Calculator
Enter your deck size, board details, joist spacing, and allowances. The calculator estimates intersections and multiplies by screws per joist connection.
Example data table
These examples show typical settings. Your results change with gaps, joist spacing, and screw pattern.
| Scenario | Deck size | Board + gap | Joist spacing | Screws/joist | Allowance | Estimated screws |
|---|---|---|---|---|---|---|
| Small garden deck | 10 ft × 8 ft | 5.5 in + 0.125 in | 16 in | 2 | 15% | ~700 |
| Wide boards, tighter joists | 14 ft × 12 ft | 7.25 in + 0.125 in | 12 in | 2 | 18% | ~1600 |
| Metric build | 4.0 m × 3.0 m | 140 mm + 5 mm | 400 mm | 2 | 15% | ~1100 |
Formula used
Step 1: Boards across deck width
boards = ceil(deck_width ÷ (board_width + gap))
Step 2: Joists along deck length (including ends)
joists = ceil(deck_length ÷ joist_spacing) + 1
Step 3: Base screw count
intersections = boards × joists
base_screws = intersections × screws_per_joist
Step 4: Add allowances
total_screws = ceil(base_screws × (1 + (waste% + allowance%) ÷ 100))
How to use this calculator
- Choose a unit system and enter deck length and width.
- Enter board width and gap to estimate board count.
- Set joist spacing to match your framing plan.
- Pick screws per joist connection, usually two.
- Add waste and borders/blocking allowances for extras.
- Press calculate, then download CSV or PDF if needed.
Practical notes for garden projects
- Use exterior-rated fasteners that resist corrosion.
- For coastal or wet areas, consider higher-grade coating.
- Pre-drilling helps prevent splitting near board ends.
- If boards are very dry or very wet, keep extra screws.
- Always follow local guidance for framing and spacing.
Board coverage and count accuracy
Board count is driven by true coverage, not nominal size. The calculator uses your board width plus the drainage gap to form a coverage strip, then rounds up. Small gap changes matter: adding 3 mm across 4.0 m can remove several boards, which also reduces intersections and fasteners. For mixed board lengths, the rounding keeps counts practical on site.
Joist spacing changes the screw total
Joists are estimated along the deck length and include both ends for fastening. Tighter spacing increases joist count and multiplies intersections. For example, moving from 400 mm to 300 mm spacing raises joists by about 33% for the same length, so the base screw count rises by a similar proportion before allowances. End blocking and stair headers can add joists beyond the estimate in practice.
Fastening pattern and edge detailing
Two screws per board per joist is a common pattern for stable boards, while one screw is sometimes used for hidden fastener clips or temporary layouts. Picture-frame borders, breaker boards, benches, and steps introduce extra blocking. Use the borders and blocking allowance to reflect these details instead of guessing late.
Allowance strategy for fewer store runs
The tool combines waste and detailing allowances, then rounds up to whole screws. For clean projects, 10–15% often covers dropped screws and short trims. Complex edging, angled cuts, or multiple penetrations can justify 18–25%. If you are unsure, it is safer to carry a small spare box.
Screw length, boxes, and budget control
Suggested screw length is based on board thickness plus the embed depth into the joist. This is a planning cue, not a substitute for fastener guidance. Once total screws are known, the calculator estimates boxes using your box size, then multiplies by price if provided. If you use two screw lengths, run the calculator twice and split the total to match your fastening plan. Record the output in CSV or PDF to share with suppliers and keep your garden build organized.
FAQs
1) How many screws per board per joist should I use?
Two is a solid default for most deck boards. One may work with clip systems or specific designs. If boards cup or twist easily, keep two and follow the fastener maker’s recommendations.
2) Does the calculator include a joist at each edge?
Yes. Joists are estimated as spaced members plus an extra one for the far end. This helps cover fastening at the first and last support points along the board run.
3) What if my boards run diagonally?
Diagonal layouts usually increase board length waste and add more cuts. Keep the same joist spacing, but raise the waste allowance and borders/blocking allowance. A small extra box is often cheaper than a second trip.
4) How do I choose the gap between boards?
Pick a gap that supports drainage and seasonal movement for your material and climate. Enter the gap you plan to install; even small changes affect board count and fastener totals across the full width.
5) What allowance values work for typical garden builds?
Start with 10% waste and 5% detailing for simple rectangles. Use 15% waste and 8–15% detailing for picture-frame borders, stairs, benches, or many penetrations.
6) Can I estimate cost if I use multiple screw types?
Yes. Run the calculator for each screw type and adjust the inputs or allowances to match the share of connections. Add a box price each time and combine the totals for a quick budget.