Deck Board & Screw Calculator
Turn deck size into boards, waste and screws
Deck board coverage and screw count FAQ
How does the calculator work out board counts?
The tool takes your deck width across the boards and divides it by an effective coverage (board face width plus a small gap). For diagonal patterns it applies an extra factor because boards run further across the deck footprint. A simple waste allowance is then added so you are more likely to have a spare board than run short on the last row.
Why don’t I enter the exact board gap?
To keep the number of boxes down, the calculator assumes a typical small gap between boards (around 1/8" or 4 mm) that you see in many span tables and manufacturer guides. If your installer uses a noticeably wider or tighter gap, it’s easy to nudge the result up or down a board or two by hand.
What if my joist spacing isn’t 16" on centre?
If you know your framing is at 12", 24" or another spacing, just type that into the joist spacing box and the screw count will adjust. When you leave the field blank, the tool assumes roughly 16" o.c., which is common for many residential decks and matches a lot of manufacturer span charts.
How accurate are the screw counts?
Screws are estimated using a simple rule: two screws per board at every joist, then a small extra allowance for dropped screws and cut ends. Real jobs may use clips, hidden fasteners or extra screws at board ends, so treat the number here as a planning target and round up to the next box of screws.
Can I use this for composite as well as timber?
Yes, as long as you use the actual exposed board width from the composite manufacturer. Many composite systems also have their own fastener spacing rules, so use this calculator for a rough count, then cross-check with the installation guide for your exact product.
Does the calculator cover stairs, picture framing or inlays?
No. Stairs, picture frame borders and decorative inlays can add a surprising amount of board length and more offcuts. For those details, use the main deck as a starting point here, then add a manual allowance or a separate take-off from your deck plan.
Why do diagonal deck boards need more material?
When boards run at 45° across the joists, each piece spans more distance than a straight board to cross the same width. Edges also need more trimming. The calculator factors in a modest uplift to board counts for diagonal layouts so your estimate feels closer to real-world usage.
Do I still need to check local code and span tables?
Absolutely. This tool only looks at coverage and fastener counts. It does not check joist spans, beam sizes, guard loadings or footing sizes. Always pair it with local deck span tables, manufacturer instructions and your building department’s deck guide.
How to use this deck board & screw calculator
This calculator turns a simple rectangular deck size into deck boards, a small waste allowance and a ballpark screw count. The idea is to get to a shopping list you can tweak, not to replace span tables or your deck drawings.
1. Pick units and board layout
Start by choosing whether you work in US units (feet and inches) or metric (metres and millimetres). Then pick your typical board layout:
- Straight across joists for standard decking.
- 45° diagonal pattern when boards run at an angle to the house.
Diagonal layouts automatically use a little more board length in the calculations.
2. Enter deck length and width
Next, enter the deck length along the boards and the deck width across the boards. If you’re working from plans, stick with the clear finished size of the deck boards, not the overall frame including overhangs or stairs.
3. Add board width and joist spacing
Type the exposed board face width (for example 5.5" for many timber and composite boards, or 140 mm in metric markets). Then either:
- Leave joist spacing blank to assume about 16" o.c. / 400 mm, or
- Enter your actual joist spacing if you know it from plans or site.
The tool uses board width and a typical small gap to work out how many boards fit across the deck width.
4. Read boards with waste and screw counts
Tap Estimate boards & screws to see:
- Deck area in square feet and square metres.
- Base and with-waste board counts for your layout.
- Total linear length of decking in feet and metres.
- A rough screw count using two screws per board at each joist.
Under the numbers you’ll see short notes reminding you what assumptions were used for gaps, waste and framing.
5. Copy and tidy your shopping list
Use Copy summary to paste the results into a quote request, a notes app or a printed checklist. If your timber yard sells only certain board lengths, divide the total linear length by your preferred length and round up to whole boards.
Treat this as a fast first pass for boards and screws. The final numbers should always be checked against your deck design, the exact board product you’re buying and whatever your builder or building department requires.
How the deck board & screw calculator math works
Behind the scenes, the calculator uses the same kind of area and coverage logic you’d use with a paper plan and a notepad, just packaged into a quick, repeatable set of equations.
1. Deck area from length and width
First, the deck size is converted into a working set of units (metres or feet) and the deck area is calculated:
Area = deck length × deck width
This gives square metres internally, then square feet are derived for a familiar reference.
2. Board coverage with a typical gap
The calculator then works out how many boards fit across the deck width by adding a small assumed gap between boards to the board face width:
Coverage width = board face width + gap allowance
Dividing the deck width by that coverage gives a base board count. For a 45° diagonal pattern, that base count is multiplied by a factor to reflect the extra board length and offcuts that come with angled layouts.
3. Waste factor and total board length
Once the base board count is known, the calculator adds a waste factor of about 10%:
Boards with waste = ceil(base boards × 1.10)
That total is then multiplied by the deck length to get total linear decking in metres and feet. From there you can quickly turn it into counts for standard 8, 12 or 16 ft boards if needed.
4. Joist spacing and screw counts
For screws, the tool estimates how many joists each board crosses and assumes two screws per board at every joist:
Joists per board ≈ floor(deck length ÷ joist spacing) + 1
Screws per board = joists per board × 2
Total screws ≈ boards with waste × screws per board
A small extra allowance is included so you’re not caught short when a few screws bend, drop into the grass or get used on cut pieces.
All of these steps are kept transparent and conservative. You can follow them with a handheld calculator if you like, tweak the assumptions for your own job and still land in the same ballpark for boards and screws.
References and further reading on deck boards and fasteners
These resources follow similar logic for deck board coverage, fasteners and overall deck planning:
- Decks.com — Deck Board & Materials Calculator — coverage and material estimates for straight and diagonal decking patterns.
- Simpson Strong-Tie — Deck Center — connection, fastener and code guidance for safe residential decks.
- American Wood Council — Prescriptive Residential Wood Deck Construction Guide (DCA 6) — prescriptive deck design details, spans and construction best practices.
Use them alongside your local deck guide, engineer drawings and board manufacturer instructions when turning these estimates into final material orders.