How to Determine Your Exercise Bra Size? | Fit That Actually Works

Most women wear the wrong sports bra size without knowing it—the band rides up, straps dig in, and support fails exactly when you need it. The fix takes two minutes with a measuring tape.

The Two Measurements You Actually Need

You need exactly two numbers: your underbust (ribcage) and your bust at its fullest point. That’s it. The difference between them determines your cup size, and the underbust measurement determines your band size.

Measure in front of a mirror with minimal clothing or an unpadded bra. REI’s guide recommends measuring close to the skin, and several brands echo that—a padded bra adds inches that throw off every result.

Band Measurement

Wrap the tape snugly around your ribcage, just under your bust. The tape should sit level all the way around—no dipping in the back. You should be able to breathe comfortably while measuring. Nike’s instructions emphasize a snug fit on this one, not tight.

Bust Measurement

Lay the tape across the fullest part of your chest, keeping it parallel to the floor. Round up to the nearest whole number.

Turning Numbers Into a Bra Size

Once you have both measurements, the math is simple: subtract your band measurement from your bust measurement. Each inch of difference equals one cup size—1 inch is an A, 2 inches is a B, 3 inches is a C, and so on.

REI’s sizing method uses a slightly different rounding approach: round your ribcage measurement down to the nearest inch for the band, then round your bust up to the nearest whole number before subtracting. Nike and other brands may round differently, which is a real source of inconsistency across companies.

Here’s the band-to-cup breakdown that most U.S. brands follow:

Some guides tell you to round your ribcage measurement down to the nearest inch for the band; others say round up to the next even number if you land on an odd or fractional measurement. When brands disagree, their own chart wins—always check it before purchasing.

Common Mistakes That Ruin the Measurement

Most sizing errors come down to four fixable mistakes. Measuring over a padded bra or bulky clothing adds inches and inflates your cup size. Letting the tape tilt or dip in the back skews the band number. Pulling the tape too tight makes the band too small, which causes discomfort during high-impact workouts. And confusing band size with cup size—cup depends on the difference between bust and band, never the bust size alone.

Beyond the tape, the right fit should feel snug but never restrictive. The band should sit level around your body and stay put when you raise your arms.

Fit Differences: Sports Bras vs. Everyday Bras

Sports bras run differently than regular bras for one big reason: support style. Compression styles press the bust against the chest and run small, while encapsulation styles support each breast independently and follow more traditional sizing. That’s why a size 34C in a daily bra might be a 36B in a high-support running bra.

When you’re ready to compare actual products against your new measurements, our roundup of top-rated exercise bras breaks down which styles fit which activities.

Because regional sizing charts can differ, double-check the brand’s chart for your country before committing. A U.S. 34C is not the same as a U.K. 34C or a European 75C, and the differences get larger as cup sizes increase.