Practical guide
Decking Board Quantities: Width, Gaps and Stock Lengths
Board area alone can hide unusable offcuts and short stock lengths. Check rows and stock pieces along each row before rounding an order.
Updated 16 October 2026
Reviewed by the 99Tools team. Editorial and corrections policy.
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.
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.
Use the related calculators
Apply the explanation above with your own figures. Open a calculator below, check every input and review the result assumptions before making a decision.
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.