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Deck Calculator

Calculate decking materials and cost

Units

Estimated Total

$1,013

Area

240 ft²

Boards

60

$/ft²

$4.22

Deck Area

240 ft²

Boards

60

Total Cost

$1,013

Cost/ft²

$4.22

Shopping List

Deck Boards60
Joists16
Beams2
Posts6
Concrete Bags (80lb)20
Deck Screws840
Joist Hangers16

Cost Breakdown

Decking$510
Framing$312
Hardware$81
Concrete$110
Total$1,013

Frequently Asked Questions

Q

How do you calculate decking materials?

Calculate deck area (length × width). Determine boards needed based on board width and length. Calculate joists based on spacing (typically 16" on center). Estimate posts every 6-8 feet along the perimeter.

Q

What is standard deck board spacing?

Deck boards are typically installed with 1/8" to 1/4" gap between boards for drainage. Standard board width is 5.5" (5/4 board) or 5.5" with spacing.

Q

How far apart should deck joists be?

Standard joist spacing is 16 inches on center (OC) for most decking materials. Some composite decking allows 24" OC spacing. Check manufacturer specifications for your decking material.

Q

How many posts do I need for a deck?

Posts are typically spaced 6-8 feet apart along the perimeter. For a 20ft deck, you'd need posts at 0ft, 6ft, 12ft, 18ft, and 20ft (5 posts per side).

Example Calculations

1Standard Deck (20 × 12 ft)

Inputs

Deck Length20 ft
Deck Width12 ft
Board Width5.5 in
Board Length12 ft
Joist Spacing16 in
Board Price$8.50

Result

Deck Area240.0 sq ft
Boards Needed52
Joists Needed16
Posts Needed10
Estimated Cost$442

Area = 20 × 12 = 240 sq ft. Board width in feet = 5.5/12 = 0.458 ft. Boards per row = ceil(12/0.458) = 27. Rows = ceil(20/12) = 2. Total boards = 27 × 2 = 54. Joists = ceil(20/(16/12)) + 1 = 16. Posts = ceil(20/6 + 1) × 2 = 10. Cost = 54 × $8.50 = $459.

2Large Deck (30 × 16 ft)

Inputs

Deck Length30 ft
Deck Width16 ft
Board Width5.5 in
Board Length16 ft
Joist Spacing16 in
Board Price$12.00

Result

Deck Area480.0 sq ft
Boards Needed70
Joists Needed24
Posts Needed12
Estimated Cost$840

Area = 30 × 16 = 480 sq ft. Board width in feet = 5.5/12 = 0.458 ft. Boards per row = ceil(16/0.458) = 35. Rows = ceil(30/16) = 2. Total boards = 35 × 2 = 70. Joists = ceil(30/(16/12)) + 1 = 24. Posts per side = ceil(30/6) + 1 = 6. Total posts = 6 × 2 = 12. Cost = 70 × $12 = $840.

Formulas Used

Deck Area

Deck Area = Length × Width

Total deck surface area.

Where:

Length= Deck length in feet
Width= Deck width in feet

Boards Needed

Boards = ceil(Width / Board Width) × ceil(Length / Board Length)

Number of boards to cover the deck based on board width (in inches, converted to feet) and board length.

Where:

Board Width= Width of each board in inches
Board Length= Length of each board in feet

Joists & Posts

Joists = ceil(Length / Joist Spacing) + 1, Posts = ceil(Length / 6 + 1) × 2

Joists spaced at regular intervals. Posts placed every 6 ft along both sides of the deck.

Where:

Joist Spacing= Distance between joists in inches (typically 16")
6= Post spacing in feet

Understanding Deck Construction

Deck construction requires careful planning of materials including boards, joists, posts, and hardware.

Standard joist spacing is 16 inches on center for most decking materials.

Always add 10-15% waste when ordering materials for cuts and errors.

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Last Updated: Feb 12, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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