How to use the deck calculator
- Enter the deck length along the house and the depth out from it.
- Enter the actual board width and the recommended gap.
- Pick board length and joist spacing (composite boards often require 12 or 16 in).
- Adjust the cost range to local quotes.
Formula
Rows = ⌈depth (in) ÷ (board width + gap)⌉; boards = rows × ⌈length ÷ board length⌉ × (1 + waste); joists = ⌊length (in) ÷ spacing⌋ + 1.
Worked example
A 16 × 12 ft deck with 5½ in boards and 3/16 in gaps: 26 rows of one 16 ft board = 26, or 29 boards with 10% waste; 13 joists. At $30–$60 per sq ft, roughly $5,760–$11,520.
Tips for using the deck calculator
- Choose board length to avoid butt joints in the middle of the deck where you can.
- Joist spacing depends on decking type; composite boards often require 12 or 16 in on center.
- Footings, beams and ledger flashing are separate items; plan them with your permit drawings.
Deck Calculator FAQs
How much does it cost to build a deck?
Industry estimates put an average of about $35 per sq ft with a typical range of $30–$60; composite decks often cost around $73 per sq ft. Other estimates give $25–$60 for a basic professionally built deck.
How far apart should deck joists be?
16 in on center is standard for wood decking; many composite boards require 12 or 16 in, and diagonal layouts need closer spacing. Follow the board manufacturer.
Do I need a permit for a deck?
Most jurisdictions require one for attached or raised decks. See our deck guide.
