Elastic Length Calculator

How much shorter, and why

Elastic holds by being under tension, so it is cut shorter than the body measurement. How much shorter is a comfort judgement rather than a formula: around ten percent for a soft waistband that should not mark, twelve to fifteen for a firmer hold, and more for a cuff or a sleeve band that must stay put against gravity.

To that, add an overlap for the join — an inch is standard, sewn with a box or a zigzag so it does not become a hard ridge.

The percentages assume a typical knitted or braided elastic. Woven elastic is firmer and usually needs less reduction to achieve the same hold, which is why a figure that works in one elastic can be too tight in another.

A worked example

A 30 inch waist at 10% reduction gives 27 inches of elastic, plus an inch of overlap, so cut 28 inches. At 15% it would be 25.5 plus overlap — two and a half inches tighter, which is a noticeably firmer waistband.

For a child's garment, where the elastic has to survive being pulled on and off, the higher reduction is usually right. For a maternity or loungewear waistband, the lower one.

Where the answer misleads

Elastics vary enormously in strength, and no percentage substitutes for testing. Wrap the cut length round the body at the position it will sit, at the tension you want it to hold: it should stay in place without leaving a mark after a minute. That test takes ten seconds and beats any table.

Braided elastic narrows when stretched and loses strength if sewn through, so it belongs inside a casing. Knitted and woven elastic keep their width and can be stitched directly to the fabric. Using braided elastic in an exposed application is a common cause of a waistband that goes slack quickly.

Elastic also relaxes with washing and time. Building in a slightly firmer fit than feels right when new is reasonable for a garment that will be laundered often.