Unit System

Material

Sets sensible defaults for joist spacing.

Board Orientation

Deck Length (ft)

Deck Width (ft)

Board Width (in)

Nominal decking ~5.5 in (140 mm).

Board Length (ft)

Gap Between Boards (in)

Typical 1/8 in (3 mm).

Joist Spacing (in)

Composite often 12 in; wood often 16 in.

Waste/Overage

Board Cost (/lf)

Per linear foot (or per meter in metric).

Fastener Cost (each)

Labor (/ft²)

Or /m² in metric.

Railings (optional)

Sides With Rail

Front (length) Back (length) Left (width) Right (width)

Post Spacing (ft)

Baluster Spacing (in)

Often ≤ 4 in (102 mm) by code.

Rail Cost (/lf)

Stairs (optional)

Include Stairs

Stair Width (in)

Deck Height (in)

Surface height above ground.

Target Rise (in)

Target Run (in)

Tread depth.

Stringer Spacing (in)

Support Posts & Footings (optional)

Support Post Spacing (ft)

Grid spacing under deck.

Footing Diameter (in)

Footing Depth (in)

Concrete Bags

Bag Price ($)

Tax %

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.

🪵 Deck Calculator