The decking calculator turns your deck dimensions and board choices into a complete material list. It returns a base board count, adjusted total, screw or hidden-clip count, deck area, fastener packs, and optional cost estimate. Homeowners use it to build an accurate shopping list before visiting the lumber yard. The result shows exactly how much to buy so you avoid a second trip for extra boards.
Results update as you type.
Enter length and width, or leave them blank and enter total area for an irregular deck. Optional price fields add a cost estimate.
Enter your deck dimensions and board choice. The calculator multiplies length by width for total area, divides by each board's coverage width (face width plus the gap), and rounds up for board count. Diagonal and herringbone layouts apply a layout factor before waste; the screw or hidden-clip count comes from how many joists each board crosses.
Multiply deck length by deck width to find total deck area. When the user enters an area override for an irregular shape, that value replaces this product.
Area = Deck Length × Deck Width
When the user supplies an area override (L-shaped or irregular decks), the override replaces L*W as A.
Add board width to the joint gap to find the on-deck footprint of one board, including the spacing to the next board.
Cover Width = Board Width + Joint Gap
C is the on-deck footprint of one board including the gap to the next board. For 5/4x6 or 2x6 (5.5 in face) with a 1/8 in gap, C is 5.625 in (0.469 ft).
Divide deck area by the area one board covers, multiply by the angle factor for diagonal layouts, then round up to whole boards.
Base Board Count = Ceiling(Area / (Cover Width × Board Length)) × Angle Factor
L_b is the chosen board length. angle_factor is 1.0 for 90-degree layouts and 1.15 for 45-degree layouts to reflect higher offcut volume on diagonals. Always rounded up to whole boards.
Increase base board count by the waste percent, then round up to the nearest whole number so the customer can buy full boards.
Total Board Count = Ceiling(Base Board Count × (1 + Waste Percent / 100))
waste is the percent waste factor. Result is rounded up so the customer can buy whole boards.
Each board gets two screws at every joist crossing. Multiply total board count by joist crossings, then multiply by two for both screws.
Screw Count = 2 × Total Board Count × Joist Crossings
Two screws per board per joist crossing is the standard prescribed by most decking manufacturers and the IRC commentary for face-screwed boards. J is the number of joist crossings each board spans, computed as ceil(run_length / spacing) + 1.
Add one clip per board at every interior joist crossing. Then add one starter clip per board at each of the two deck ends.
Clip Count = Total Board Count × Joist Crossings + 2 × Boards Per Row
One clip per board per interior joist plus starter clips at each end of the deck. J here is interior joist crossings (joists - 1).
Divide the perpendicular span by joist spacing, round up, then add one to include the far end joist.
Joist Count = Ceiling(Perpendicular Span / Joist Spacing) + 1
Used internally to compute J. span_perpendicular is the dimension perpendicular to the boards (W when boards run lengthwise, L when boards are at 90 degrees to width).
The calculator takes deck length and deck width to find the deck area, which drives every output that follows. Board width and joint gap determine the coverage each board provides. The calculator divides deck area by that coverage to reach the base board count. A diagonal layout activates the angle factor, raising the base board count to cover extra cuts at each board end. The waste percent then increases the base board count to produce the adjusted total you should order. The adjusted total feeds the screw count at a standard fastener rate per board. Finally, board length and joist spacing determine how many joists each board crosses, which sets the fastener count.
Mark is building a 12 × 16 foot deck off his back door this summer. The deck area comes to 16 × 12 = 192 square feet. He chose 5.5-inch cedar boards with 0.125-inch expansion gaps. Each board plus its gap covers 5.5 + 0.125 = 5.625 inches, or 0.469 feet wide. Dividing the 12-foot span by 0.469 gives 25.59, which rounds up to 26 base boards. Adding 10% for waste, 26 × 1.10 = 28.60, which rounds up to 29 boards total.
Tom is adding a cedar platform deck next to his garage, 14 feet long and 10 feet wide, with joists running across the 10-foot width and spaced 16 inches on center along the 14-foot length. He plans to use 5.5-inch boards with 0.125-inch gaps. Each board plus its gap covers 5.5 + 0.125 = 5.625 inches, or 0.469 feet of span. To cover the 10-foot width, 10 ÷ 0.469 = 21.32, rounding up to 22 base boards. Adding 10% for waste, 22 × 1.10 = 24.20, which rounds up to 25 boards to purchase. The 14-foot length at 16-inch spacing yields ceil(14·12/16) + 1 = 12 joists, and every board crosses all 12. With 2 screws at each crossing, the full job needs 25 × 12 × 2 = 600 screws.
A new deck build is the most common moment to reach for this tool — enter your deck length, deck width, board width, joint gap, and board length before ordering any material. The tool also handles repair jobs — enter dimensions for only the damaged section to get a targeted count, not a full-deck guess. Contractors use it at bid time to firm up board count and fastener count before writing a quote.
Board width and joint gap work together to set how many boards fit across your deck. A wider board covers more surface with each run, reducing the total board count. A larger joint gap shrinks the effective width of each board, which slowly raises the count. Most pressure-treated decks use a 3/16-inch or 1/4-inch gap to allow for drainage and wood movement.
A diagonal board layout gives a deck more visual interest but increases your board order. Boards run at an angle, so each end gets trimmed and those trimmings are wasted. The angle factor captures this extra material need. A higher angle factor tells the calculator to increase the base board count before waste percent is applied.