What this calculator works out
This calculator adds an overlap allowance to your area and divides by the coverage of one roll, giving the number of rolls to buy.
Before ordering, it is worth deciding whether membrane is the right approach at all. It works well under paths and gravel and causes long-term problems in planted beds.
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Before you order
- Roll coverage is on the packaging, commonly 15 to 30 m² for domestic rolls.
- The overlap allowance covers the 10 to 15 cm overlap between adjacent strips and the waste around edges and obstacles.
- Membrane needs pegging and, on a path, covering with at least 5 cm of gravel or bark to protect it from ultraviolet light.
- Cut crosses rather than holes when planting through it, and fold the flaps back rather than removing them.
Under gravel, bark paths and hard landscaping it does a real job: it keeps the surfacing separate from the soil and stops it sinking in. In a planted bed it prevents organic matter from reaching the soil, roots grow through and bind to it, weed seeds germinate in the mulch on top regardless, and after some years it degrades into fragments that are very difficult to remove.
How this calculator works
Area plus overlap, divided by roll coverage:
Rolls = area × (1 + overlap allowance ÷ 100) ÷ roll coverage, rounded upThe allowance is applied as a percentage of area, which is a simplification — real overlap depends on the number of seams, so a long thin area needs proportionally more than a square one of the same size.
For an area with many obstacles, increase the allowance rather than trusting the calculated figure.
Worked example: 20 m² of path
Using the default figures — 20 m² with 15 m² rolls and a 10% overlap allowance:
- Area including overlap: 20 × 1.1 = 22 m²
- Rolls: 22 ÷ 15 = 1.47, rounded up to 2
Two rolls gives 30 m² for a 22 m² requirement, which is generous — the rounding up is doing most of the work here. That spare is not waste: membrane is awkward to join neatly, and having enough to run full-width strips rather than piecing together offcuts produces a much better result.
Common mistakes
- Using membrane in a planted bed. It causes more problems than it solves over time.
- Leaving it exposed to sunlight. Ultraviolet light degrades it within a season or two.
- Not overlapping seams. Weeds find every gap.
- Cutting large holes for plants. Cut a cross and fold the flaps back.
- Expecting it to stop perennial weeds. Bindweed and couch grass grow through or around it.
Frequently asked questions
Does weed membrane actually work?
Under gravel and hard surfacing, yes — it keeps the two layers separate and prevents the surfacing sinking into the soil, which is arguably its more valuable function. As a weed control in planted beds it works for a few years and then progressively fails as weed seeds germinate in the mulch above it and perennial roots grow through.
What is the alternative in a planted bed?
Mulch. A 5 to 7 cm layer of compost or bark suppresses germinating weed seeds, feeds the soil as it breaks down, and can simply be topped up. It requires occasional hand weeding, which membrane also does once weeds establish in the surface layer.
What is the difference between membrane and plastic sheeting?
Membrane is permeable — water and air pass through it. Solid plastic sheeting is not, and using it under planting creates waterlogging and kills soil life. Solid sheeting has a legitimate short-term use for clearing weedy ground by excluding light, but it is lifted afterwards rather than left in place.
How do I remove old membrane?
With difficulty, which is the main argument against installing it in a bed. Once roots have grown through and the material has begun to fragment, it comes up in pieces mixed with soil and mulch. Allow considerably more time than expected, and accept that some fragments will remain.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Royal Horticultural Society — weed control methods, mulching and membrane guidance
- GOV.UK — garden waste, plastics and disposal guidance
Sources are checked at publication and can change — how I choose and check references.