How a fabric is wash tested

Short answer: The sample is washed a fixed number of times at a fixed temperature together with a standardised amount of laundry. Afterwards, size, colour and surface are measured and compared with how the sample looked before. Everything is set in advance so the results can be compared.

Everything is fixed in advance

Water volume, temperature, wash time and the amount of fabric in the machine are set in the method before the sample gets wet. Otherwise two laboratories cannot compare their results. What matters is not the wash itself but that it runs the same way every time, whoever carries it out.

The sample gets company

The sample is washed with a standardised amount of laundry, so the machine has a fixed load. A nearly empty machine treats fabric differently from a full one, because the fabric moves and rubs more or less. Fixing the amount removes one source of uneven results.

A standard detergent

A standard detergent is used, not the one you have at home, so the result does not depend on the brand. A detergent can affect colour and surface, and then it would be unclear whether a change comes from the fabric or the product. With the same detergent every time, the fabric is the only variable.

Measuring before and after

The sample gets marks at a known distance before washing. After the wash the same distances are measured again, and the difference is given as a percentage. See Boody to compare fabrics, and read how the composition is stated before you judge how much a fabric changes in the wash.

Colour and surface are judged too

Besides size, colour and surface are examined. Has the colour faded, or has the sample pilled or turned rough? The judgement is often made against a scale or an unwashed sample. That lets you put a number or grade on something that would otherwise be only an impression.

More washes show the trend

One wash shows the first shrinkage. Five washes show whether the fabric keeps moving or settles down. The trend says most about how a garment behaves over time. See Lindbergh and JBS if you want to follow the care label yourself at home.

Drying counts too

The method also states how the sample is dried after washing, for example air drying or a tumble dryer. Drying can shift the size as much as the wash itself, so it must be fixed like everything else. Otherwise you cannot know what caused the change in the fabric.

Why there are several samples

A laboratory often tests several pieces of the same fabric and looks at the average. A single piece can be slightly different by chance, and the average gives a more reliable picture. That is why a report is rarely one number from one sample, but a summary of several measurements under the same conditions.

Using this at home

The test is meant to resemble ordinary use, but it is more uniform than a wash at home. Follow the care label, use the same temperature each time and avoid overfilling the machine. Then you get the changes in the fabric that the test predicts, and no more than there is reason for.