Sports Bra Sizing Guide | Measure Once, Buy Right

Most women wear the wrong sports bra size, and the fallout is predictable — chafing, sore shoulders, and support that simply isn’t there. The fix for getting proper sizing for a sports bra is two measurements and a few minutes with a tape measure. Here’s the process, plus the brand-by-brand differences that trip up most shoppers.

How To Measure For A Sports Bra

You need a soft tape measure and a well-fitting non-padded bra (or go braless). Take two measurements while standing straight with your arms relaxed at your sides.

First, the band: wrap the tape snugly around your rib cage, just under your breast, keeping the tape parallel to the floor. Nike’s sports bra size chart stresses that the tape should feel snug — loose measuring adds inches and ruins the fit. Second, the bust: measure around the fullest point of your chest, again keeping the tape level.

Write both numbers down in inches. These two numbers — not your dress size — drive everything else.

Converting Measurements To A Size

Subtract your band measurement from your bust measurement. That difference maps to a cup size:

Here’s where brands part ways, and it matters:

• Round up consistently:
• Round differently:
• Use the brand’s own chart: never assume one universal conversion works everywhere.

That last point is the one that gets most people. Nike’s women’s alpha range runs XXS through 4X, while adidas maps bra sizes A through G against band sizes 30 through 46 and alpha sizes from 2XS through 4X — similar ranges, different conversion logic. adidas’ sports bra size chart spells out its own mappings, and you need that specific table, not a generic one.

Common Sizing Mistakes That Ruin Fit

Nearly every fit problem traces back to one of a handful of measurement errors. The most common ones, per the brands’ own guidance:

• Measuring too loosely. A slack tape adds phantom inches. Keep it snug, as Nike instructs.
• Letting the tape sag. It must stay parallel to the floor front and back, not dip at the spine.
• Using the wrong rounding rule. Rounding up when a chart wants you to round down shifts your size by a full cup or band.
• Assuming brand charts are interchangeable. REI’s expert advice on sports bra sizing differs from Nike’s in rounding and conversion, and Under Armour and New Balance use their own systems as well. Check each brand’s chart.

One more trap: region-specific sizing. Nike publishes a separate Asian Sizing chart with different band and cup mappings, so a size that fits a U.S. shopper won’t match that regional chart. Always confirm you’re reading the chart for your market.

Between Sizes? Here’s The Rule That Counts

A slightly larger band or cup gives better coverage and support during activity, and a too-small sports bra is both uncomfortable and ineffective at controlling motion.

Sports bra sizing is a starting point, not a guarantee. A chart tells you which band and cup to try first; the bra’s intended support level — light, medium, or high — plus the activity you’re doing should steer your final pick. If you’re shopping for a new one, our tested roundup of top-rated athletic bras for every workout narrows down which models actually hold up.