Geomembrane Cost per Square Foot: Converting and Comparing Unit Rates

Geomembrane cost per square foot is a common way to ask for a price, especially from buyers who work in sq ft on building sites. Indian suppliers usually quote per square metre, per kilogram or per roll, and those three rates hide different assumptions about thickness, freight and what is included. This article shows how to convert between units, how to turn a per-kg rate into a per-sq-m or per-sq-ft figure using thickness and density, and what to check so two unit rates are measuring the same thing.

It does not give prices. Polyethylene sheet is priced off resin, and resin moves with crude oil and petrochemical supply, so any figure printed here would date quickly. Use the arithmetic below on the dated quotes you receive.

Square feet and square metres

One square metre is 10.764 sq ft. The conversions you need:

• Rate per sq m = rate per sq ft × 10.764
• Rate per sq ft = rate per sq m ÷ 10.764
• Area in sq m = area in sq ft ÷ 10.764

A rate per sq ft looks about ten times smaller than the same rate per sq m. That is the most common reason two quotes appear wildly different when they are close. Before comparing anything, convert every quote to one unit. On Indian civil works that is normally the square metre, which is also the unit used in tenders and in measurement books.

From thickness to weight

Geomembrane is a solid sheet, so its mass per unit area follows directly from thickness and density:

Mass (kg per sq m) = thickness (mm) × density (g/cm³)

HDPE geomembrane to GRI-GM13 has a density of at least 0.940 g/cm³. Using 0.94:

Two things follow. First, a 1.5 mm sheet contains three times the polymer of a 0.5 mm sheet, so its material cost per sq m is roughly three times as much at the same per-kg rate. Second, you can check any per-sq-m rate against a per-kg rate:

Rate per sq m = rate per kg × kg per sq m

Rate per sq ft = rate per kg × kg per sq m ÷ 10.764

If a supplier quotes per kg, ask for the thickness and the nominal density so you can do this conversion yourself.

Mils, microns and millimetres

Some specifications, especially those copied from US documents, give thickness in mils. One mil is a thousandth of an inch, or 0.0254 mm. So 30 mil is 0.76 mm, 40 mil is 1.02 mm, and 60 mil is 1.52 mm. Indian quotes normally round these to 0.75, 1.0 and 1.5 mm. Confirm which one the rate is for: a sheet sold as “1.5 mm” and a sheet specified as “60 mil” are within about 1.5% of each other, which is fine, but a quote for 1.0 mm against a 60 mil spec is a different product.

What a unit rate includes

A rate per sq m or per sq ft can mean very different scopes. Ask the supplier to tick each line:

A supply-and-install rate per sq ft is only comparable with another supply-and-install rate built on the same list. For whole-job budgets, including subgrade work, protection layers and testing, see how much a geomembrane project costs.

Net area or gross area

Check which area the rate is applied to. Net area is the developed surface of the bed, slopes and anchor trench. Gross area adds welding overlaps between panels and trimming waste. Double-wedge welding needs an overlap of roughly 100–150 mm between rolls, and cutting around corners and pipes adds offcuts. A per-sq-ft rate applied to gross area and one applied to net area will give different totals for the same pond.

Illustrative example: putting two quotes on one basis

The figures below are symbols, not prices.

• Quote A: 1.0 mm HDPE, supply only, ex-works, rate *a* per sq ft of net area.
• Quote B: 1.0 mm HDPE, supply only, delivered, rate *b* per kg.

Convert both to rupees per sq m of net area, delivered:

1. Quote A per sq m = *a* × 10.764. Add freight per sq m (ask for a lump sum and divide by the area).
2. Quote B per sq m = *b* × 0.94 (kg per sq m at 1.0 mm). Then check whether B bills on net or gross area; if gross, multiply by the overlap and waste factor the supplier uses.

Only now can you say which is lower. The same method works for comparing a 1.0 mm quote with a 1.5 mm one: divide each per-sq-m rate by its kg per sq m to see whether you are paying the same per-kg rate for both.

Thickness tolerance and what you are paying for

A unit rate assumes the sheet is the thickness you ordered. GRI-GM13 measures thickness by ASTM D5199 for smooth sheet and allows no single reading more than 10% below nominal. Ask for the roll test certificate, and on large orders measure a few samples from delivered rolls. A sheet that runs thin gives you less polymer, and less puncture resistance, at the same per-sq-m price. Our HDPE sheets page lists GM13 minimum properties by thickness. For guidance on picking the grade in the first place, see how to choose geomembrane thickness.

Protection layer cost per sq ft

On angular or gravelly subgrade, a nonwoven geotextile cushion goes under the liner, and its rate is quoted per sq m by GSM. Add it to the liner rate as a separate line so you can see the combined cost per sq m of lining. At Port Blair, a 1 mm HDPE liner was laid over 400 GSM PET geotextile for a water storage pond; at Jaipur, 5,500 sq m of 750 micron HDPE was supplied for water retention. Each job has its own combination of liner grade and underlay, and that combination, not the liner alone, is the unit rate that matters.

For pond buyers, our guide to pond liner price works out the liner area for a farm pond before any rate is applied.

Frequently asked questions