Inputs
Example data table
| Deck (L×W) | Board width | Gap | Stock length | Waste | Boards to purchase |
|---|---|---|---|---|---|
| 16 ft × 12 ft | 5.5 in | 0.125 in | 16 ft | 10% | 27 |
| 20 ft × 14 ft | 5.5 in | 0.1875 in | 12 ft | 12% | 45 |
| 6 m × 4 m | 140 mm | 4 mm | 3.6 m | 8% | 42 |
Formula used
1) Boards across the span
Each board covers its width plus a gap after it, except the last board. We solve for N where:
2) Pieces per row
3) Boards to purchase with waste
Fasteners are estimated using joist count and screws per joist.
How to use this calculator
- Pick your units, then enter deck length and width.
- Enter board width and your preferred gap size.
- Choose orientation to match your planned board direction.
- Set stock length and a realistic waste allowance percentage.
- Optionally add joist spacing and screws per joist.
- Add a price to estimate material cost by your purchase method.
- Press Calculate and review results above the form.
- Use CSV or PDF exports for ordering and documentation.
Professional field guide for deck board planning
1) Start with the finished deck footprint
Confirm the clear length and width after fascia, picture-frame borders, and stair openings. A 16 ft × 12 ft platform is 192 sq ft, which helps you compare wood versus composite bids and delivery loads.
2) Choose board width that matches your layout
Common “5/4” deck boards install near 5.5 in wide, while 2× boards install near 7.25 in. Wider boards reduce row count, but can show more cupping in wet climates. Narrower boards hide movement and look cleaner on small decks.
3) Set realistic gaps for drainage and movement
Typical gapping is 1/8–1/4 in (about 3–6 mm) depending on moisture content, material, and local humidity. Composite often uses a manufacturer-specific spacer, while dry kiln lumber may need a slightly tighter initial gap.
4) Decide board direction before ordering
Boards can run along the deck length or width. Direction changes the run length and the number of boards across the span. It also changes seam locations, where you may want blocking and a consistent stagger pattern.
5) Stock length drives waste and splice count
Longer stock reduces seams and labor. For example, a 20 ft run with 12 ft stock requires 2 pieces per row, while 16 ft stock may still need 2 pieces but leaves less offcut. Use the calculator’s pieces-per-row output to plan butt-joint spacing.
6) Use a waste allowance tied to job complexity
Straight decks with full-length boards often need 5–10% waste. Diagonal patterns, picture frames, curves, or multiple penetrations can push waste to 12–15% or more. Waste also covers end trimming, damaged boards, and color sorting.
7) Fastener quantity depends on framing spacing
A practical estimate is two screws per joist per board. With 16 in (≈400 mm) joist spacing, more joists are crossed, increasing fasteners and labor. If you move to 12 in spacing for composite, expect a noticeable rise in screw counts and boxes required.
8) Translate quantities into purchase and cost decisions
If you buy by the board, use the “boards to purchase” figure based on stock length. If you buy by linear foot, use installed linear footage plus waste. Pair exports with supplier SKUs so your takeoff, order, and field cuts stay aligned from bid to build.
Before finalizing, verify local code requirements for span, joist spacing, and stair geometry, and confirm any manufacturer limits for hidden fasteners. Field-check squareness so your edge boards land cleanly.
FAQs
1) What gap should I use between deck boards?
For many decks, 1/8–1/4 in (3–6 mm) works well. Use tighter gaps for dry lumber and follow manufacturer spacers for composite. Always prioritize drainage, debris clearance, and local humidity conditions.
2) How much waste should I add?
Use 5–10% for straightforward, square decks with minimal cutouts. Use 12–15% for diagonal layouts, picture frames, multiple penetrations, or heavy trimming. Add extra if you must match color or grain.
3) Why does board direction change the quantity?
Direction changes the run length and the span you cover with board widths and gaps. That shifts the boards-across count, the number of splices per row, and the total linear footage you need to buy.
4) How are fasteners estimated?
The calculator estimates joists crossed by dividing run length by joist spacing, then multiplies by boards across and screws per joist. It’s a planning number; add allowance for perimeter blocking, stairs, and rim boards.
5) Can I price by linear foot instead of per board?
Yes. Select price per linear foot to multiply installed footage plus waste by your rate. If your supplier sells fixed stock lengths, price per board is usually closer to the invoice total.
6) What if my stock length is longer than the run length?
Pieces per row becomes 1, and the calculator will show fewer seams and lower waste. You may still trim ends for squareness and consistent overhang, so keep a modest waste percentage.
7) Does this include picture-frame borders and stairs?
Not automatically. Enter the main deck area for the field boards, then separately calculate border runs, stair treads, and landings. Add those linear feet and fasteners to your exported takeoff.
Measure twice, plan carefully, build safely, and enjoy outdoors.