# How many deck boards do I need?

Computes the rows of deck boards across a deck, the linear feet and number of boards to buy with extra for waste, the joists under them, and the cost.

- Page: https://www.acalculator.org/construction/deck-calculator
- JSON spec: https://www.acalculator.org/construction/deck-calculator.json
- Version: a19e49950988

## Default answer

Example with the default inputs (Deck length 16 ft, Deck width 12 ft, Board width 5.5 in, Gap between boards 0.1875 in, Board length 16 ft, Joist spacing 16 in, Extra for waste 10%): A 192 ft² deck needs 29 boards in 26 rows, over 13 joists.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| len | Deck length | The length of the deck along the boards. |
| wid | Deck width | The width of the deck across the boards. |
| bw | Board width | The actual width of one board. A nominal 6 in board is 5 1/2 in wide. |
| gap | Gap between boards | The gap between the long edges of two boards. |
| bl | Board length | The length of the boards you buy. |
| joist | Joist spacing | The distance from the center of one joist to the next. |
| waste | Extra for waste | Extra board length for cuts and offcuts, as a percent. |
| price | Price per board | The price of one board. Leave empty to skip the cost. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| boards | Deck boards | The boards to buy: linear feet × (1 + waste) ÷ board length, rounded up. |
| rows | Rows of boards | How many boards side by side cover the width, gaps included. |
| linear | Linear feet of board | The rows × the deck length, before waste. |
| joists | Joists | The joists across the length at the spacing, one at each end. |
| area | Deck area | The length × the width. |
| cost | Cost of boards | The boards × the price per board. |

## Method

rows = ⌈(width + gap) ÷ (board width + gap)⌉; linear = rows × length; boards = ⌈linear × (1 + waste ÷ 100) ÷ board length⌉; joists = ⌈length ÷ spacing⌉ + 1

## Assumptions

- Boards run along the deck length and the joists run across it, square to the boards. The last row is ripped to fit.
- Boards are counted by total length, not by a cutting plan. Offcuts and end joints come out of the waste allowance.
- The joist count spaces joists from one end, with one at each end. It does not size the joists, beams or footings.
- A count within 0.000000001 of a whole number counts as that whole number, so exact fits are not rounded up.

## Worked examples

1. len = 4.8768, wid = 3.6576, bw = 0.1397, gap = 0.004762, bl = 4.8768, joist = 0.4064, waste = 10%, price = $42.00 gives rows = 26, linear = 126.7968, boards = 29, joists = 13, cost = $1,218.00. Source: hand calculation in content.mdx: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35, so 26 rows; 26 × 16 = 416 ft; × 1.10 ÷ 16 = 28.6, so 29 boards; 192 ÷ 16 + 1 = 13 joists; 29 × $42 = $1,218.
2. len = 6.096, wid = 3.048, bw = 0.1397, gap = 0.00635, bl = 3.6576, joist = 0.3048, waste = 5% gives rows = 21, linear = 128.016, boards = 37, joists = 21. Source: hand calculation in content.mdx: (120 + 0.25) ÷ 5.75 = 20.91, so 21 rows; 21 × 20 = 420 ft; × 1.05 ÷ 12 = 36.75, so 37 boards; 240 ÷ 12 + 1 = 21 joists.
3. len = 3.048, wid = 0.2921, bw = 0.1397, gap = 0.0127, bl = 3.048, joist = 0.6096, waste = 0% gives rows = 2, boards = 2, joists = 6. Source: hand calculation in content.mdx: (11.5 + 0.5) ÷ (5.5 + 0.5) = 2 rows exactly; 20 ft ÷ 10 = 2 boards; 120 ÷ 24 + 1 = 6 joists.

## FAQ

### How do I calculate how many deck boards I need?

Divide the deck width, plus one gap, by the board width plus the gap, and round up: that is the number of rows. Multiply by the deck length for the linear feet, add waste, and divide by the board length. A 16 × 12 ft deck with 5 1/2 in boards and 3/16 in gaps takes 26 rows, 416 linear feet, and 29 boards of 16 ft with 10% waste.

### How wide is a deck board?

Deck boards are sold by nominal size. Under the American Softwood Lumber Standard (PS 20), a nominal 6 in board is 5 1/2 in wide when dressed, and 5/4 × 6 decking is about 1 in × 5 1/2 in. Composite boards are usually close to 5 1/2 in; check the product.

### How big should the gap between deck boards be?

Follow the board maker. Trex, for example, recommends 3/16 in between the long edges of its composite boards in all weather, and 1/8 in to 3/16 in end to end depending on the temperature at installation.

### How far apart should deck joists be?

The International Residential Code (IRC 2021, Table R507.7) allows joists up to 16 in on center under 1 1/4 in (5/4) wood decking laid square to the joists, and up to 24 in under 2 in wood decking; diagonal decking needs closer joists (12 in and 16 in). Composite boards have their own limits in the maker’s instructions.

### How much extra decking should I buy?

Enter your own allowance in Extra for waste. The calculator counts boards by total length, so end joints, offcuts and bad boards all come out of that allowance; a deck with angles or a picture frame border uses more.

### Does this size the joists and beams?

No. It counts joists at the spacing you choose. Joist and beam sizes, spans, ledgers and footings come from your local building code (IRC section R507 in many US areas).

## Sources

- Trex, How to Space Deck Boards: 3/16 in width-to-width; 1/8 in end to end above 40°F, 3/16 in below. https://www.trex.com/deck-ideas/how-to-space-deck-boards/
- International Code Council, International Residential Code 2021, R507.7 Decking and Table R507.7: maximum joist spacing 16 in o.c. (12 in diagonal) for 1 1/4 in wood decking and 24 in (16 in diagonal) for 2 in wood decking.
- NIST Voluntary Product Standard PS 20, American Softwood Lumber Standard: a nominal 6 in board is 5 1/2 in wide dressed.