From ground area to geotextile rolls
A roll labelled 20 m² does not necessarily cover 20 m² of ground. Overlaps and edges matter: the calculation considers roll dimensions, not just the area on the label.
Why might you need an extra strip?
Strips run along the area’s length in this tool. The first contributes the full roll width. Each additional strip adds the roll width minus the overlap.
Two 2 m strips overlapping by 20 cm cover only 3.80 m, not 4 m. This layout therefore needs three strips. The 20 cm is a worked-example input, not a specification; use the overlap required for the product and ground.
Covered width = roll width + (strips − 1) × (roll width − overlap)
Worked example: a 5 × 4 m area
With 2 × 10 m rolls and an example overlap of 20 cm, the result is three strips of 5 m: 15 linear metres. Buy two 10 m rolls. At a fictional price of €30 per roll, material costs €60 before installation.
| Quantity | Result |
|---|---|
| Ground area | 20 m² |
| Strips before trimming | 3 × 5 × 2 = 30 m² |
| Purchased area | 2 × 10 × 2 = 40 m² |
| Material left on rolls | 40 − 30 = 10 m² |
What the estimate does not cut yet
For an irregular plan, the actual area is shown but strips are estimated over its surrounding rectangle. Recesses and reusable offcuts are not optimised. The displayed offcut figure is material left on the rolls after the strips, not all overlap and edge waste.
Roll count is total strip length divided by roll length, rounded up. The engine does not arrange each cut within individual rolls. Strips longer than a roll, end joints, upstands or cutting restrictions may require more material. Check a cutting plan before buying.
Choose by function, not weight alone
Choose a geotextile suited to its intended function, such as separation or filtration. Grams per square metre alone do not establish whether it suits the job.
Use the manufacturer’s requirements for overlap, joints, ground preparation and installation. The calculator does not prescribe these, and a geotextile is not a substitute for a suitable foundation.
Compare roll formats without changing your zone
Try another roll width or length and compare whole rolls, leftover material and total price. A larger roll is not automatically the cheaper purchase. Replace example prices with your supplier’s figures.
Continue the project with gravel or paving using the same zone. Changing the roll does not change the terrace geometry. Save to the active project to keep your choices.
Common buying mistakes
- Dividing ground area by roll area without considering strips.
- Forgetting the overlap between strips.
- Confusing leftover roll material with all cutting waste.
- Assuming end joints and upstands are included.
Before buying geotextile
Will a 20 m² roll cover 20 m² of ground?
Not necessarily. Width, shape, overlaps and trimming also matter. The worked example explains why area alone can mislead.
Does it optimise an L-shaped layout?
No. It uses the plan’s area but estimates strips from its bounding rectangle. Reusable pieces are not automatically allocated.
Which overlap should I enter?
Use the requirement for your product and installation conditions. The example’s 20 cm is not a universal recommendation.
Does the price include fitting and ground preparation?
Roll cost is the whole-roll count multiplied by your price. Any labour estimate is separate; excavation and preparation are not automatically included.
Check the installation guidance
Terram explains separation, filtration, relevant properties and overlaps suited to the site. This reference does not establish a universal fabric weight or overlap. Terram — Standard Geotextiles