Deck Calculator

The formula

Coverage and yield assumptions

A 5/4 × 6 or 2 × 6 deck board is about 5.5 in wide. Wood boards are typically gapped about 1/8–3/16 in once dry; many composite brands specify 3/16–1/4 in side gaps. Two screws per board per joist is standard face fastening; hidden fastener systems use one clip per joist crossing instead.

Joist spacing

Most wood decking and many composites allow joists at 16 in on center when boards run perpendicular. Diagonal decking or some thinner composites require 12 in spacing. The American Wood Council's DCA 6 guide gives joist spans by lumber size, species and spacing; for example, 2×8 southern pine joists at 16 in can span roughly 13 ft in many cases. Always check the deck board maker's maximum span to keep the warranty.

Board length and seams

Choosing a board length that matches the deck length avoids butt joints. If the deck is longer than your boards, stagger seams across the deck and land every seam on a doubled joist or blocking so both ends are supported. Picture-frame borders look finished and hide end grain, but they add boards and cutting.

Material choices

Pressure-treated pine is the least expensive. Cedar and redwood resist decay naturally and weather to gray. Composite and PVC boards cost more but need little upkeep beyond cleaning. Hardwoods such as ipe are very durable but dense and need pre-drilling.

Building safely

• Attached decks need a properly flashed and bolted ledger; most codes require a permit.
• Guards are required on decks more than 30 in above grade.
• Use hot-dip galvanized or stainless fasteners rated for treated lumber.

Common mistakes to avoid

• Letting board seams land between joists.
• Using joist spacing wider than the decking allows.
• Attaching a ledger without flashing or proper bolts.
• Gapping wet pressure-treated boards too wide; they shrink as they dry.
• Forgetting railing requirements on raised decks.

Waste factor

10% covers end trimming and defects. Use 5% when board length matches the deck exactly, and 15% for diagonal decking, picture-frame borders or angled corners.

Imperial and metric

Switch the toggle above the inputs to enter ft ↔ m, in ↔ cm. The page converts metric entries to imperial before calculating, using NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm), and shows key results in both systems.

Worked examples

Deck 16 × 12 ft, 16 ft boards

Inputs: deck length (along boards) 16 ft; deck width (across boards) 12 ft; board width (actual) 5.5 in; gap between boards 0.1875 in; board length: 16 ft; joist spacing: 16 in on center; waste factor 10%.

Result: You need 29 deck boards.

• Units to buy (rounded up): 29 boards
• Deck area: 192 sq ft (17.8 m²)
• Board rows: 26
• Board length total: 416 linear ft (126.8 m)
• Joists: 13
• Deck screws (2 per joist crossing): 676

Composite deck 20 × 14 ft

Inputs: deck length (along boards) 20 ft; deck width (across boards) 14 ft; board width (actual) 5.5 in; gap between boards 0.25 in; board length: 20 ft; joist spacing: 16 in on center; waste factor 10%; cost per unit $58.

Result: You need 33 deck boards.

• Units to buy (rounded up): 33 boards
• Deck area: 280 sq ft (26 m²)
• Board rows: 30
• Board length total: 600 linear ft (182.9 m)
• Joists: 16
• Deck screws (2 per joist crossing): 960
• Estimated material cost: $1,914.00

Diagonal deck 12 × 12 ft

Inputs: deck length (along boards) 12 ft; deck width (across boards) 12 ft; board width (actual) 5.5 in; gap between boards 0.1875 in; board length: 12 ft; joist spacing: 12 in on center; waste factor 15%.

Result: You need 30 deck boards.

• Units to buy (rounded up): 30 boards
• Deck area: 144 sq ft (13.4 m²)
• Board rows: 26
• Board length total: 312 linear ft (95.1 m)
• Joists: 13
• Deck screws (2 per joist crossing): 676

Quick reference for common sizes

The table repeats the first worked example at smaller and larger sizes, keeping every other setting the same, so you can see how the deck calculator result scales.

Limitations

This estimates the surface only. Beams, posts, ledgers, stairs and railings are separate. Joist count assumes joists run across the full length; adjust for double rim joists.