SettingsSettings for this calculationUS
The width of the slope to be covered, measured across the direction the rolls will be laid.
This is the dimension that decides how many ROLLS, and therefore how many overlapped seams run down the slope. Roll width is fixed by what is manufactured, so waste here is all or nothing: a slope a hand's breadth wider than a whole number of roll widths costs a complete extra roll, and it is worth checking whether the layout can be rotated before accepting that.
The slope length in the direction each roll runs (e.g. down-slope distance).
Along the roll, so this decides the overlaps WITHIN a run rather than between runs. On a slope longer than a roll, a transverse joint is unavoidable — and it must be shingled, upper piece lapping over lower, exactly like roofing, or water running down the slope finds the seam and gets underneath the fabric. A joint lapped the wrong way is worse than no joint, because it channels.
The manufactured width of the geotextile roll.
4.5m (about 15 ft) is a common standard roll width.
The minimum overlap required between adjacent rolls.
12 in (0.3m) is a commonly specified minimum for riprap underlayment; check your project spec.
Fabric needed
3,680 sq ft
Doesn't include extra fabric for anchor trenches at the slope crest/toe or for irregular slope shapes — add a margin for those if applicable.
- Number of roll widths needed
- 5 rolls
- Effective coverage width per roll
- 14.02 ft
They open the calculator with your figures already in it
Geotextile Riprap Fabric Overlap Calculator: 3,675 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard geotextile installation requires adjacent rolls to overlap by a minimum margin, reducing the effective coverage width per roll below its full manufactured width
Inputs used
- Slope Width
- 66 ft
- Slope Length (Along the Roll)
- 49 ft
- Fabric Roll Width
- 15 ft
- Required Overlap Width
- 11.75 in
Intermediate steps
- Number of roll widths needed
- 5 rolls
- Effective coverage width per roll
- 14.02 ft
Confidence note: Doesn't include extra fabric for anchor trenches at the slope crest/toe or for irregular slope shapes — add a margin for those if applicable.
What this calculation does not cover
- Roll length is not an input. The total assumes every roll runs the full slope length in one continuous piece, so if the run is longer than the rolls you are supplied you also need transverse end laps, and the real fabric requirement is higher than this figure.
- Nothing is added for anchor trenches. Fabric keyed into a trench at the crest, at the toe, or along the outer edges consumes extra length and width beyond the area being covered.
- The two slope dimensions are used exactly as entered. The model does not convert a horizontal or plan measurement into the longer distance measured down the face, and it adds no cut waste for curved, benched or irregular slopes.
- This is a material take-off, not a filter or erosion-control design. It does not check the fabric's opening size, permittivity or survivability class against the soil and the stone, it does not size the riprap or the bedding layer, and the lap width it uses is whatever you type rather than a value it verifies against a specification.
- It assumes plain shingled laps. Sewn or heat-bonded seams, and the pins, staples or ballast used to hold fabric on the slope while stone is placed, are not counted.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-23 · v1.1.0
Regulatory standards & verification citations1
- Standard geotextile installation requires adjacent rolls to overlap by a minimum margin, reducing the effective coverage width per roll below its full manufactured width
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