Almost every fabric moves after washing. If the pattern does not account for that movement, the garment fits correctly exactly once.
How to test
- Cut a marked square of known dimensions from the actual production fabric
- Wash and dry it using the care method you will print on the label
- Measure again and calculate change in length and width separately
- Repeat across dye lots if the run uses more than one
Length and width move differently
Knits in particular often shrink far more in one direction than the other. A single average figure hides that and produces a garment that becomes shorter and wider, or narrower and longer, than intended.
Building it into the pattern
Once measured, shrinkage is added to the pattern so the garment lands on specification after washing. This has to happen before grading, because grading a pattern with the wrong base compounds the error across every size.
Test what you will actually sell
Test the production fabric, not the swatch from six months ago, and test after any dye or wash treatment. Treatments change shrinkage substantially.
Match the care label to the test
If you test on cold wash and tumble dry low, that is what the care label should say. A label that contradicts your test transfers the shrinkage to your customer.