To calculate how many Trex deck boards you need for a 10 x 10 deck, you need to consider the total deck area, the width of each board including the gap, and then calculate the number of boards required. Here’s how to do it:

**Calculate the Total Deck Area:**
The deck is 10 feet by 10 feet, so the total area is 10×10=100 square feet.
**Convert the Board Width and Gap to Feet:**
Each board has a finished width of 5.5 inches and there is a 3/16 inch gap between boards.
Convert these measurements to feet (since the deck dimensions are in feet). There are 12 inches in a foot, so:
Board width in feet = 5.5 inches12≈0.4583 feet
Gap width in feet = 3/16 inches12≈0.0156 feet
**
Calculate the Combined Width of Board and Gap:
**
Combined width = Board width + Gap width = 0.4583 + 0.0156 = 0.4739 feet
**Calculate the Number of Boards Needed:**
Divide the width of the deck by the combined width of a board and a gap.
Number of boards = 10 feet (deck width)0.4739 feet (combined board and gap width)0.4739 feet (combined board and gap width)10 feet (deck width)

**Given:**

```
Deck width = 10 feet
Board width = 5.5 inches
Gap width = 3/16 inches
```

**Convert inches to feet for the board and gap widths:**

```
Board width in feet = 5.5 inches ÷ 12 = 0.4583 feet
Gap width in feet = (3/16) inches ÷ 12 ≈ 0.0156 feet
```

**Calculate the combined width of a board and a gap:**

`Combined width = 0.4583 feet (board) + 0.0156 feet (gap) ≈ 0.4739 feet`

**Finally, divide the deck width by the combined width to find the number of boards:**

`Number of boards = 10 feet ÷ 0.4739 feet ≈ 21.09`

**Since you can’t use a fraction of a board, round up to the nearest whole number:**

`You would need 22 boards for the deck.`

Please note that the numbers are rounded for simplicity, and the actual requirement might slightly vary based on precise measurements and onsite adjustments.