How to use the deck calculator
- Enter the deck length along the house and its depth out from the house. Boards are assumed to run along the length, across the joists.
- Enter the actual board width (a 5/4 × 6 board is 5.5 in wide) and the gap the board maker recommends.
- Choose the board length you will buy; lengths that divide the deck neatly waste less.
- Set the joist spacing, checking the decking maker’s maximum span, especially for composite boards.
- Add prices per foot or metre for boards and framing, and per box for screws.
How the materials are worked out
Boards = rows × deck length × (1 + waste %) ÷ board length, rounded up
Joists = deck length ÷ joist spacing, rounded up, + 1
Screws = rows × joists × 2
The board count assumes offcuts are reused in other rows. When the deck is longer than one board and offcuts are too short to use, each row needs whole boards, so round up per row or raise the waste allowance. The joist count includes one at each end. The ledger fixed to the house and the rim board on the outside edge are listed separately, each as long as the deck. Beams, posts, footings, stairs and railings depend on height, span tables and local code, so they are not included.
Worked example
A 16 × 12 ft deck (16 ft along the house), 5.5 in boards with 3/16 in gaps, 16 ft boards, joists at 16 in centres, 10% waste, boards at $3 per foot and framing at $2.50 per foot:
- Rows: 144 in ÷ (5.5 + 0.1875) in = 25.32, so 26 rows.
- Board length: 26 rows × 16 ft × 1.10 = 457.6 ft, which is 29 × 16 ft boards.
- Joists: 192 in ÷ 16 in + 1 = 13 × 12 ft, plus a ledger and rim board along the 16 ft length.
- Screws: 26 rows × 13 joists × 2 = 676 (2 boxes).
- Materials for the 192 sq ft deck: $1,932.00.
Tips and common mistakes
- Get the permit first. Most decks attached to a house, or above a certain height, need a permit and inspections. The approved plans set joist sizes, beam spans and footings.
- Flash the ledger. Water getting behind the ledger board is a leading cause of rot and deck failures. Use proper flashing and the bolts or structural screws your code requires.
- Stagger the butt joints. Where boards meet end to end, land the joint over a joist and avoid lining joints up in neighbouring rows.
- Use the right fasteners. Treated lumber needs corrosion-resistant screws, and many composite boards are made for their own hidden clips.
US, UK and Canada differences
In the US and Canada, decks are usually framed with 2×8 to 2×12 joists at 16 in centres, and boards are sold in 8 to 20 ft lengths, priced per board or per linear foot. UK decking boards commonly come in 3.6 m and 4.8 m lengths, joists are often set at 400 mm centres, and many garden decks are low, free-standing structures. Canadian codes generally require footings below the frost line, which can be over 4 ft deep in some provinces.
Metric units and other countries
In metric mode, deck sizes are in metres, boards in centimetres and gaps in millimetres, with prices per metre of board. In very hot climates, choose boards rated for sun and heat, and leave the larger gap the maker recommends, because composite and PVC boards expand more than timber. Type your local prices.