A bra size calculator determines your accurate band size and cup letter by comparing two simple torso measurements. To find your bra size, measure your underbust band circumference snugly below your breasts, then measure your bust circumference around the fullest part of your chest. Subtracting the band size from the bust measurement reveals your cup size, where each 1-inch difference equals one cup letter progression.
Measure snugly around your torso, directly under your bust
Measure around the fullest part of your bust while wearing a well-fitted bra
Band Size: Measure around your ribcage, directly under your bust. Keep the tape level and snug but not tight.
Bust Size: Measure around the fullest part of your bust while wearing a well-fitted, non-padded bra.
Tips: Stand straight, breathe normally, and round measurements to the nearest ¼ inch or ½ cm for best results.
Wearing the correct bra size is critical for proper posture, physical comfort, and garment fit. Studies show that over 70 percent of women wear the wrong bra size, most often selecting a band that is too loose and a cup that is too small.
When your band is too large, the shoulder straps carry the weight of your bust, leading to shoulder grooves, neck strain, and back pain. The bra band should provide about 80 percent of your total support. An accurate fit ensures the weight is evenly distributed around your ribcage rather than pulling on your shoulders. You can also explore our sister size calculator to see how cup volume transfers across different band numbers.
Before using the tool, gather a few basic items to make sure your numbers are accurate:
If you do not have a soft tape measure, wrap a piece of string or ribbon around your body, mark the endpoint with a pen, and measure that length against a flat ruler.
Follow these straightforward steps to get precise inputs for the calculator.
Stand upright with your arms relaxed at your sides. Wrap the measuring tape around your ribcage directly beneath your breast tissue. Pull the tape snug against your skin, making sure it stays level across your back. Exhale normally and record this number in inches or centimeters.
Wrap the measuring tape around the fullest part of your chest, which is usually across the nipple line. Keep the tape parallel to the floor. Do not pull the tape too tight, as compressing your breast tissue will cause an incorrect small cup calculation.
Subtract your band size from your bust size. The resulting difference in inches correlates to your cup letter:
| Bust minus Band Difference | US/UK Cup Size | EU Cup Size |
|---|---|---|
| Less than 1 inch | AA | AA |
| 1 inch | A | A |
| 2 inches | B | B |
| 3 inches | C | C |
| 4 inches | D | D |
| 5 inches | DD / E | E |
| 6 inches | DDD / F | F |
| 7 inches | G | G |
Sister sizes are groups of bra sizes that share the exact same cup volume, even though their band numbers and cup letters are different. If a bra fits well in the cups but feels too tight or too loose around your ribs, switching to a sister size solves the problem without changing the cup space.
The golden rule of sister sizing is simple. If you decrease your band size by one step, you must increase your cup size by one letter. Conversely, if you increase your band size, decrease your cup letter.
For example, if a 34C feels too tight around your ribcage, your sister size with a larger band is a 36B. If a 34C feels too loose in the band, your sister size with a firmer fit is a 32D. Knowing your sister size is useful when shopping across brands that run small or large, or when using our sports bra size calculator for high-impact activity gear.
Bra sizing standards differ around the world. A size 34D in the United States uses a different band label in Europe or Australia. Use this quick overview table to compare standard regional band sizes.
| US and UK Band | European (EU) Band | French (FR) Band | Australia (AU/NZ) Band |
|---|---|---|---|
| 30 | 65 | 80 | 8 |
| 32 | 70 | 85 | 10 |
| 36 | 80 | 95 | 14 |
| 38 | 85 | 100 | 16 |
When converting cup sizes, US and UK sizes remain identical up to a DD cup. Above a DD cup, UK sizing uses double letters like FF and GG, while US sizing continues up the alphabet like F, G, and H. For specialized garments, check our maternity bra calculator to accommodate body changes.
Paying attention to how your bra sits on your body makes it easy to spot fit issues.
You should measure your bra size every six to twelve months. Body changes caused by weight shifts, workout routines, hormonal cycles, or pregnancy frequently alter your band and bust measurements.
Yes, measure while wearing a thin, unlined, non-padded bra. Measuring over a padded bra adds fake inches to your chest, while measuring bare-breasted can lead to inaccurate cup numbers if there is sagging tissue.
US and UK bra sizing use the same numbers for band sizes, but their cup progression diverges after a DD cup. UK sizing follows DD, E, F, FF, G, GG, while US sizing moves to DDD/F, G, H, I.
Gapping happens if the cup letter is too large, but it also occurs when the bra style does not suit your breast shape. For example, full-coverage cups may gap on shallow breast shapes even if the volume math is correct.
Always size your bra cup to fit the larger breast comfortably. You can then use a small insert or tighten the shoulder strap on the smaller side to balance out the fit.