Deck Material Calculator
Boards, joists and area — rough material estimate.
Deck area:
Deck boards needed:
Joists needed:
How to use this calculator
Enter deck length, width, board width and joist spacing, then press Calculate. Deck area, boards and joists are shown as a rough material estimate.
Worked example
For a 16 ft by 12 ft deck using 6-inch boards, each board covers 0.5 ft of width, so 12 ft / 0.5 = 24 boards across, each 16 ft long. Joists at 16 inches on center need 12 ft x 12 / 16 + 1 = 10 joists spaced along the 16 ft side. Add framing for the rim and any stairs separately.
What the result means
Boards are the surface you walk on; joists are the frame underneath that carries the load. Ordering both together - and a little extra for cuts - prevents a mid-build trip to the lumber yard.
Board width and joist spacing work together
Wider boards span joists spaced farther apart; narrow boards need closer joists. If you change the board width, the joist spacing rule changes with it. Composite decking often has its own manufacturer spacing requirement - check it before framing.
Frequently asked questions
What's the usual joist spacing?
16 inches on center is the standard for residential decks. Heavy materials or large hot tubs may call for 12-inch spacing.
Boards and joists are only part of the materials list - you will also need posts, concrete footings, a ledger, rim boards, fasteners and railing. Use this tool for the two big line items, then add roughly 30-40% on top for the supporting materials and hardware.