Decking Calculator

How to use this decking calculator

What this calculator does. It estimates the finished deck surface area, the number of full decking boards to purchase, an approximate fastener count, board cost, and total decking-material cost. It is intended for early material planning, supplier comparisons, and a quick order check. It does not size beams, joists, posts, footings, guards, or connections, and it does not replace a code-compliant structural plan. For structural context, review the American Wood Council's Prescriptive Residential Wood Deck Construction Guide.

When to use it. Use the calculator when budgeting a new rectangular deck, comparing two board widths, checking whether a supplier's board count is reasonable, or estimating how many screw or hidden-fastener packs to buy. For irregular decks, divide the plan into rectangles, calculate each section separately, and combine the quantities while allowing for cuts and layout direction.

How to calculate. The calculator opens with a ready-to-use 20 ft by 13 ft demonstration, 6 in by 8 ft boards, screws or nails, a $7 board price, and $98 of fasteners. The example workbook is immediately available. Follow these steps:

1. Replace Deck length and Deck width with the finished rectangular footprint in feet.
2. Select Board width; choose Other to reveal Custom board width. Enter Board length in feet.
3. Choose Fastener type. Add Cost per board and Total fastener cost when a cost estimate is needed.
4. Read the live results and material table, then use Download Excel to export the current validated model.
5. Use Reset to clear the demonstration and all calculated content. Export is disabled after reset until a complete valid state is entered again.

Input guide. Deck length and Deck width are required positive decimals in feet; examples are 20 and 13. Increasing either increases area, boards, fasteners, and usually cost. Do not type feet marks or use a decimal comma. Board width is required and is the installed face width in inches; the example is 6 in. A wider board generally reduces the board count. Custom board width is required only when Other is selected and must be a positive decimal in inches. Board length is required in feet; the example is 8 ft. Longer boards cover more area per piece, though real layouts can create offcuts that this area model cannot predict. Fastener type selects either 3.5 screws/nails per square foot or 1.75 hidden fasteners per square foot. Cost per board and Total fastener cost accept nonnegative U.S. dollar amounts; enter 0 when price is unknown rather than leaving a required quantity field blank.

Output guide. Total decking cost is the sum of boards cost and the entered total fastener cost. Deck area is the exact rectangular identity length × width in square feet. Number of boards divides deck area by one board's face area, adds a fixed 10% waste allowance, and rounds up to a whole board. Fasteners needed applies the selected rule of thumb and rounds up. Boards cost multiplies whole-board quantity by cost per board. The material table repeats those values with quantities and costs so you can compare them against a supplier quote. Zero cost is meaningful when pricing is unknown; zero dimensions are invalid because they describe no deck.

Worked example. A 20 ft × 13 ft deck has 260 ft² of surface. A 6 in board is 0.5 ft wide, so an 8 ft board covers 4 ft². The raw board requirement with 10% waste is 260 ÷ 4 × 1.10 = 71.5, which rounds up to 72 boards. Screws or nails are estimated at 260 × 3.5 = 910. At $7 per board, boards cost $504; adding $98 for fasteners gives the first-open total of $602.00.

Learn more. Material estimates should be checked against local code, manufacturer installation instructions, and board moisture behavior. The USDA Forest Products Laboratory's Wood Handbook explains wood properties and moisture movement, while its guide to pressure-treated wood discusses selection and use considerations.