Free Deck Board Calculator — Boards, Joists & Fasteners

How to Use the Deck Board Calculator

How to Calculate Deck Boards

The fundamental deck board formula is Number of Boards = Deck Length ÷ (Board Width + Gap), where deck length is the dimension perpendicular to the direction the boards run, board width is the actual face width of the decking, and gap is the spacing between adjacent boards. For example, a 20' × 12' deck using standard 5/4×6 decking (5-1/2" actual width) with a 1/8" gap between boards requires 12' (144") ÷ (5.5" + 0.125") = 25.6, rounded up to 26 boards, each 20' long. If 20' boards are not available, you will need to stagger joints across joists and add a waste allowance for butt joints and trimming. Enter your deck dimensions into this decking calculator and it handles the math instantly, including waste adjustments for diagonal and herringbone patterns.

Board Orientation Options

The direction you run your deck boards affects both the look of the finished deck and the amount of material you need. There are three common orientations, each with different waste characteristics:

• Perpendicular (Standard): Boards run perpendicular to the joists. This is the most common orientation and the most material-efficient, typically requiring only a 5% to 10% waste allowance for trimming and defective boards. It is the simplest to install and provides the strongest connection to the joist framing.
• Diagonal (45°): Boards run at a 45° angle to the joists. This creates a visually striking pattern that can make small decks appear larger, but it increases material consumption by approximately 15% because every board that meets the deck perimeter must be angle-cut, producing a triangular offcut that is often too short to reuse. Diagonal decking also requires blocking between joists for adequate support at the board ends.
• Herringbone / Chevron: Boards alternate direction in a V-pattern or zigzag. This decorative layout increases waste by approximately 20% or more due to the high number of angled cuts and the need for a center joist or beam at each direction change. Herringbone patterns require careful layout planning and are best suited for experienced builders.

Joist Spacing

Joist spacing is the on-center distance between the framing members that support your deck boards. The two standard spacings are 16" on-center and 24" on-center, and the correct choice depends on the decking material and thickness.

16" OC is the most common and is required for most composite decking products, 5/4×6 wood decking, and any diagonal board layout. It provides more support points per board, reduces bounce and flex underfoot, and is the default for residential construction. 24" OC is acceptable only for full 2×6 (1-1/2" thick) wood decking installed perpendicular to the joists. Thinner boards and composite products will sag or feel springy at 24" spacing. Always check the decking manufacturer's installation guide for the maximum allowable joist spacing — exceeding it voids the warranty and can create a structural hazard.

To calculate the number of joists you need, divide the deck length (in the direction the joists run) by the joist spacing and add 1: Joists = (Deck Length ÷ Spacing) + 1. A 20' deck at 16" OC requires (240" ÷ 16") + 1 = 16 joists.

Fastener Estimation

Deck boards require two fasteners at every joist intersection — one near each edge of the board. For face-screwed installations at 16" OC joist spacing, this works out to roughly 8 screws per square foot of decking. A 20' × 12' deck (240 sq ft) needs approximately 1,920 screws, or about four 5-pound boxes of #9 × 2-1/2" deck screws.

Hidden fastener systems offer a cleaner appearance by attaching boards from the side or bottom rather than driving screws through the face. Systems like Camo, Tiger Claw, and Mantis use proprietary clips or edge-driven screws that sit in the gap between boards. Hidden fasteners typically cost 2× to 3× more than face screws and require more installation time, but they eliminate visible screw heads on the deck surface. Many composite decking manufacturers require or recommend specific hidden fastener systems to maintain the product warranty.

Pro Tips

• Stagger board end joints across at least two joist bays so that no two adjacent rows end on the same joist. This distributes loads evenly, prevents weak lines across the deck, and looks much better than aligned butt joints.
• Leave a 1/8" gap between deck boards for drainage and airflow. Use a 16d nail or a dedicated spacing tool as a gauge while fastening. Composite boards may require different gap sizes — check the manufacturer's specifications.
• Use stainless steel or coated deck screws rated for the decking material. Standard zinc-plated screws corrode quickly in treated lumber due to the copper-based preservatives, leaving black stains and weakened connections. ACQ-compatible or stainless fasteners prevent this problem.
• Crown boards up — sight down the length of each board and install it with the slight natural bow (crown) facing upward. The weight of foot traffic will flatten the crown over time, resulting in a flatter surface. Boards installed crown-down will develop a noticeable sag between joists.
• Check your local building code for railing requirements. Most jurisdictions require a 36" guardrail on any deck surface more than 30" above grade, with balusters spaced to pass the 4" sphere test. Factor railing materials into your budget and timeline before starting the decking installation.