A bra size has two parts: a number, the band size, which comes from the measurement around your ribcage just under the bust, and a letter, the cup size, which comes from the difference between that measurement and the measurement around the fullest part of the bust. Neither part is a guess - both come off a soft measuring tape in about a minute, and knowing how the system works makes any brand's size chart much easier to read.
Measure wearing a non-padded, unlined bra or no bra, standing relaxed - padded styles add volume that distorts the bust measurement. Take each measurement twice; if the two readings differ, use the average. The result is a starting size: fit checks at the end of this page confirm whether it is right, because cup and band shapes vary between brands even at the same labeled size.
Step 1: The Underbust Measurement (Your Band)
Wrap the tape around your ribcage directly under the bust, where the band of a bra sits. The tape should be level all the way around - parallel to the floor at the back, not riding up - and pulled snug, the way a new band feels. Breathe out normally and read the number.
Your band size is that underbust number rounded to the nearest even number. An underbust of 29 inches points to a 30 band; 31 inches points to a 32 band. If you land exactly between, try both bands in the fitting room - band firmness varies by brand, and the band provides most of a bra's support, so this number matters more than the cup letter. The general body-measuring technique is covered in the body measurements guide.
Step 2: The Bust Measurement and Cup Size
Wrap the tape around the fullest part of the bust, usually across the nipple line, keeping it level and resting on the skin without compressing. The tape should touch all the way around but not indent. Read the number, then subtract your underbust measurement from it. The difference, in inches, gives the cup letter:
| Bust minus underbust | Cup size (US/UK) |
|---|---|
| 0 in | AA |
| 1 in | A |
| 2 in | B |
| 3 in | C |
| 4 in | D |
| 5 in | DD (E) |
| 6 in | DDD (F) |
| 7 in | G |
Worked example: an underbust of 31 inches and a bust of 35 inches give a 32 band and a 4-inch difference - a 32D. Cup letters are relative to the band, so a 32D and a 38D do not hold the same volume; the letter only has meaning together with the number.
Sister Sizes: Your Backup Options
Sister sizes are sizes with the same cup volume on a different band: when you go up one band size, you go down one cup letter to keep the volume, and the reverse when you go down a band. The sister sizes of a 34C, for example, are 32D and 36B. They are genuinely useful - a brand whose bands run firm may fit you better in the sister size up, and a style cut with a loose band may work in the sister size down.
Sister sizes are a fitting tool, not an exact equivalence: cup spacing and strap placement shift with the band, so a sister size can solve a band problem but not a cup-shape problem. If neither your measured size nor its sisters fit, the issue is usually the style's cup shape rather than your measurements.
Fit Checks That Confirm the Size
- Band: it should sit level and firm on the loosest hook when new, and stay put when you raise your arms. A band that rides up is too big; one you cannot close comfortably is too small.
- Cups: breast tissue should fill the cup without spilling over the top or sides and without gaping at the top edge. Spillage means go up a cup; gaping usually means a different cup shape or a smaller cup.
- Center gore: on underwire styles, the center front should sit flat against the sternum. A gore that floats away from the body usually means the cups are too small.
- Straps: they should stay in place without digging. Straps do a small share of the work; if they are carrying the load, the band is too loose.
Re-measure when your weight changes, after pregnancy, and at least once a year otherwise - the two numbers drift, and a size that was right two years ago may not be now. Treat the tape result as the start of fitting rather than a verdict: among several close sizes, the right one is the one that passes all four checks.
Common Questions
- What is a sister size?
- A sister size keeps the same cup volume on a different band: up one band and down one cup letter, or down one band and up one cup letter. The sister sizes of a 34C are 32D and 36B. They are useful when a brand's band runs firmer or looser than standard.
- How often should I measure for a bra size?
- At least once a year, and whenever your weight changes noticeably or after pregnancy. Band and bust measurements drift over time, so an old size is only a rough starting point.
- Why do two bras in the same size fit differently?
- Because the labeled size only fixes band length and cup volume. Cup shape - shallow or projected, wide or narrow wires, molded or seamed - differs by brand and style, and shape mismatches feel like size problems even when the numbers are right.
- Do I measure for a sports bra the same way?
- Yes - the same underbust and bust measurements are the starting point. Sports bras are then chosen by support level for the activity, and many are sold in band-and-cup sizes using exactly these numbers, while compression styles in S/M/L still fit better when you know your band size.
Measure & Convert: The Full Series
This page is part of the Apparel Search measuring and size conversion series. Browse the full series below, or start from the size guides hub.