Deck Calculator

1. Choose your area input method. Enter deck length and width in feet, or switch to "Known area" if you already measured the total square footage. For L-shaped or multi-section decks, calculate each rectangle separately and run the deck calculator for each section.
2. Select board length and actual board width. Most residential decks use 5/4×6 boards (5.5 in actual width) in 12 ft or 16 ft lengths. Pick the board length closest to your deck span to reduce end cuts. A common mistake is using the nominal 6 in width instead of the actual 5.5 in face dimension.
3. Set the board gap. Standard wood deck boards use 1/8 in (0.125 in) spacing for drainage. Composite boards often need 3/16 in (0.1875 in) per manufacturer instructions. The gap affects coverage per board and total board count.
4. Choose board direction and joist spacing. Straight (90-degree) layouts waste about 10%. Diagonal (45-degree) layouts look great but produce 15% or more waste from angle cuts. Most residential decks use 16 in on-center joist spacing per IRC R507.
5. Select your fastener type. Standard deck screws work for face-fastened wood boards. Hidden clip systems work with grooved composite boards from brands like Trex, TimberTech, and Fiberon. This deck calculator adjusts the fastener count based on your choice.
6. Add optional pricing. Enter a price per board and total fastener cost for a quick materials budget. Leave both at zero if you only need quantities. The deck calculator shows board cost and fastener cost as separate line items so you can compare supplier prices.

Pro tip: Always order 2 to 3 extra boards beyond what the calculator says. On my last 12 × 20 deck, I dropped one board off the sawhorses and cracked it, and two others had factory defects I didn't notice until I was fitting them. The $36 in extra lumber saved me a second trip to the yard.

This deck calculator converts your deck dimensions into a board count, fastener estimate, and optional cost using straightforward area math. The formulas apply equally to pressure treated wood and composite decking boards. The main difference between wood and composite is the gap setting and fastener type.

Deck area:

Deck area (sq ft) = Deck length (ft) × Deck width (ft)

If you already know your total square footage, enter it directly using the "Known area" mode.

Board coverage:

Effective width (in) = Board actual width (in) + Gap (in)

Board coverage (sq ft) = Board length (ft) × Effective width (in) / 12

A 5/4×6 board at 12 ft with a 1/8 in gap covers 12 × 5.625 / 12 = 5.625 sq ft per board. Many homeowners use the nominal 6 in width, which overestimates coverage and leads to under-ordering. Always use the actual face dimension.

Board count with waste:

Base boards = ceil(Deck area / Board coverage)

Waste boards = ceil(Base boards × Waste %)

Boards to buy = Base boards + Waste boards

Straight layouts typically use 10% waste. Diagonal layouts need at least 15% because angle cuts produce shorter, unusable offcuts. I bumped waste to 20% on a herringbone pattern and still came up 2 boards short.

Fasteners:

For screws: Crossings per board = floor(Board length (in) / Joist spacing (in))

Total screws = Boards to buy × Crossings per board × 2 screws per crossing

For hidden clips: Total clips = Boards to buy × Crossings per board

Most deck boards get 2 screws at each joist. A 12 ft board across 16 in joists crosses 9 joist lines, needing 18 screws per board.

Example:

A 12 ft × 16 ft deck with 5.5 in × 12 ft boards, 1/8 in gap, straight layout, 16 in joist spacing, 10% waste.

• Deck area = 12 × 16 = 192 sq ft
• Effective width = 5.5 + 0.125 = 5.625 in
• Board coverage = 12 × 5.625 / 12 = 5.625 sq ft
• Base boards = ceil(192 / 5.625) = ceil(34.13) = 35
• Waste boards = ceil(35 × 0.10) = 4
• Boards to buy = 35 + 4 = 39
• Screws per board = floor(144 / 16) × 2 = 9 × 2 = 18
• Total screws = 39 × 18 = 702

Variables:

• Deck length = total deck length in feet
• Deck width = total deck width in feet
• Board length = length of one deck board in feet (8, 10, 12, 16, or 20)
• Board actual width = face width of one board in inches, not the nominal lumber size
• Gap = space between boards for drainage, in inches (1/8 in standard, 3/16 in for composite)
• Joist spacing = center-to-center joist distance in inches
• Waste factor = extra percentage for cuts, defects, and layout losses

Assumptions:

• Deck surface is a single rectangle. For L-shaped or multi-level decks, run each section separately
• Board gap is added to actual width for coverage calculation, reflecting real installed spacing
• Screw count assumes 2 fasteners per board at each joist crossing. Hidden clip systems use 1 clip per crossing
• Stairs, picture frame borders, breaker boards, railings, footings, beams, and substructure need separate estimates. Use the stair calculator and baluster spacing calculator for those components
• Cost estimate covers boards and fasteners only. Pressure treated 5/4x6 boards run $8 to $15 each and composite boards run $12 to $22 each in 2026. Delivery, tax, joists, beams, posts, and concrete footings are additional

Limitations:

This deck calculator handles rectangular deck surfaces. It does not estimate substructure lumber (joists, beams, posts), stair stringers, railing materials, concrete footings, or permits. Multi-level or wraparound decks should be calculated as separate rectangles. Actual lumber prices vary by region, season, and supplier. Always verify your deck framing plan against local building codes before purchasing materials.