Deck Calculator – Boards, Joists, Rails, Stairs & Costs
Use this free Deck Calculator to estimate the materials and cost for your outdoor deck. Enter length and width, pick your board size and joist spacing, then toggle optional railings, stairs, and footings. You’ll get board counts, joists, screws, railing length, concrete bags, and an optional cost breakdown.
How to Use
- Choose Unit System and Material (wood or composite).
- Enter deck length and width, then set board width, board length, and gap.
- Pick a joist spacing preset (12 in for many composites, 16 in for many woods).
- Optional: enable Railings, Stairs, and Footings with your preferred spacing.
- Optional: add costs per linear foot/meter, fastener, and labor to estimate a budget.
- Click Calculate to see materials and totals instantly.
Formulas & Assumptions
- Board rows = floor(deck span ÷ (board width + gap)).
- Boards per row = ceil(other span ÷ board length).
- Total boards = rows × boards per row, then add your waste %.
- Joists = floor(perpendicular span ÷ joist spacing) + 1; joists run perpendicular to board direction.
- Fasteners ≈ boards × joists × 2 (two per joist intersection).
- Rails = sum of selected sides; posts ≈ floor(side ÷ post spacing) + 1 per side (minus shared corners); balusters ≈ rail length ÷ spacing.
- Stairs = risers = ceil(height ÷ target rise); treads = risers − 1; stringers by width ÷ stringer spacing.
- Footing volume (each) = π × (diameter/2)² × depth; bags = total volume ÷ yield per bag size.
Always verify spans, fastener schedules, railing requirements, and footing sizes against local building codes and manufacturer guidance.
Pro Tips to Reduce Waste
- Pick a board length that divides one deck dimension cleanly (e.g., 12 ft boards on a 24 ft span).
- Consider a picture frame border to hide cut ends and allow full-length runs.
- Diagonal layouts look great but can increase waste — add a higher waste % if you plan a pattern.
- Confirm joist spacing with the board manufacturer; composites often need tighter spacing.
Common Decking Sizes & Spacing
- Board width: ~5.5 in (140 mm) nominal for many boards.
- Board lengths: 8, 10, 12, or 16 ft are common stock sizes.
- Gaps: ~1/8 in (3 mm) for drainage and expansion.
- Joist spacing (o.c.): 16 in for many wood boards; 12 in for many composites.
FAQs
How much waste should I add?
Use 10% as a baseline. Increase to 12–15% for diagonal layouts, picture frames, or lots of obstacles.
What joist spacing is best?
Many wood boards allow 16 in o.c.; many composites specify 12 in o.c. Always check the manufacturer’s span tables.
Do I need railings?
Often required once the deck is about 30 in (760 mm) above grade. Check your local code.
How many screws do I need?
About two per joist intersection per board. The calculator multiplies boards × joists × 2.
How many concrete bags per footing?
We compute footing volume by diameter and depth and convert it to bags based on your selected bag size.
Related Tools
Try our Concrete Calculator, Gravel Calculator, Sand Calculator, and Topsoil Calculator.
Important Estimate Disclaimer
The results provided by this calculator are intended as general estimates only and should not be considered precise engineering measurements. Actual material requirements, costs, and quantities can vary significantly based on site conditions, local building codes, soil type, reinforcement requirements, climate exposure, and the intended use of your project.
These calculators are designed to help you plan and budget your project more effectively, but they do not replace professional advice from a qualified contractor, engineer, or building inspector. Always verify your calculations and assumptions with trained professionals before placing material orders or commencing construction.
For more information on the assumptions, formulas, and data used in these tools, see our Data Sources and Methodology pages.
Material estimates, cost figures, and volume calculations are approximate and may not reflect your actual requirements. Always double-check dimensions and project-specific needs, and consider adding a contingency allowance for waste, over-excavation, and unforeseen conditions.
If you are unsure about any aspect of your project — including structural requirements, reinforcement needs, drainage, or load-bearing capacity — consult with a licensed professional before proceeding.