Practical guide
Should fabric be prewashed
Prewashing reduces later shrinkage and removes some finishes.
A reliable method
Use a temperature and drying method similar to future care. Delicate and dry-clean-only materials require manufacturer guidance. Secure fraying edges before washing.
Use the calculation in practice
Measure the real usable width after prewashing.
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Fabric, pattern and finishing instructions take priority over a general online estimate.