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Quick answer
An illustrative 6 m × 4 m clear deck, 140 mm board width and 5 mm side gap gives 28 rows. With 6 m usable boards and an entered 10% allowance, purchase 31 stock boards. These are arithmetic inputs, not universal product dimensions or spacing advice.
How it works
Estimate boards for straight rectangular deck zones using actual face width, stock length and specified gaps. Keep whole-board purchases, spare material and the optional fixing schedule separate from structural and installation design.
Formula
Rows = ceil((clear width + side gap) ÷ (board width + side gap)). Single-board rows need adequate usable stock length. Joined rows use n × usable stock length + (n − 1) × specified end gap.
Assumptions and common mistakes
Check units, input values and the method before relying on an estimate. Review the existing tool notes and official sources below.
Material quantity formula
Rows = ceil((clear width + side gap) ÷ (board width + side gap)). Single-board rows need adequate usable stock length. Joined rows use n × usable stock length + (n − 1) × specified end gap.
| Quantity | Basis |
|---|---|
| Rows across each zone | ceil((width + gap)/(board width + gap)) |
| Usable stock length | Ordered stock length − two specified end trims |
| Pieces per joined row | max(1, ceil((clear length + end gap)/(usable stock + end gap))) |
| Base boards | Rows × stock pieces per row × identical zone count |
| Purchased boards | Round total base plus entered spare allowance up |
| Optional fixings | Entered schedule for one hardware product, rounded to its pack size |
How the decking board calculator works
Enter each non-overlapping rectangular deck zone with its clear boarded length along the boards and clear width across them. These dimensions must already reflect the planned perimeter clearances and any separately detailed borders. Repeat counts apply only to identical zones. Use the actual board face width and selected stock length. The tool counts rows and whole stock boards for each row independently; it does not assume that offcuts from one row can cover another.
Decking board widths and side spacing
For clear width W, board face width b and specified side gap g, row count is ceil((W + g) ÷ (b + g)), with all dimensions in the same unit. This counts only the gaps between rows, not an extra gap outside the deck. A row count alone is not a final rip-cut layout: the first and last board widths, allowable narrow strips, supported edges and real installation tolerances still need checking. There is no universal decking board width, thickness or gap.
Board length, trimming and joined rows
Single-board mode requires the usable length of one stock board to reach the clear boarded length. Usable stock length equals ordered length minus the entered trim at each end. Joined-row mode is available only for a specified permitted joint layout: covered run = n × usable stock length + (n − 1) × entered internal end gap. The calculator rounds pieces per row to whole stock boards. It does not choose butt-joint positions, stagger joints, provide their supports or add a breaker board automatically.
Composite decking board calculator example
For a rectangular clear boarded area 6 m long by 4 m wide, an illustrative actual board width of 140 mm and side gap of 5 mm give 28 rows. With 6 m usable stock length and single-board rows, the base is 28 boards. An entered 10% spare allowance rounds the purchase to 31 full boards, or 186 purchased linear metres. These are arithmetic dimensions and allowances, not a standard product size or a recommended gap. Picture-frame borders, stairs and fascia need a separate schedule.
Decking board sizes and product descriptions
Check the selected product’s actual face width, thickness, available length, groove profile and intended use. Composite decking can use different polymer and fibre formulations; solid, semi-solid, hollow and capped are product descriptions, not interchangeable strength grades. A cap or outer layer does not by itself establish load capacity, slip performance or all-round protection of cut ends. Plastic decking, composite and pressure-treated pine have different material and installation requirements. Use the exact technical specification rather than treating every wood-look board as the same product.
Decking board spacing and supporting structure
Side gaps, internal end gaps and clearances against walls or posts can serve different purposes and are not interchangeable. Composite movement can depend on the product and installation conditions; timber dimensions can change with moisture. Use the current manufacturer’s instructions for the actual board, clip, framing material and layout. This tool does not pick joist spacing, spans, overhang, foundations, drainage fall, ventilation or railings. Angled boards, borders and joints can need different support details from straight uninterrupted rows.
Composite decking board clips and screws
A hidden clip can engage adjacent grooved boards, while a face-fixed system can use a different fixing pattern. Starter, finishing, butt-joint and edge details may need distinct hardware. The optional fixing estimate therefore covers one specified hardware product only, entered either as a planned count or as total specified fastening positions multiplied by units of that product per position. Those positions must come from the checked layout. Do not assume one clip equals one face screw or combine different product types in one pack count.
How many decking screws per board?
For an approved face-fixed arrangement, a board segment’s fixing quantity can be calculated from its actual supported fastening positions multiplied by the specified number of screws at each position, with any additional details counted separately. Neither figure is universal. A short cut, butt joint or doubled support can change the schedule. Enter total fastening positions from the actual layout if using rate mode; the calculator does not derive them from an assumed joist interval. Board purchase spares do not automatically create installed fixing positions.
Decking board prices and total budget
Enter a current quote per full stock board or per purchased linear metre. Material cost uses the whole-board order including spare boards. Optional hardware pricing uses rounded purchasing packs of one specified product. Add separately quoted framing, installation, fascia or other finishes, and delivery without repeating included costs. Installation per m² uses clear boarded zone area, including internal gaps, rather than the nominal face area of all purchased boards. Zero prices remain unpriced, and no extra VAT is added.
Choosing a design and checking the order
Straight rectangular rows are the scope of this estimator. Herringbone, diagonal cuts, curves, picture frames, breaker boards, steps, edge boards, fascia and penetrations require their own cutting and support details. A decking board bender or a curved appearance does not establish that a product can be bent or heated safely; use a manufacturer-approved method and specialist fabrication where applicable. Check the order against the drawings and selected product before buying. No current retailer price or UK stock range, including Wickes, is assumed to match South African availability.
Reviewed by the 99tools team. Editorial and corrections policy.
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Decking board guides
Frequently asked questions
How many decking boards do I need?
Count rows from clear width, actual board face width and the specified side gap, then count whole stock pieces for each row and add any chosen spare allowance.
How wide, long or thick is a decking board?
Dimensions vary by product. Enter the actual face width and stock length; confirm thickness and profile separately from the technical specification.
Why does the row formula add a gap to the deck width?
There is one fewer internal gap than rows. Rows × board width + (rows − 1) × gap must cover the clear width.
Can I join short boards along a row?
Only where the selected system and designed layout permit it. Joined-row mode estimates pieces but does not design joint locations, supports or breaker boards.
Does the calculator reuse offcuts?
No. It rounds stock independently for each row and zone. A checked cutting plan may reduce the conservative purchase.
What gap should I leave between decking boards?
Use the selected product’s current instructions and installation conditions. Side, end and perimeter gaps are different inputs and details.
How many screws or clips are needed per board?
Use the actual fastening layout and the specified hardware rate. The tool does not assume a universal screw or clip count.
Is capped or semi-solid decking automatically stronger?
No. Product construction labels alone do not establish capacity. Check the specific profile, test information and permitted support arrangement.
Are joists, fascia and stairs included?
Not as designed quantities. Their separately quoted costs can be added, but the board estimate covers the entered rectangular surface zones only.
Does it show live decking board prices?
No. Costs use your current entered quotes and whole-board or hardware-pack purchases.
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Helpful guides
Decking Board Quantities: Width, Gaps and Stock Lengths
Count decking rows and whole stock boards for clear rectangular zones, allowing for actual width, specified gaps, trimming and a chosen spare allowance.
Composite Decking Boards: Sizes, Spacing, Clips and Screws
Compare composite and timber decking specifications, product-specific gaps and fixing schedules without assuming universal board sizes or support spacing.