Why roll layout can beat a simple area calculation
A roll's printed area is only its width multiplied by its length. Once you install membrane as neighbouring strips, the overlap at each seam reduces the new ground covered by every strip after the first. That makes roll width, overlap and strip direction important.
For example, a 5m × 4m rectangle covered with a 1m-wide roll needs four 5m strips when there is no overlap: 20 running metres. Add a 100mm overlap and each later strip contributes only 900mm of new width, so five strips are needed: 25 running metres. A 25m roll is still one roll; a 10m roll becomes three whole rolls. This is why “20m² ground ÷ 10m² roll” can be misleading.
Choose the roll size from your actual product
UK membrane is sold in different widths and lengths. Current Wickes listings include domestic weed-control fabric in 1m and 2m widths and lengths such as 10m, 20m and 25m. Other manufacturers and landscape suppliers sell different formats. Enter the dimensions printed for the roll you intend to buy rather than relying on a “standard” size.
Wider fabric can reduce seams on broad areas, but it is not automatically more economical: roll length, garden dimensions, handling and cuts all matter. The comparison mode shows both orientations because rotating a 1m-wide roll by 90 degrees can change the number of strips and total running metres.
How much overlap should weed membrane have?
There is no single overlap number that should be forced into every calculation. Product instructions and site conditions matter, so the calculator leaves overlap editable. Some current landscaping calculators and suppliers discuss overlaps around 100–150mm in ordinary installations, with different advice for particular products or situations. Treat any example as context, then follow the membrane manufacturer's instructions for the product and use you have chosen.
Under gravel, bark and ornamental beds
Membrane is commonly sold for landscaping beneath decorative gravel and around planted beds. Whether it is appropriate, which permeability you need and how it should be installed depend on the product and the garden. For gravel, this calculator can solve the fabric roll layout while the Gravel Calculator deals with aggregate volume and weight. For bark, the Bark Calculator deals with surface depth and bag quantity.
Irregular beds, curves and obstacles
A circle, curved border or bed wrapped around shrubs cannot honestly be converted into a precise strip count without knowing how you intend to cut and position the fabric. That is why the Area estimate mode keeps the geometry simple and lets you add an explicit cutting/seam allowance instead. For a genuinely awkward garden footprint, first use the Garden Area Calculator to establish the area, then return here to plan the roll purchase.
Common membrane ordering mistakes
This ignores seam overlap and strip geometry.
An offcut may be the wrong width or shape for the next section.
Only include surfaces that will actually receive membrane.
Use the installation guidance for your chosen fabric.