What is a Decking Calculator?
A decking calculator helps you determine the exact number of deck boards needed for your project, along with associated hardware and estimated costs. Proper planning prevents material shortages that delay construction and excess purchasing that wastes money. The calculation accounts for board width, deck dimensions, gaps between boards, and a waste factor for cuts and defects.
Deck boards are available in various materials including pressure-treated lumber, cedar, redwood, and composite materials. Each has different widths, lengths, and cost profiles. Pressure-treated pine is the most economical choice at $1.50-3.00 per linear foot, while composite decking ranges from $3-10 per linear foot but requires less maintenance over its lifetime.
Board Calculation Formula
Each board runs the length of the deck (or a portion if shorter boards are used). The number of boards needed across the width is determined by the board width plus the gap between boards.
Decking Material Comparison
| Material | Cost/Lin Ft | Lifespan | Maintenance |
|---|---|---|---|
| Pressure-Treated Pine | $1.50-$3.00 | 15-20 years | Stain every 2-3 years |
| Cedar | $3.00-$5.00 | 20-25 years | Seal every 1-2 years |
| Redwood | $5.00-$8.00 | 25-30 years | Seal every 1-2 years |
| Composite (basic) | $3.00-$6.00 | 25-30 years | Minimal (wash only) |
| Composite (premium) | $6.00-$10.00 | 30-50 years | Minimal (wash only) |
Joist Spacing Guide
Deck joists are the structural members that support the deck boards. Standard joist spacing is 16 inches on center for most deck boards. Composite manufacturers often require 12-inch spacing for diagonal installations or 16-inch for perpendicular layouts. Always verify the joist spacing requirement for your specific decking product before framing.
Worked Example
A 20 ft × 12 ft deck using 5.5" wide boards with 1/8" gaps and 10% waste:
With waste: 26 × 1.10 = 28.6 → 29 boards
Frequently Asked Questions
How much gap should I leave between deck boards?
For wood decking, leave 1/8" to 3/16" between boards for drainage and expansion. For composite decking, follow the manufacturer's specifications, typically 3/16" to 1/4". In dry conditions during installation, use a smaller gap as boards may expand with moisture; in wet conditions, use a larger gap.
How many screws per deck board?
Use 2 screws per board at each joist location. With 16" joist spacing on a 12-foot board span, that means about 18 screws per board. Use #8 or #10 deck screws that are at least 2.5 inches long for 5/4 decking or 3 inches for 2x lumber.
Should I run deck boards parallel or perpendicular to the house?
Most decks have boards running perpendicular to the house (parallel to the joists' short direction). This is the standard orientation that maximizes board support. Diagonal patterns add visual interest but require closer joist spacing and create more waste due to angled cuts.
How do I account for deck board waste?
Add 10% waste for straightforward rectangular decks with boards running in one direction. Add 15-20% for angled or diagonal patterns. Add extra for complex shapes with many cuts around posts, stairs, or irregular edges. Always round up to the next whole board.