Deck Calculator — ft, boards, joists, cost
How to use this calculator
Enter deck dimensions in feet, inches, and fractions. Choose rectangle or L-shape, select decking board size, joist size and spacing, beam and post configuration, and waste allowance. Results update instantly with material counts, board feet, and an interactive plan view.
Cost — enter price per board, per joist, per beam, per post to get material cost.
Hardware — joist hangers, post bases, screws (lbs), and carriage bolts are estimated automatically based on the framing layout.
Saved Calculations
| Time | Shape | Area ft² | Boards | Joists | Beams | Posts | BF | Total $ |
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How to Calculate Materials for a Wood Deck
This deck calculator estimates the number of decking boards, joists, beams, posts, and hardware needed for a rectangular or L-shaped wood deck. Enter dimensions in feet, inches, and fractions (⅛″ precision), select lumber sizes and board lengths, set joist spacing (12″, 16″, or 24″ on center per IRC R507.5), and adjust waste allowance from 5% to 20% depending on layout complexity.
Formulas
Decking boards = deck span perpendicular to boards / (board face width + gap), rounded up. Each board must span the parallel dimension; partial lengths require additional boards per row.
Joists = (span / spacing) + 1, plus 2 rim joists. Beams run perpendicular to joists with posts at the specified spacing. Board feet = (nominal thickness x nominal width x length) / 12.
Screws ≈ 350 screws per 100 ft² (about 1 lb per 75 screws for #8 x 2.5″). Joist hangers = 1 per joist. Carriage bolts = 2 per beam-to-post connection.
Tips
Use 5/4 x 6 for residential decking (lighter, less expensive). Choose 2 x 6 or 2 x 8 joists at 16″ o.c. for most spans up to 12 ft. Double beams (e.g., 2x doubled 2 x 10) allow longer post spacing. Always use pressure-treated or naturally rot-resistant lumber for ground contact.
FAQ
What spacing for composite decking? Most composite manufacturers require 16″ o.c. joist spacing (some allow 12″ for diagonal installation).
How much waste for a simple rectangle? 10% is standard. Add 15-20% for angled or diagonal patterns.
Do I need double rim joists? Double rim joists are recommended for decks wider than 12 ft or when supporting stairs.
