 16 x 12 = 192 sq ft. Joists run along 12 ft; boards run 16 ft, across the joists.
- Rows: (12 ft + 0.125 in gap) ÷ (5.5 + 0.125 in) = 25.622, rounded up to 26.
- Pieces: 16 ÷ 10 ft, rounded up to 2 per row; 26 x 2 = 52 boards. With 10% waste: 57.2, rounded up to 58.
- Joists: 16 ft ÷ 16 in on center, rounded up, plus 1 = 13.
- Fasteners: 26 rows x 13 joists x 2 = 676; with waste 744.
Result: 58 boards of 10 ft
How the deck materials are worked out
rows = ceil((joist side + gap) ÷ (board width + gap))
boards = rows × ceil(board run ÷ board length)
joists = ceil(board run ÷ spacing) + 1
fasteners = rows × joists × 2
Joists run along one side of the deck and are spaced across the other. Deck boards lie across the joists, so each board runs along the side where the joists are spaced, and the rows of boards stack along the side the joists run.
Take a 16 ft by 12 ft deck with joists running along the 12 ft width. With 5.5 inch boards and a 1/8 inch gap, each row takes 5.625 inches, so the 12 ft stack needs (144 + 0.125) ÷ 5.625 = 25.6, rounded up to 26 rows.
With 10 ft boards each 16 ft row needs 2 pieces, so the deck takes 26 × 2 = 52 boards, or 58 with 10 percent waste. Joists at 16 inches on center across 16 ft are 192 ÷ 16 = 12 spaces plus 1, which gives 13 joists.
See how we calculate for the formulas, sources and review schedule behind every calculator.
Also useful alongside this calculator: Concrete Calculator, Square Footage Calculator and Length Converter.
Joist direction and board direction
The joist direction setting decides which way the framing lies, and the boards always run across it. Turning the joists on the same 16 ft by 12 ft deck gives 35 rows of boards 12 ft long, 2 pieces each of 10 ft boards, and 10 joists. That is 70 boards, or 77 with waste, instead of 58.
Board lengths and fitting
The calculator offers 8, 10, 12 and 16 feet, or 2.4, 3.0, 3.6 and 4.8 meters as rounded equivalents. Switching units picks the closest stock length.
Each row is fitted on its own, so a 16 ft row of 10 ft boards counts as 2 pieces even though the offcut might be reused. That makes the count cautious. With 16 ft boards the same deck needs 26 pieces, or 29 with waste.
Waste, diagonal patterns and fasteners
Choosing diagonal only changes the suggested waste from 10 percent to 15 percent, because angled cuts leave more offcuts. The row and board counts are always for boards laid straight across the joists, so a diagonal result is rougher. These are typical allowances, not standards. Change the percentage freely, from 0 to 100.
Fasteners are counted as 2 per board per joist crossing. The 16 ft by 12 ft deck has 26 rows and 13 joists, so 676 fasteners, or 744 with 10 percent extra. Hidden clips or other systems may use a different number, so check the product instructions.
What this estimate does not cover
Joist size and spans, beams, posts, footings, ledger attachment, stairs and guards are structural and safety matters. They must follow your local building code and a qualified professional. The calculator only counts boards, joists implied by the spacing, and fasteners.
The 16 inch spacing is an editable assumption, within 8 to 48 inches or 200 to 1,200 millimeters. Some decking products need closer spacing, so read the manufacturer instructions.
How to use the Deck Calculator
- Choose imperial or metric units.
- Enter the deck length and width, then the board width and the gap between boards.
- Select a stock board length and the joist spacing on center.
- Choose which way the joists run and the board pattern, then check the waste percentage.
- Read the rows, boards, joists and fasteners, then use Copy, Print or Share.
Worked example
A 16 ft by 12 ft deck, 192 square feet, uses 5.5 inch boards with 1/8 inch gaps and joists at 16 inches on center running along the 12 ft width. It needs 26 rows of boards and 13 joists. With 10 ft boards that is 52 pieces, or 58 with 10 percent waste, and 676 fasteners, or 744 with waste.
| Deck | Board length | Rows | Boards with waste | Joists |
|---|---|---|---|---|
| 12 ft × 10 ft | 12 ft | 22 | 25 | 10 |
| 16 ft × 12 ft | 10 ft | 26 | 58 | 13 |
| 16 ft × 12 ft | 16 ft | 26 | 29 | 13 |
| 20 ft × 12 ft | 16 ft | 26 | 58 | 16 |
Frequently asked questions
How many deck boards do I need for a 12 by 12 deck?
Enter 12 and 12 with your board width, gap and length. Each row is 12 ft, so 12 ft boards need one piece per row.
How far apart should deck joists be?
The calculator assumes 16 inches on center but you can edit it. The right spacing depends on the decking product, joist size and local code.
How much waste should I add for decking?
Ten percent is a typical allowance for a straight layout and 15 percent for diagonal. Choose more for many cuts or a complex shape.
Does this design the structure of my deck?
No. It is a material estimate only. Footings, beams, joist size and span, ledger attachment and guards must follow local building code and a professional.
Why is the board count higher than area divided by board size?
It fits whole boards row by row with no offcut reuse, adds gaps between boards and then adds waste, so it is deliberately cautious.