Title: Metric Conversions

Metric Conversions

Metric Conversions are dependant on the test method that is being converted. For Geotextiles and Geomembranes we need to know what the test method is before we can convert.

Converting values from US/Imperial values to metric and back can sometimes require interpretation. An understanding of both the test method and the material being tested is often required for correct interpretation. This tech note discusses some of the more usual conversions. A conversion chart at the end of this tech note shows the various conversions.


1. TENSILE TEST. A number of Tensile tests are used: ASTM D751, D638, and D882. Each measures the force to break a unit of plastic (lbs converts to N, or ppi converts to N/mm). For force measured in psi you will also need to know the thickness of the sample tested (psi converts to kPa).

2. TEAR STRENGTH. The standard tear test for fabric supported materials is ASTM D751. This test always provides values in unit pounds of force (lbs converts to N). Unsupported lining materials are tested with ASTM D1004. The actual pounds of force used to tear the sample can be reported, or the force (in pounds) divided by the sample thickness can be reported (ppi converts to N/mm). An example for 30 mil PVC would look like this.


8 pounds of force divided by .030 inches of thickness = 267 pounds per inch of thickness (ppi).

3. PUNCTURE RESISTANCE. There are two widely accepted puncture tests for Geomembranes, FTMS 101C method 2065 and method 2031. These are very different tests and provide different values. Both values are normally reported in pounds of force (lbs converts to N) but occasionally a manufacturer will report pounds per inch of thickness (ppi converts to N/mm) as in the tear strength example.


A. TENSILE TESTS. There are two basic styles of Geotextile Tensile tests, the grab tensile test (ASTM D751 or ASTM D4632) and the wide width Tensile test (ASTM D4595). Design calculations using Geotextile strengths should always use wide width Tensile test results. Grab tensile values are always reported in pounds of force (lbs converts to N) while wide width tensile values are reported in pounds per inch of width (ppi converts to kN/m), or pounds per foot of width (lbs/ft converts to kN/m).

B. TEAR TEST. Geotextiles use the trapezoidal tear test (ASTM D4533) which reports values in pounds of force (lbs converts to N).

C. PUNCTURE TEST. Geotextiles use the ASTM D4833 Puncture test with values reported in pounds of force (lbs converts to N).

D. BURST TESTS. Geotextile burst tests measure the pressure required to burst a Textile on a specific bursting machine. The Ball Burst Test (ASTM D751 Method 18.2) and the Mullen Burst (ASTM D751 Method 18.3) are quite different tests within the same ASTM designation. Ball Burst values are reported in pounds of force (lbs converts to N), Mullen Burst values are reported in pounds per square inch (psi converts to kPa). There is no correlation between the two tests.

E. APPARENT OPENING SIZE (AOS). AOS is a measure of the filtration size of a Geotextile. Conversion from US sieve sizes to metric opening sizes should be done with a conversion chart.


Metric Conversion Chart
US / Imperial Unit Conversion Factor Metric Unit
To convert to Metric, multiply by the factor > Factor < to convert to US values, divide by the factor
mil Thickness (mil) (thousandths or an inch) 0.0254 Millimeters (mm)
Inches (in) 25.4 Millimeters (mm)
Feet (ft) 0.3048 Meters (m)
Square Feet (ft2) 0.09290304 Square Meter (m2)
Dollars per Square Foot ($/ft2) 10.76391 Dollars per Square Meter ($/m2)
Pounds of Weight (lbs) 0.4536 Kilograms (kg)
Ounces per Square Yard (oz/yd2) 33.90575 Grams per Square Meter (g/m2)
Pounds per Foot of Length (lbs/ft) 1.4882 Kilograms per Meter (kg/m)
Pound per Square Foot of Area (lbs/ft2) 4.8824 Kilograms per Square Meter (kg/m2)
Pounds of Force (lbs) x 4.44822 Newtons (N)
Pounds per inch (ppi) x 0.175127 Newton/mm (N/mm)
Pounds per inch (ppi) x 0.175127 KiloNewtons/m (kN/m)
Pounds per Square inch (ppi) x 6.89476 KiloPascals (kPa)
Pounds per Square inch (ppi) x 0.00689476 MegaPascals (mPa)
Feet per Second (ft/sec) 0.3048 Meters per Second (m/sec)
Gallons per Minute per Foot Squared (gpm/ft2) 40.746 Litres per Minute per Meter Squared (l/min/m2)
Note: Factors in bold type are exact conversions. Factors in normal text are accurate to the number of significant figures shown.


AOS Conversions
US Sieve mm US Sieve mm
18 1.00 60 0.250
20 0.841 70 0.210
25 0.707 80 0.177
30 0.595 100 0.149
35 0.500 120 0.125
40 0.420 140 0.105
45 0.354 170 0.088
50 0.297 200 0.074