The grams per square meter (GSM) number of a knit fabric is a measurement of the fabric’s weight and plushness. To find the GSM of a length of knit fabric, divide the fabric’s weight by its surface area. The GSM of a piece of circular knit fabric is more difficult to calculate than the GSM of a flat piece of fabric because the formula for the surface area of a cylinder is more complex than the formula for the surface area of a rectangle.
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Lay your circular fabric flat on a work surface with one of the open ends facing you. Measure across the open end to get the diameter of the circle. Divide this number in half to get the radius. Measure from one open end to the other in a straight line to get the height of the cylinder. Convert both of these numbers to meters. A radius of 10 inches converts to .254 meters. A height of 36 inches converts to .9144 meters.

Use the formula 2 x pi x radius x height to find the surface area of the knit cylinder. For example, if your radius is .254 meters, and your circular fabric is .9144 meters tall, multiply 2 x pi (3.14159) x .254 x .9144 to get a surface area of 1.4593 square meters.

Weigh your fabric and convert the weight to grams.

Divide the weight in grams by the surface area in meters to get grams per square meter. For example, a cylinder of fabric that weighs two pounds (907.18 grams) and has a surface area of 1.4593 square meters has a GSM of 622.
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