Trex Decking Calculator: Square Feet Guide

This calculator works out the square footage of a deck surface, which is the first number you need when planning a Trex decking project. You enter the deck’s length and width in feet, and it returns the total area in square feet, along with the number of deck boards needed based on a standard board width. The result gives you a solid starting point for estimating materials and budget, before you account for waste, joist spacing, and fasteners.

Formula

The core calculation is straightforward: Area = Length × Width. For the number of boards, the formula is Boards = (Length × Width) ÷ (Board Width / 12).

Where:

• Length (l) – the longer side of the deck, in feet.
• Width (w) – the shorter side of the deck, in feet.
• Board Width (bw) – the actual width of one deck board, in inches. Trex boards typically have a face width of 5.5 inches (for a 6-inch nominal board).
• Area – total deck surface, in square feet.
• Boards – the total linear feet of board required, which you then divide by the board length you plan to buy (e.g., 12 ft, 16 ft, 20 ft) to get the number of individual boards.

Worked Example

Using the default values from the calculator above:

1. Deck length = 20 ft
2. Deck width = 12 ft
3. Board width = 5.5 in

First, calculate the area: 20 × 12 = 240 sq ft.

Next, convert board width to feet: 5.5 ÷ 12 = 0.4583 ft.

Then, find the total linear feet of board: 240 ÷ 0.4583 = 523.64 linear ft.

The calculator rounds to two decimals, showing 240.00 sq ft and 523.64 boards (linear feet). If you buy 12-ft boards, you need 523.64 ÷ 12 ≈ 43.6, so round up to 44 boards.

Why Square Footage Matters for Trex

Trex composite decking is sold by the linear foot, not by the square foot, but the square footage is what you use to compare prices, estimate under-deck waterproofing, or calculate the amount of hidden fastener clips. Most Trex products come in 12-ft, 16-ft, and 20-ft lengths, and the actual board width is 5.5 inches (for the standard 6-inch board) or 7.25 inches (for the 8-inch board). Knowing the total square footage lets you quickly estimate how many boards you need, but you also have to plan for cuts, angles, and the small gap between boards (typically 1/8 inch for proper drainage and expansion).

How to Use the Calculator for a Real Project

Follow these steps to get a realistic material estimate:

• Measure the longest and widest points of your deck frame, in feet. If the deck is a rectangle, multiply them. If it has an L-shape or other irregular form, split it into rectangles, calculate each, and add them together.
• Choose your board width. Standard Trex boards are 5.5 inches wide; if you are using a grooved edge board for hidden fasteners, the width is the same.
• Decide on the board length you will buy. Common lengths are 12, 16, and 20 feet. Divide the total linear feet from the calculator by that length, then round up to the nearest whole board.
• Add at least 10% extra for waste – cuts, mistakes, and warped boards. For diagonal layouts, add 15–20%.

Common Mistakes When Estimating

The most frequent error is mixing up board width and board length. The calculator asks for board width in inches, but many people enter the board length (e.g., 12 or 16) by mistake, which throws off the board count by a factor of 12. Another common issue is forgetting the gap between boards. While the gap is small (1/8 inch), it reduces the number of boards you need by about 2% – not huge, but it adds up on a large deck. Also, remember that the square footage is the deck surface, not the framing lumber. If you are using the result to buy joists or beams, you need a separate structural calculation based on joist spacing (typically 12 or 16 inches on center).

How to Read the Results

The first output, Total area, is the flat surface area of your deck. This is what you would use to estimate the cost of Trex per square foot (most retailers list price per linear foot, so multiply by 12 and divide by board width to get per square foot). The second output, Boards needed (linear ft), is the total length of deck board you must purchase. Divide that by the board length you intend to buy to get the number of boards. For example, with 523.64 linear feet and 20-ft boards, you need 27 boards (523.64 ÷ 20 = 26.18, round up).

Limitations of This Calculation

This calculator gives a rough material quantity, not a cutting plan. It assumes a simple rectangular deck with no stairs, no angled cuts, and no picture framing. Real decks have waste from cutting boards to length, ripping boards to fit at edges, and working around posts or vents. It also does not account for the hidden fastener clips (you need two clips per joist per board) or the color-matched screws for face-fastening. For a precise quote, a contractor or Trex’s own project planner will measure the actual framing and layout, but this calculator is a fast way to get a ballpark figure before you go shopping.

Frequently Asked Questions

How many square feet does one Trex board cover?

A single 5.5-inch-wide, 12-foot-long board covers 5.5 sq ft (5.5 in × 12 ft = 66 sq in = 0.458 sq ft, times 12 ft = 5.5 sq ft). A 16-ft board covers 7.33 sq ft, and a 20-ft board covers 9.17 sq ft. This is the face area without the gap; with a 1/8-inch gap, the actual coverage is slightly less.

Should I add waste to the calculator result?

Yes, always. Add at least 10% for a straight, rectangular deck. For diagonal layouts, 45-degree corners, or decks with many cutouts, add 15–20%. This covers miscuts, defective boards, and future repairs. It is better to have one extra board than to wait for a replacement.

Does Trex sell by square foot or linear foot?

Trex is sold by the linear foot. The price per linear foot varies by product line and retailer, but you can convert to square feet by dividing the linear foot price by the board width in feet (e.g., a $3.00 per linear foot board that is 5.5 inches wide costs $3.00 ÷ 0.458 = $6.55 per square foot).

What is the difference between 5.5-inch and 7.25-inch boards?

The 7.25-inch board is wider, so it covers more area per linear foot. For the same deck, you need fewer boards and fewer fasteners, but the boards may be heavier and more expensive each. The 5.5-inch board is the most common and offers more design flexibility for borders and patterns.

Can I use this calculator for a non-rectangular deck?

Yes, if you split the deck into rectangles. Measure each section separately, calculate the area of each, and add them together. For triangles, use the formula (base × height) ÷ 2. Then enter the total area into the calculator as the product of length and width – or simply enter the total area directly by setting length and width to values that multiply to that area (e.g., 1 ft by 240 ft for 240 sq ft).

How do I account for the gap between boards?

The gap is typically 1/8 inch, which reduces the total board count by about 2%. To be precise, multiply your total linear feet by 0.98 (or divide by 1.01). In practice, most people just add 10% waste, which covers the gap and cuts. For a 240 sq ft deck, the gap saves about 5 linear feet of board, which is less than one board.

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in l | Deck length | ft | 20 | 0
in w | Deck width | ft | 12 | 0
in bw | Board width | in | 5.5 | 1
out area | Total area | sq ft | l * w | 2
out boards | Boards needed (linear ft) | boards | (l * w) / (bw / 12) | 2