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Weed Membrane Calculator — How Much Landscape Fabric Do I Need?
Calculate weed membrane rolls, overlap allowance and fixing pins for driveways, borders, paths and gravel areas.
Length of the area to cover. Strips are laid along this dimension
Width of the area to cover. Strips are laid side by side across this dimension
100mm min for suppression, 150mm typical (and under gravel), 200–300mm for slopes/stabilisation
On (default): part rolls are used up by joining strips end to end with the same overlap and pinning the join, which is standard geotextile practice. Off: every strip is one uncut run, so a part roll shorter than one strip is left spare and you will need more rolls
Conservative ~6 pins/m² default — flat ground often needs 2–4/m², slopes/curves 5–6/m², plus edge pins at ~300mm centres
Enter your supplier price for a cost estimate
Rule of thumb: 2–4/m² on flat ground, 5–6/m² on slopes/curves; this calc defaults to a conservative 6. Always pin all edges and overlap joints at ~300mm centres
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How We Calculate This
Our weed membrane calculator works out how many strips of membrane you need to cover your area, accounting for the overlap between adjacent strips, then calculates the number of rolls to purchase and fixing pins required.
The formula
Strips needed = 1 + ceil((Area width - Roll width) ÷ (Roll width - Overlap))
The first strip is laid full width; only the strips after it lose the overlap to their neighbour. If a single roll already spans the width, you need just 1 strip.
Whole strips per roll = Roll length ÷ Strip length (rounded down)
Rolls needed = the strips laid into rolls one at a time until they all fit, so the count is a plan that exists rather than a division that might not pack.
A roll is a fixed length of fabric, not a pool of divisible metres. Dividing total metres by roll length assumes every last metre can be laid, and it only can be if you are willing to join strips end to end: a 25m roll cut into one 13m strip leaves 12m, too short to be a second whole 13m strip. Each end join costs one extra overlap of fabric, which is why the total can come out a roll higher than the division suggests.
By default the calculator does use those part rolls, joining strips end to end and telling you how many joins that takes. End joints are ordinary geotextile practice: lap them by the same amount as a side joint and pin them at around 300mm centres. If you would rather every strip came off the roll in one uncut piece, switch “Allow end-to-end joins” off and the roll count rises to suit. If a strip is longer than the roll, end joins cannot be avoided either way and the results say so.
Strips run along the Area Length. Swapping the two dimensions gives more, shorter strips, which often wastes less of each roll but leaves more side joins to lap and pin.
Fixing pins = Area (m²) × pins per m² (default 6, rounded up)
Recommended overlaps
- Standard gardens: 150mm overlap
- Heavy weed areas: 300mm overlap
- Under driveways: 150-200mm overlap
Fixing pin guidance
Pin density is a rule of thumb, not a fixed standard — it varies with ground conditions and exposure. The calculator defaults to a conservative 6 per m² so you do not under-order; you can lower it in the advanced options.
- Flat ground: roughly 2–4 pins per m²
- Sloped, curved or windy areas: about 5–6 pins per m²
- Edges & overlap joints: always pin at around 300mm centres
Overlap and pinning figures follow common UK weed-membrane supplier and installer guidance; always follow the manufacturer’s instructions for your specific fabric.
Frequently Asked Questions
Overlap adjacent strips by at least 100mm for basic weed suppression; 150mm is a sensible all-round default and is what most installers use under gravel. For exposed, sloped or ground-stabilisation jobs, increase the overlap to 200-300mm. Always secure overlapping joins with fixing pins at around 300mm centres along the join. Note: standard weed membrane does not stop Japanese knotweed. That needs a purpose-made, tested root barrier (welded seams or a 2m+ overlap, buried deep), not a wider fabric overlap.
There is no single fixed rate. UK supplier and installer guidance gives a range. On flat ground roughly 2-4 pins per square metre is typical; on slopes, curves and exposed sites use about 5-6 per m². This calculator defaults to a conservative 6 per m² so you do not under-order, and lets you adjust it. Whatever the field rate, always pin all the edges and every overlap joint at around 300mm centres. Standard U-shaped membrane pins are 150mm (6 inch) long for most garden applications.
Because a roll is a fixed length of fabric, not a pool of divisible metres. Dividing total metres by roll length assumes you can use every last metre, and you can only do that by joining strips end to end, which costs one extra overlap of fabric per join. Four 13m strips are 52m and look like three 25m rolls, but each roll yields only one whole 13m strip and the rest has to come out of the 12m offcuts with two end joints. That still fits on three rolls; on a tighter geometry the laps tip it to one more. If you would rather not join at all, switch "Allow end-to-end joins" off: every strip is then one uncut run, a part roll shorter than a strip is left spare, and the roll count goes up accordingly. End joints themselves are normal practice for geotextile, lapped and pinned exactly like a side joint, and the results always state how many the roll count assumes.
Use 2m wide membrane wherever possible. It means fewer joins and overlaps, which gives better weed suppression and uses less material. Use 1m wide rolls for narrow borders, paths under 1m, or areas with many curves where a narrower roll is easier to handle and wastes less.
For most garden landscaping, use at least 100g/m² weed membrane. This is the practical minimum UK suppliers recommend for reliable, long-term weed suppression. Under driveways and heavy traffic areas, use 125-150g/m² heavy-duty membrane. Lighter 50-80g/m² fabrics are sometimes used for short-term, permeable cover in flower beds under a thick mulch. They tear easily and weeds can push through over time, so they are a false economy for anything you want to last. Always use a woven or needle-punched geotextile rather than thin plastic sheeting, which traps water and causes puddling.
Yes, weed membrane under gravel is strongly recommended. Without it, weeds will grow through within a season, and the gravel will gradually sink into the soil. Lay the membrane on level ground, pin it securely, and add gravel on top. Use our Gravel Calculator to work out how much gravel you need.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.