Deck Calculator
Enter your deck size, the decking board you're using and the joist spacing. You'll get the number of boards (with 10% for cuts and waste), the joist count, and roughly how many deck screws to buy.
Results
How the deck calculator works
Decking is estimated in two steps: how many rows of board cover the deck, and how much linear board that adds up to. The calculator lays the boards along the deck length, stacks rows across the width using your board width plus the gap, then adds 10% for cuts, waste and the occasional bad board.
Formula: rows = ceil(width in inches ÷ (board width + gap)), then rows × deck length × 1.10 ÷ board length = boards
| Board length | Boards per 100 sq ft |
|---|---|
| 8 ft | ≈ 30 |
| 12 ft | ≈ 20 |
| 16 ft | ≈ 15 |
| 20 ft | ≈ 12 |
The calculator also sizes the joists — one every 16 inches on center plus one to close the end — and estimates deck screws at two per board where it crosses each joist, which lands close to the common rule of roughly 350 screws per 100 square feet.
Tips before you order decking
- Use 16" joist spacing for most wood decking, but drop to 12" for composite boards or when running boards diagonally — check your board's span rating.
- A 3/16" gap between boards lets water drain and gives wood room to move; kiln-dried boards can be butted a touch tighter.
- Buy boards long enough to cross the deck in one piece where you can — butt joints must land on a joist and use up extra board.
- Diagonal or picture-frame layouts waste more: bump the allowance to 15% instead of 10%.
- This tool covers the deck surface and joists only. Posts, footings, beams and the ledger are separate — the gravel and concrete-block calculators help with the footing side.
Frequently asked questions
How many deck boards do I need for a 16x12 deck?
A 16 × 12 ft deck (192 sq ft) with 5½" boards and a 3/16" gap needs about 26 rows of board — roughly 29 sixteen-foot boards including 10% waste, or about 416 linear feet of decking before waste.
How do I calculate decking board quantity?
Divide the deck width (in inches) by the board width plus the gap to get the number of rows, multiply by the deck length for linear feet, add about 10% for waste, then divide by the length of board you're buying.
How many screws do I need for a deck?
Plan on about two screws per board at every joist crossing — roughly 350 screws per 100 square feet of decking with 16" joist spacing. Hidden fastener systems use one clip per board per joist instead.
What joist spacing should I use for decking?
Sixteen inches on center suits most solid-wood decking laid straight. Use 12" for composite boards, for thinner 5/4 boards under heavy use, or when boards run diagonally across the joists.