Bra Size Calculator

Finding the right bra size can be frustrating, especially when sizes vary between brands and countries. This free bra size calculator takes the guesswork out of bra fitting by converting your body measurements into standardized sizes across four international sizing systems, with visual guides and shape-based fit recommendations.

Step 1: Take Your Measurements

You need two measurements: underbust (band) and bust. For the underbust, wrap a soft measuring tape snugly around your ribcage directly below your bust line — not around the breast tissue itself. For the bust measurement, wear a non-padded bra and wrap the tape around the fullest point of your bust, keeping it parallel to the floor. Enter measurements in either inches or centimeters using the toggle switch.

Step 2: Use the Visual Measurement Guide

The diagram above the calculator shows exactly where to place the tape. The red dashed line indicates the underbust position (just below the breasts), and the blue dashed line shows the full bust position (at the fullest point). Both lines should be parallel to the floor when measuring.

Step 3: Get Your Sizes and Sister Sizes

Click Calculate to see your bra size in US, UK, EU, and AU formats. The calculator rounds your band measurement to the nearest even number and determines cup size based on the difference between bust and band. Sister sizes appear below your primary size — these have equivalent cup volume with different band sizes. If your size feels slightly off, try a sister size first.

Step 4: Select Your Shape for Personalized Fit Tips

Below the calculator, select your breast shape from five options: Round, Teardrop, East-West, Side-Set, or Close-Set. Each selection shows specific style recommendations. For example, East-West shapes benefit from front-close bras that center tissue, while Close-Set shapes should avoid high-bridge styles that cause discomfort at the center gore.

Step 5: Check Brand Fit Notes and the Fit Issues Guide

The brand comparison chart shows how your calculated size translates to popular brands — brands like Victoria's Secret run slightly small in cups, while ThirdLove offers half-cup sizes for a more precise fit. The Common Fit Issues section helps troubleshoot specific discomforts: if your band rides up in the back, try a smaller band size; if cups overflow, go up one cup size.

Understanding International Size Differences

US and UK bra sizes share the same band numbers but differ in cup progression after D. The US system uses DD, DDD, G, H while the UK system uses DD, E, F, FF, G. European sizes use centimeter-based bands (65, 70, 75, 80) with standard cup letters. Australian sizes match UK cup letters with a different band numbering system starting at 8. The bra size conversion chart at the bottom of this page shows common equivalents for quick reference.

Step-by-Step Measuring Guide

Getting accurate measurements makes the difference between a bra that fits and one that causes daily discomfort. Follow these steps exactly:

1. Wear a non-padded bra (or no bra). A padded bra adds volume and distorts your bust measurement. A wireless, non-padded bra gives the most accurate base measurement.

2. Measure your band size (underbust). Wrap a soft measuring tape snugly around your ribcage, directly below your bust and over your shoulder blades. Keep the tape level and parallel to the floor. Breathe normally — don't exhale to make it smaller. Note the measurement in inches and round to the nearest even number. This is your band size. If the number is odd, round up to the next even number.

3. Measure your bust at the fullest point, leaning forward 90 degrees. This is the technique professional fitters use and it's the most important step most people skip. Lean forward at the waist so your torso is parallel to the floor. Let your breast tissue fall forward naturally. Now wrap the tape around the fullest part of your bust. This forward-lean position gives a more accurate cup measurement because gravity pulls the tissue to its natural hanging position — the position the bra cup needs to accommodate. Enter this measurement as your bust.

The difference between bust and band measurements determines your cup: 1 inch = A, 2 = B, 3 = C, 4 = D, 5 = DD/E, and so on.

Common Bra Fitting Mistakes

Studies suggest that up to 80% of people wear the wrong bra size. These are the most common mistakes:

Band too loose: The most frequent error. Many people go up a band size to compensate for cup discomfort, when the real issue is cup size. A correctly fitting band should allow only two fingers to slide underneath comfortably. If you can pull the back band more than 1 inch away from your back, it's too loose.

Cup size confusion (cup is relative to band): A D cup on a 32 band is a very different volume than a D cup on a 38 band. The cup letter only describes the volume difference between your underbust and bust measurements — it's relative, not absolute. This is why sister sizes exist and why "D cup" means very different things at different band sizes.

Not scooping and swooping: After putting on a bra, reach into each cup with the opposite hand and physically scoop all breast tissue forward and into the cup. Then sweep the side tissue that may be behind the underwire into the cup. This technique — called "scoop and swoop" — is how professional fitters ensure all tissue is correctly housed in the cup before assessing fit. Skipping this step makes most bras look and feel too small.

Understanding Sister Sizes

Sister sizes are bra sizes that share the same cup volume despite having different band and cup letter combinations. The rule: going up one band size means going down one cup letter to maintain the same volume. Going down one band size means going up one cup letter.

Example: 34C has the same cup volume as 32D and 36B. If you're a 34C and the band is too tight, try a 36B instead of a 36C (which would have a larger cup). If the band is too loose, try a 32D instead of a 32C (which would have a smaller cup). Sister sizes solve band fit problems without sacrificing cup volume — one of the most useful tricks in bra shopping that most people never learn.

The sister-size range typically extends 2 band sizes in either direction: a 34C's extended sister family includes 30E, 32D, 34C, 36B, and 38A.

When to Get Remeasured

Bra size isn't permanent. Get remeasured (or use this calculator again) when: your weight changes by more than 10 pounds in either direction; during and after pregnancy; after breastfeeding ends; after any chest or breast surgery; and as a general best practice every 6–12 months. Bodies change gradually and most people go years wearing a size that no longer fits simply because they haven't remeasured.

For a detailed walkthrough, see our guide: How to Measure Your Bra Size at Home.