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Deck Board Calculator
Estimate the decking boards, joists and fasteners for a rectangular deck. Enter the deck size, the board width and length, the gap and the joist spacing, and get a shopping list with an allowance for cuts and waste.
Results are estimates. Soil, stone and mulch settle, weigh differently and vary by supplier. Confirm quantities and local requirements before ordering.
How to calculate it
Boards run one way and joists run across them. First work out how many rows of boards fit across the width: deck width in inches ÷ (board width + gap). Each row is as long as the deck, so the total length of board is rows × deck length. Divide by the length of the board you buy and add about 10% for cuts.
Formula: boards = rows × (deck length ÷ board length) × waste factor.
Joists are spaced along the length of the deck: joists = deck length in inches ÷ spacing + 1. Each board is fixed to every joist it crosses with two screws, or one hidden clip.
This covers decking only. Beams, posts, footings, ledger and railings depend on your design and local building code, so check them with your building department.
Worked example
A 16 ft × 12 ft deck with 5½ in wide, 12 ft long boards and joists 16 in on center:
| Deck area | 192 sq ft |
|---|---|
| Rows of boards | 26 at 5.5 in wide plus a 0.188 in gap |
| Deck boards to order | 39 boards 12 ft long, includes 10% extra |
| Joists (16 in on center) | 13 each about 12 ft long (156 linear ft) |
| Screws (2 per joist crossing) | 744 about 3 box(es) of 350 |
| Hidden fastener clips (alt.) | 372 one per board per joist |
Common decking board sizes
| Nominal size | Actual width |
|---|---|
| 2×4 | 3½ in |
| 2×6 (or 5/4×6) | 5½ in |
| 2×8 | 7¼ in |
Tips
- Choose a board length that divides the deck length evenly to reduce waste. A 12 ft deck suits 12 ft boards.
- Boards joined end to end should meet on a joist, and joints should be staggered.
- Composite and PVC decking often needs closer joist spacing, such as 12 in, especially on diagonal layouts. Check the manufacturer's span table.
- Diagonal patterns waste more, so allow 15% or more instead of 10%.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5½ in boards and a 3/16 in gap, about 26 rows are needed across the width. With 12 ft boards that is roughly 26 boards before any extra for cuts.
How far apart should deck joists be?
16 inches on center is the usual standard for wood decking. 12 in is common for composite or diagonal layouts, and 24 in is allowed by some products. Always check the board maker and local code.
How many screws do I need for a deck?
Two screws per board at each joist. This calculator counts them and adds the same allowance as for boards.
How big a gap should be left between deck boards?
Typically 1/8 to 1/4 in. Wet pressure-treated lumber shrinks as it dries, so it is often installed with a smaller gap than dry boards and composites.