Bra Size Calculator

How Bust and Underbust Measurements Determine Bra Size

The bra size calculator uses two key measurements to determine your perfect fit: the underbust (band) measurement and the bust (overbust) measurement. Your underbust measurement, taken snugly around your ribcage directly beneath your breasts, determines your band size. The bust measurement, taken loosely around the fullest part of your chest, provides the data needed to calculate cup size when compared against the band measurement.

The difference between your bust and underbust measurements determines your cup size. Each inch of difference corresponds to one cup size: 1 inch equals an A cup, 2 inches equals a B cup, 3 inches equals a C cup, and so on. For example, if your underbust measures 32 inches and your bust measures 35 inches, the 3-inch difference indicates a C cup, giving you a 32C bra size.

Cup Size Difference Chart

Difference (inches)	US Cup	UK Cup
Less than 1"	AA	AA
1"	A	A
2"	B	B
3"	C	C
4"	D	D
5"	DD/E	DD
6"	DDD/F	E
7"	G	F

What Do Band and Cup Size Numbers Mean?

A bra size consists of two parts: a number representing the band size and a letter indicating the cup size. The band size (32, 34, 36, etc.) corresponds to your underbust measurement in inches, typically rounded to the nearest even number. The cup letter represents the volume difference between your bust and band measurements, with larger letters indicating more volume.

Band size provides the primary support in a properly fitted bra, carrying approximately 80% of the weight. The band should fit snugly around your torso without riding up in the back or digging uncomfortably into your skin. Cup size is relative to band size, meaning a 32D cup holds less volume than a 38D cup despite sharing the same letter designation.

Understanding Sister Sizes for Better Fit Options

Sister sizes are bra sizes with different band and cup combinations that share the same cup volume. Moving down a band size requires moving up a cup size (and vice versa) to maintain equivalent cup volume. For example, 34C, 32D, and 36B are sister sizes because their cups hold identical volume despite different size labels.

Sister sizes prove invaluable when shopping across different brands or when your usual size feels slightly off. If a 34C feels too loose in the band but cups fit well, try the 32D sister size. Conversely, if the band feels tight, the 36B sister size offers the same cup volume with a more comfortable band. For body measurements, try our body fat calculator.

How to Measure Yourself for Accurate Bra Sizing?

Accurate measurements require a soft measuring tape and ideally a mirror to ensure proper tape placement. Measure while braless or wearing a non-padded, well-fitting bra for best results. Stand straight with arms relaxed at your sides during measurement.

For the underbust measurement, wrap the tape snugly around your ribcage directly beneath your breasts, keeping it level all around. Exhale normally and note the measurement without rounding. For the bust measurement, wrap the tape around the fullest part of your chest, keeping it parallel to the floor. The tape should be snug but not compressing breast tissue - you should breathe comfortably.

Signs of an Incorrectly Fitted Bra

Studies suggest 70-85% of women wear incorrectly fitted bras, often choosing bands too large and cups too small. Common signs of poor fit include: the band riding up in the back (indicating the band is too loose), red marks or indentations on shoulders (straps carrying too much weight), breast tissue spilling over cup edges (cups too small), gaps between cup and breast (cups too large), and the center gore not lying flat against your sternum.

International Bra Size Differences Explained

Bra sizing systems vary significantly between countries, making international shopping challenging. US and UK sizes use similar band numbering but differ in cup progression above D. European (EU) sizing uses centimeter-based band numbers (70, 75, 80, etc.) and different cup letter sequences. Australian and New Zealand sizing resembles UK sizing but with slightly different cup progressions.

To convert between systems: a US 34C equals UK 34C, EU 75C, and AU 12C. Our calculator automatically provides these conversions, eliminating guesswork when shopping international brands.

Bra Fitting Tips for Optimal Comfort

When trying on bras, fasten on the loosest hook initially since bands stretch over time - this gives tightening room as the bra ages. Bend forward and scoop breast tissue into cups for proper positioning. The band should sit level around your body, parallel to the floor, not riding up in back. Straps should support without digging - two fingers should fit comfortably underneath. The center gore should rest flat against your sternum without floating or pressing uncomfortably.

Frequently Asked Questions

What is a bra size calculator?

A bra size calculator is a tool that determines your correct bra size using bust and underbust measurements. It calculates the band size from your underbust measurement and cup size from the difference between bust and underbust, providing size recommendations for US, UK, and EU sizing systems along with sister size alternatives.

How do I find my correct cup size?

Cup size is determined by subtracting your band measurement from your bust measurement. Each inch of difference corresponds to a cup size: 1 inch = A, 2 inches = B, 3 inches = C, 4 inches = D, and so on. For example, a 36-inch bust with a 32-inch band gives a 4-inch difference, indicating a D cup.

What are sister sizes in bras?

Sister sizes are bra sizes that share the same cup volume but with different band sizes. Moving up one band size requires moving down one cup size to maintain equivalent cup volume. For example, 34C, 32D, and 36B are sister sizes - all have the same cup volume with different band lengths.

Why do my bra sizes vary between brands?

Bra sizes vary between brands due to lack of standardization in the lingerie industry. Each manufacturer uses slightly different sizing standards, patterns, and construction methods. A 34C in one brand may fit differently than 34C in another. Always try on bras when possible and use sister sizes to find the best fit.

How often should I measure my bra size?

Measure your bra size at least once per year since body weight fluctuations, hormonal changes, aging, and lifestyle changes can affect your measurements. Also re-measure after significant weight changes (5+ pounds), pregnancy, breastfeeding, or starting new exercise routines that affect your body composition.

Should I measure wearing a bra or braless?

For the most accurate measurements, measure while braless or wearing a thin, non-padded bra. Padded or push-up bras can artificially inflate bust measurements, leading to incorrect size calculations. If measuring braless feels uncomfortable, wear your oldest, thinnest bra without padding.

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