Deck Calculator
Calculate decking boards, joists, beams, posts, and footings for your deck project. See also Fence Calculator, Board Foot Calculator, and Concrete Calculator.
Deck Anatomy Diagram
How to Use the Deck Calculator
Enter your deck's length and width in feet. Joist spacing is typically 16 inches on center for standard decking or 12 inches for composite materials. Board width is 5.5 inches for standard 2x6 decking or 3.5 inches for 2x4 boards. Check the railing box if your deck is 30 inches or more above grade (required by code). The calculator provides a complete material list for planning and purchasing.
Formula
Joists = (Deck Length x 12 / Joist Spacing) + 1
Decking Boards = Deck Length x 12 / Board Width
Beams = Deck Width / 6 ft max span (rounded up)
Posts = (Beams + 1) x 2
Railing Posts = Perimeter / 6 ft spacing
Example Calculation
Deck: 16 ft x 12 ft, 16" OC joists, 5.5" boards
Joists = (16 x 12 / 16) + 1 = 13 joists
Decking boards = 16 x 12 / 5.5 = 35 boards
Beams = 12 / 6 = 2 beams
Posts = 3 x 2 = 6 posts
Railing = (2 x 16 + 12) / 6 = 8 railing posts
Reference Table
| Lumber Size | Actual Size | Max Span | Common Use |
|---|---|---|---|
| 2x6 | 1.5 x 5.5 in | 9 ft 9 in | Decking boards, short joists |
| 2x8 | 1.5 x 7.25 in | 13 ft 1 in | Joists (16" OC) |
| 2x10 | 1.5 x 9.25 in | 16 ft 5 in | Joists, beams |
| 2x12 | 1.5 x 11.25 in | 19 ft 1 in | Long-span joists |
| 4x4 | 3.5 x 3.5 in | N/A | Railing posts |
| 6x6 | 5.5 x 5.5 in | N/A | Structural posts |
Frequently Asked Questions
What size joists do I need for a deck?
For spans up to 10 feet, 2x8 joists at 16" OC are sufficient. Spans of 10-14 feet require 2x10 joists. For spans over 14 feet, use 2x12 joists or add a mid-span beam. Always check local building codes for specific requirements.
How deep should deck footings be?
Deck footings must extend below the frost line, which varies by region (36-48 inches in northern states, 12-18 inches in southern states). Minimum footing diameter is typically 12 inches for single-story decks and 16 inches for elevated decks.
Do I need a permit to build a deck?
Most jurisdictions require a building permit for decks over 200 square feet or more than 30 inches above grade. Permits ensure structural safety and proper attachment to the house. Check with your local building department before starting.
What is the best decking material?
Pressure-treated pine is the most affordable ($2-4/sq ft). Cedar offers natural rot resistance ($4-8/sq ft). Composite decking ($6-12/sq ft) requires minimal maintenance and lasts 25-50 years. Choose based on budget, maintenance preference, and aesthetics.
How far apart should deck posts be?
Structural deck posts should be spaced no more than 8 feet apart along beams. For most residential decks, 6-foot spacing provides adequate support. Post spacing depends on beam size, joist span, and total deck load.
When is railing required on a deck?
Building codes require railings on decks 30 inches or more above grade. Railings must be at least 36 inches high (42 inches in some jurisdictions). Balusters must be spaced no more than 4 inches apart to prevent children from passing through.