1. All fabrics loose some of their weight and volume during processing. These factors are taken into account by the manufacturer when the dimensions for the width and the pattern repeat are given, though in reality both may vary slightly either way
2. Washing and laundering also cause shrinkage. Before making up any fabric that will be washed or laundered on a regular basis, test a small piece. Measure it carefully across the width and the length–20 cm is a reasonable size–and wash it at a slightly higher temperature than you would normally use, press it before it is fully dry and then re-measure it. From this test piece you’ll be able to work out how much you need to allow for shrinkage in the project dimensions.
Alternatively, you can make a generous allowance at the outset and overcut each piece ready to wash and shrink them before you start work