Bra Size Calculator

About This Tool

The Bra Size Calculator helps you find your correct bra size from two simple body measurements: underbust circumference and bust circumference. It calculates your size in four major international sizing systems (US, UK, EU, and Australian) and provides sister size recommendations for when your exact size is not available. Studies suggest that up to 80% of women wear the wrong bra size, leading to discomfort, poor posture, and inadequate support. This tool aims to help you find your best-fitting size as a starting point for your bra shopping journey.

Understanding Bra Sizing Basics

A bra size consists of two components: the band size (a number) and the cup size (a letter). The band size corresponds to the circumference around your ribcage just below your breasts. In the US and UK, this is measured in inches and rounded to the nearest even number. The cup size is determined by the difference between your bust measurement (at the fullest point) and your band size. Each inch of difference equals one cup size increment: 1 inch = A, 2 inches = B, 3 inches = C, 4 inches = D, and so on. A common misconception is that cup size is an absolute measure of breast size, but it is actually relative to band size. A 32D has much smaller cups than a 40D because both the band and cup scale together.

How to Measure Correctly

Accurate measurements are the foundation of finding the right bra size. For the underbust measurement, stand in front of a mirror and wrap a soft measuring tape around your ribcage, directly under your breasts where the bra band would sit. The tape should be snug and level all the way around, firm enough to stay in place but not so tight that it digs into your skin. You should be able to slide one finger underneath. For the bust measurement, wrap the tape around the fullest part of your bust, typically across the nipple line. The tape should be level and not pulled tight. Lean forward slightly to let gravity help position the tape at the fullest point. Wear a well-fitting, unpadded bra or no bra for the most accurate results. Take each measurement two or three times and use the average.

International Size Conversion

Bra sizing varies significantly around the world, which creates confusion when shopping from international brands. The US and UK systems both use inches for band sizes and share the same sizes up to D cup. Above D, the systems diverge significantly. The US system uses DD, DDD (or F), G, H, and so on. The UK system uses DD, E, F, FF, G, GG, H, HH, J, and so on, with double letters indicating intermediate sizes. The European (EU) system uses centimeters for band sizes in increments of 5 (60, 65, 70, 75, 80, etc.) and letter-based cup sizes that roughly correspond to the US system. The Australian system uses its own band numbering (US band minus 22) and follows the UK cup progression. French sizing is identical to EU sizing but offset by one band increment. Italian sizing uses an entirely different numbering scheme. Always verify conversions with the specific brand's size chart.

The Concept of Sister Sizes

Sister sizes are groups of bra sizes that share approximately the same cup volume but differ in band size. When you increase the band size by one increment (e.g., from 34 to 36), you decrease the cup letter by one to maintain the same volume (e.g., from C to B). Conversely, going down a band size means going up a cup letter. So 32D, 34C, and 36B are all sister sizes with similar cup volumes. This concept is invaluable when shopping: if a store does not carry your exact size, you can try a sister size as an alternative. However, sister sizing works best for one size up or down. The band length, strap width, underwire shape, and overall fit change with each band size, so sister sizes are approximations, not perfect substitutes. The band should always provide firm support without being uncomfortable.

Signs of a Good Fit

A well-fitting bra has several telltale signs. The band should sit level all the way around your torso, parallel to the floor, and should not ride up in the back. You should be able to fit two fingers comfortably under the band. The center gore (the piece between the cups) should lie flat against your sternum. The underwire, if present, should follow the natural crease under your breast and rest on your ribcage, not on breast tissue. The cups should be smooth without any wrinkling (too big) or spillage (too small). The straps should sit comfortably on your shoulders without digging in or slipping off. The straps provide about 20% of the support while the band provides 80%, so if you find yourself tightening the straps for support, the band is likely too loose.

Common Fitting Mistakes

The most common bra fitting mistake is wearing a band that is too large and cups that are too small. This often happens because of outdated fitting methods that add 4-5 inches to the underbust measurement, resulting in an artificially large band size. When the band is too loose, it rides up in the back and fails to provide adequate support, forcing the straps to do most of the work, which causes shoulder pain and strap digging. Other common mistakes include not trying on bras before buying, wearing the same size for years without re-measuring (body shape changes with age, weight, and life events), and assuming that cup size is universal across brands. Our calculator uses the modern fitting method that does not add extra inches to the underbust measurement, providing a more accurate starting point for your bra shopping.