Bra Sewing Calculator

Underbust:Wrap the tape measure directly under your bust, where the band will sit. Pull it snug — exhale and let the tape settle naturally. The tape should be level all the way around. This measurement determines your band size.

Full bust:Measure at the fullest point of your bust, keeping the tape parallel to the floor. Don't compress the tissue — the tape should just touch the skin. The difference between full bust and underbust determines your cup size.

Apex span:The distance between your two bust points (apex to apex) typically ranges from 7–9" for average frames. Measure this to position cups correctly on your pattern — off-the-rack bras often assume a narrower span than many people actually have.

The underwire should follow the natural crease (inframammary fold) under your bust. It must extend far enough to the side to encapsulate all breast tissue without sitting on it. A wire that's too narrow presses into tissue and causes discomfort; a wire that's too wide gaps at the sides.

Tip-to-tip is the distance from one end of the wire to the other, measured straight across. Width is the widest horizontal distance when the wire is in its curved shape. Depthis how far the wire projects from the chest wall. Together these three measurements define the wire's profile.

Wire sizes vary between manufacturers. Always compare the actual dimensions of the wire to your body — the wire number is a starting point, not gospel. Bend a pipe cleaner or soft wire to your body shape to check fit before committing to a wire size.

Sister sizes are bra sizes that share the same cup volume but with a different band. For example, 34C, 36B, and 32D are all sister sizes — the cup holds the same amount of tissue, but the band is tighter or looser.

This matters for bra sewing because your calculated band size might not match available wire sizes or pattern gradings. If your wire sits better one size up or down, you can adjust the band and shift the cup accordingly without changing the cup volume.

Band up, cup down: a 36B becomes 38A. The cup volume stays the same but spreads over a wider band. Use this if the band feels too tight or you prefer a looser fit.
Band down, cup up: a 36B becomes 34C. Same cup volume, snugger band. Use this if you want more support or your fabric has more stretch than expected.

The band provides roughly 80% of a bra's support — the straps only handle the remaining 20%. A well-fitting band is snug enough to stay in place without riding up, which means the cut length of the band must be shorter than your underbust measurement.

How much shorter depends on the fabric's stretch. For typical bra fabrics (power net, duoplex, firm mesh), 15–25% stretch reduction works well. For swim fabrics or very stretchy knits, you may go up to 30–40%. For woven bands (like in a longline bra), use only 0–5%.

Test your fabric:Cut a 10" strip, stretch it comfortably, and measure how far it goes. If it stretches to 12", that's 20% stretch. Use that percentage in the calculator to get the correct band cut length. Always make a test band in your actual fabric before cutting the final pieces.