Elastic Length Calculator
Elastic should measure smaller than your body to stay up — how much smaller depends on the fit you want.
Why elastic is cut shorter than your body measurement
Elastic needs to stretch to go on and then pull the fabric in smaller than your relaxed body measurement to actually stay up without slipping. Cutting it at 100% of your waist measurement produces something that fits like a loose ring rather than a snug waistband.
The percentages here are a starting range
85% of the body measurement is a common starting point for a comfortable everyday waistband. Go toward 90% for something you want to sit gently, or 80% for activewear that needs to stay firmly in place during movement — but elastic brands and widths vary in how much recovery they have, so these are a starting point to test, not a guarantee.
Always test-fit before finishing the casing
Overlap and stitch the elastic into a loop, then try it around the actual body (or waistband opening) before sewing it permanently into the casing. It's much easier to unpick one seam and adjust the loop than to redo an entire finished waistband.