Decking board calculator
Estimate whole boards from deck area, board width, gap, available length and waste.
20 LAUNCH TOOLS
Choose a calculator
When to use it
Use for a first-pass surface-board quantity after the deck footprint, board direction, exact product and workable layout are known.
How it works
The board-plus-gap module converts area to linear coverage; stock boards are rounded up.
Worked example
Assume a 10.0 m² net deck area, 120 mm actual board width, a manufacturer-specified 5 mm side gap, 3.6 m stock boards and an illustrative 10% cutting allowance. One stock board represents 0.125 m × 3.6 m = 0.45 m² of area-based coverage; 10.0 ÷ 0.45 = 22.222 boards; × 1.10 = 24.444, so round up to 25 boards. The real cutting plan can change that order.
Measurement guidance
Net deck area in m², actual board face width in millimetres, the selected product’s side gap, available stock length in metres and a layout-specific cutting allowance.
Buying and specification checks
This is an area-based surface-board estimate. It does not model actual row lengths, end joints, borders, steps or reusable offcuts, and it does not establish joist spans, supports, fixings, guarding or structural suitability.
Set out the boards and check the system
Use the calculator as the start of a cutting and ordering plan, not as a deck design.
- Draw and label the net deck footprint, board direction, full row lengths, borders, steps, access panels and fixed obstacles. Test the first and final rows and record where every board end or change of direction will be supported.
- Use the exact board range’s actual face width, available lengths and current installation instructions. Side, end and perimeter gaps can differ by material, product and installation conditions; do not transfer one timber or composite rule to another range.
- Choose an allowance for the layout, cuts, defects and matching spares. Create a board-by-board cutting plan, identify reusable offcuts, then round to the supplier’s sale unit once and check batch or colour consistency, delivery access, storage and returns.
- Keep the surface order separate from the structural and component schedules. Confirm joist sizes, centres and spans, beams, supports, foundations, connections, fixings, ventilation, drainage, steps and guarding with the responsible designer, installer and board manufacturer.
Common mistakes
- Ignoring gaps
- Using nominal board width
- Treating board count as frame design
FREE TOOL AND PROJECT PACK
What the free calculator provides
The free calculator converts net deck area, one board width, one side gap, a stock length and a visible allowance into a rounded whole-board estimate. It does not produce a cutting plan or specify the support frame, fixings, gaps, steps or guarding.
What the dedicated pack adds
The Decking Project Pack helps plan and retain the wider domestic garden-decking project.
- Brief, site, levels, permissions, exact board system and support-frame decisions.
- Layout, board and fixing schedules, quote comparison, order gate, delivery and concealed-frame checks.
- Board installation, troubleshooting, completion, care and a retained deck passport.
19-page printable digital workbook (PDF). Immediate digital delivery with 90-day access; no subscription.
Practical FAQs
Does the board total include the frame and fixings?
No. Board coverage is only the surface. Joists, beams, posts, supports, concrete, connectors, screws or clips, fascia, steps and guarding need their own specified schedules.
What gap should I enter between boards?
Use the current installation instructions for the exact board range and conditions. Timber moisture movement and composite temperature movement can require different side, end and perimeter details, so do not adopt a universal gap.
Will the board count work for a diagonal layout or picture-frame border?
It is only a first area estimate. Diagonal boards, borders, short rows, butt joints and steps alter cuts and can require extra support. Draw the layout and calculate separate components before ordering.
Does this calculator tell me the joist spacing or span?
No. Board support centres come from the exact board instructions, while joist spans, beams, posts, foundations and connections depend on material, grade, section, loads and design. Have the complete support frame specified and checked separately.
Planning guidance, not a quotation or a substitute for project-specific design and competent advice.