Deck Material Calculator
Estimate decking boards, joists, posts, concrete, screws, and cost from deck length and width. Free deck material calculator for DIY builds.
How the calculation works
A deck material calculator turns a rectangle into a shopping list: decking, joists, rim joists, posts, footings, and fasteners. Defaults match common pressure-treated 5/4×6 decking (about 5½″ actual width) with a ⅛″ gap and 16″ joist spacing. Change board width for 2×6 or composite planks.
Framing here is a typical single-span deck with beams along the length and joists across the width. Always follow local code, span tables, and your lumber grade—this is a materials estimate, not an engineered plan. Add extra boards if you picture-frame the perimeter or run boards at 45°.
Formula
Worked example
A 16×12 ft deck with 5.5″ boards, ⅛″ gaps, 16 ft boards, and 10% waste needs about 29 decking boards, 13 joists at 16″ OC, and roughly 6 posts.
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Frequently asked questions
How many decking boards do I need?
Divide the deck width by the laid width of one board (board + gap), then multiply by how many board lengths fit the run, and add waste. This tool does that math.
How much waste should I add?
10% is a solid default for straight runs. Use 12–15% for diagonal decking, picture frames, or lots of cuts.
16″ or 24″ joist spacing?
16″ on center is the usual residential choice, especially under composite. 24″ is only for lumber and spans the joist tables allow.
How many concrete bags per footing?
About two to three 80 lb bags per 12″-class footing, more for frost-depth tubes. This estimate uses three bags each.
Does this include railing and stairs?
No. Add posts, balusters, and stair stringers separately. The square footage of the walking surface is the starting point.
Results are estimates for informational purposes and are not financial or professional advice.