Deck Material Calculator

Calculate deck materials: boards, joists, posts, beams, screws and cost from size, board dimensions and spacing.

Deck Material Calculator

Calculate deck materials: boards, joists, posts, beams, screws and cost from size, board dimensions and spacing.

Deck length (feet)

Deck width (feet)

Unit system

Board width (inches)

Board length (feet)

Joist spacing (inches)

Waste %(%)

Price per board ($, optional)($)

Enter values above and click Calculate — results will appear here with the formula explained.

Deck area = length×width; Boards = ceil(width / (boardWidth/12) × length / boardLength) with waste; Joists = ceil(length×12 / spacing)+1; Screws ≈ boards×joists×2.5

How does this work?

Deck boards run along length; number across width is width divided by board width plus gap (1/8 inch), times rows along length. Joists are placed every 16 inches (12 for diagonal, composite). Waste 10% for cuts. Screws are 2 per board per joist plus 10% extra.

Posts every 6-8 feet, beams doubled. For metric, convert: 1 ft = 0.3048 m, board width in cm.

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Quick Answer

Deck Material Calculator: Calculate deck materials: boards, joists, posts, beams, screws and cost from size, board dimensions and spacing. Formula: Deck area = length×width. Example: 16×20 ft deck (320 sq ft), 5.

How to Use This Calculator

1Enter deck length and width, board width and length, joist spacing and waste.
2Add price per board to see material cost and full shopping list.
3For stairs, add 3-4 boards per step and stringers separately.

Formula Used

Deck area = length×width; Boards = ceil(width / (boardWidth/12) × length / boardLength) with waste; Joists = ceil(length×12 / spacing)+1; Screws ≈ boards×joists×2.5

Posts every 6-8 feet, beams doubled. For metric, convert: 1 ft = 0.3048 m, board width in cm.

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Worked Example

16×20 ft deck (320 sq ft), 5.5-inch boards, 16 ft long, 16-inch joists, 10% waste: about 44 boards, 13 joists, 1,430 screws, cost $1,540 at $35/board.

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Frequently Asked Questions

QWhat board width?

Nominal 6-inch decking is 5.5 inches actual. 8-inch is 7.25. Measure actual, not nominal.

QJoist spacing 12 or 16?

16 inches standard for wood, 12 for composite or diagonal decking.

QHow many posts?

Posts every 6-8 feet on beams. A 16×20 deck needs 6 posts (3 per beam ×2 beams).

QWaste 10%?

10% for simple rectangle, 15% for picture frame or diagonal, 5% for very simple.

Formula Used

Deck area = length×width; Boards = ceil(width / (boardWidth/12) × length / boardLength) with waste; Joists = ceil(length×12 / spacing)+1; Screws ≈ boards×joists×2.5

Posts every 6-8 feet, beams doubled. For metric, convert: 1 ft = 0.3048 m, board width in cm.

Frequently Asked Questions

QWhat board width?

Nominal 6-inch decking is 5.5 inches actual. 8-inch is 7.25. Measure actual, not nominal.

QJoist spacing 12 or 16?

16 inches standard for wood, 12 for composite or diagonal decking.

QHow many posts?

Posts every 6-8 feet on beams. A 16×20 deck needs 6 posts (3 per beam ×2 beams).

QWaste 10%?

10% for simple rectangle, 15% for picture frame or diagonal, 5% for very simple.