Deck Calculator
Build a materials list for an attached or freestanding deck: decking, joists, rim, ledger, beams, posts, hangers and screws.
How it's calculated
Deck boards run parallel to the house. The number of rows is the deck depth divided by the board width plus gap; each row takes enough boards to cover the length. Joists run out from the house at your spacing, and posts are spaced along the beam.
Worked example
A 16 × 12 ft deck with 5-1/2" boards and 1/8" gaps has 26 rows of 16 ft boards: 29 boards with waste. At 16" o.c. it needs 13 joists.
Field notes
- Flash the ledger and fasten it with structural screws or through-bolts, never nails.
- Size joists, beams and footings from the IRC deck span tables for your species and grade.
- Composite decking often requires 12" joist spacing, especially on diagonals.
- Use hot-dipped galvanized or stainless hardware with treated lumber.
Common questions
- How many deck boards do I need?
- Divide the deck depth in inches by the board width plus gap (5-5/8" for 5/4×6 with a 1/8" gap) to get rows, then multiply by boards per row and add 10%.
- How far apart should deck joists be?
- 16" on center for most wood decking, 12" for many composites and diagonal patterns.
- How far apart should deck posts be?
- Typically 6–8 ft, depending on beam size and joist span. Check the IRC beam span table.