How to Determine Your Sports Bra Size for the Perfect Fit

Measure your underbust and bust with a soft tape, subtract the two numbers, and check the band, cups, and straps for a secure fit.

Finding your sports bra size starts with two measurements and ends with a fit check that proves the numbers right. A correctly fitted sports bra keeps the band level, the cups fully covered, and the bounce under control—whether you’re running, lifting, or doing yoga. Here’s the step-by-step method.

Measuring Your Band Size: Underbust First

Wrap a soft measuring tape snugly around your rib cage, directly under your bust. Keep the tape level all the way around your back—no dipping, no gaping.

No tape measure? Nike’s guide covers that too: use a piece of string, wrap it the same way, then lay it against a ruler. The result is the same measurement.

Measuring Your Cup Size: The Fullest Point

Now measure the fullest part of your bust, keeping the tape parallel to the floor. Round up to the nearest whole number. Your cup size is the difference between this bust measurement and your band size.

For example, if your band is 34 inches and your bust measures 37 inches, you’re a 34C.

Measure in a regular bra—not a sports bra—that gives your natural shape without padding or push-up effects. A too-tight or too-loose tape throws the whole calculation off.

Fit Checks That Confirm the Size Is Right

Numbers only get you partway. The real test is how the bra behaves on your body. Run through this quick checklist before committing to a size:

• Band: sits level all the way around, doesn’t ride up in the back, and allows about two fingers between the band and your body—not more, per REI’s guidance.
• Cups: fully contain your breast tissue with no spillage over the top or sides, and no gaping at the top edge.
• Straps: stay stable on your shoulders without digging in. You shouldn’t need to adjust them mid-workout.
• Bounce: minimal movement when you jump in place. Nike’s guidance says your chest should feel covered and secure with minimal bounce.

The most common sizing mistake happens before the tape even touches your body: measuring over breast tissue instead of directly under the bust for the band. That adds inches and gives you a band that’s too loose to do its job. The second most common mistake is inconsistent rounding—rounding the underbust up and the bust down, which shrinks the cup difference. Pick a direction and stick with it.

Between Sizes and Final Fit Notes

If you measure between sizes, round up. A slightly roomier band is more comfortable for breathing and movement, and you can always adjust the hooks tighter. A too-small band will pinch, roll, and ride up—none of which a workout needs.

Try the bra on and move before you buy. Jump, reach overhead, and bend side to side. If the band shifts or the cups lose coverage during movement, go up a size. If the bra feels snug at the loosest hook setting, go down a band size but keep the same cup letter to preserve the ratio.

For a roundup of tested options from a brand that fits true to size, check our guide to the best yellow sports bras that balance comfort and support.

The size you measured is a starting point, not a sentence. Different brands run different ways, and the same brand can fit differently across styles—compression versus encapsulation, racerback versus traditional straps. Between technical models, your usual bra size or the measurement method above will land you close; the fit check decides the rest.