Bra Size Calculator

Most people wear the wrong bra size because they guess instead of measure. This bra size calculator turns two simple tape-measure numbers - your underbust and your bust - into a US band and cup size in seconds. As a concrete example: if your underbust measures 31 inches, that rounds up to a 32 band; if your bust measures 36 inches, the difference is 4 inches, which is a D cup - so your size is 32D. Below you'll also see sister sizes, which keep the same cup volume on a different band so you can dial in the fit.

How bra size is calculated

The math is two short steps. First the band, then the cup:

So a 33-inch underbust rounds to a 34 band, and a 40-inch bust gives a 6-inch difference (DDD/F), for a size of 34DDD. Cup letters are relative to the band: a D cup on a 32 band is a smaller volume than a D cup on a 40 band.

Understanding sister sizes

Cup volume stays roughly constant if you move the band and cup in opposite directions. Drop one band size (minus 2 inches) and go up one cup letter, or add one band size and drop one cup letter. That's why 32D, 34C and 30DD are "sisters" - the cups hold a similar amount, but the band fits a different ribcage. If your size is correct on volume but the band feels too tight or too loose, the sister size on the row above or below is usually the fix.

Getting accurate measurements

• Band (underbust): wrap the tape snugly around your ribcage right under the bust, keeping it level front and back. Exhale and read the number.
• Bust: measure loosely around the fullest part of the chest, wearing a thin, non-padded bra so the tape isn't lifted or compressed.
• Stand normally: don't slouch or puff your chest - measure relaxed and upright.
• Re-measure over time: weight changes, pregnancy and bra wear all shift your size, so check again every 6-12 months.

Signs your bra doesn't fit

The band should sit level and do most of the supporting work, the underwire should lie flat against your ribcage, and the cups should fully contain the breast with no spillage or gaping. A band that rides up usually means it's too big (try a smaller band, larger cup); straps that dig in usually mean the band isn't supporting enough. Use the calculator's result as your starting point and adjust with sister sizes.

How to use this calculator

You only need a tape measure and two numbers. Work through it in order:

1. Measure your underbust: wrap the tape snugly around your ribcage directly under your bust, level all the way around, and read the number on an exhale.
2. Measure your bust: measure loosely around the fullest part of the chest while wearing a thin, non-padded bra so the tape isn't lifted or flattened.
3. Enter both numbers in inches. The calculator rounds the underbust to the nearest even band and subtracts it from your bust to find the cup.
4. Read your result at the top, then scan the sister-size row to find adjacent sizes with the same cup volume.

If a result feels off, double-check that you measured the band snugly - a loose underbust reading is the single most common cause of a too-large band.

A second worked example

Suppose your underbust measures 29 inches and your bust measures 35 inches. The underbust rounds up to a 30 band. The difference between bust and band is 35 − 30 = 5 inches, which is a DD/E cup - giving you a size of 30DD. Now look at the sisters: if the 30 band feels too tight on the loosest hook, step up to 32D (one band bigger, one cup smaller); if it feels too loose, step down to 28DDD. All three hold a similar cup volume, so you are choosing the band that hugs your ribs best, not changing how much the cup contains.

Who this calculator is for

This tool is for anyone who wants a reliable starting size instead of a guess. That includes:

• First-time shoppers who have never been professionally fitted and need a baseline before buying online.
• Online buyers who can't try things on in a store and want to narrow down to two or three sizes to order.
• Anyone whose body has changed after weight change, pregnancy, breastfeeding, or hormonal shifts and suspects their old size no longer fits.
• People stuck on the old "+4" rule whose bands have always felt loose and who want the modern, snugger band method.
• Gift buyers working from someone else's measurements who need to translate them into a size to order.

Why the old "+4" method gives the wrong band

For decades the standard advice was to add four (or five) inches to your underbust to get your band size. That habit dates from an era of less stretchy fabrics, and on today's materials it almost always produces a band that is too big - which is why so many people wear bands that ride up and rely on the straps for support. The modern method this calculator uses adds nothing: the band is your underbust rounded to the nearest even inch. If you have always added four inches, expect your true band to come out one or two sizes smaller, with a correspondingly larger cup letter to keep the volume the same.

US, UK and EU sizing differences

Band numbers and the first few cup letters line up reasonably well across countries, but they diverge above a D cup. In US sizing the run is A, B, C, D, DD/E, DDD/F, G, and so on. UK sizing uses single letters more consistently (DD, E, F, FF, G, GG), so a US DDD often maps to a UK F or G. EU bands are measured in centimetres (a 32 band is roughly an EU 70, a 34 is about a 75), and EU cup letters track closer to the UK run. This calculator returns US sizing - if you are buying from a British or European brand, find your equivalent on that brand's own size chart rather than assuming the letters match.

What changes your bra size

Your measured size is not fixed for life. Several things move the numbers, sometimes by a full band or cup:

• Weight change: gaining or losing even 5-10 pounds can shift your ribcage measurement and your bust differently, changing band, cup, or both.
• Pregnancy and breastfeeding: the bust and ribcage both expand, and they keep changing through nursing and weaning, so a fitting before pregnancy rarely holds afterward.
• Hormonal cycles: many people swell by up to a cup in the days before a period, then settle - measure on an average day, not at the peak.
• Exercise and posture: building back and chest muscle changes the ribcage; a stronger upright posture also alters how a band sits.
• Bra age: the elastic in a band relaxes over months of washing and wear, so an old bra reads "bigger" than its label - replace rather than resize when it stops gripping on the tightest hook.

Tips for a better fit

Once you have your calculated size, these habits help you land the right bra rather than just the right number on the label:

• Try the calculated size and both nearest sisters - for example 34C, plus 32D and 36B - and keep whichever band stays level without riding up.
• Fasten new bras on the loosest hook so you can tighten inward as the elastic stretches over time.
• Do the scoop: lean forward and settle all breast tissue fully into each cup before judging spillage or gaping.
• Check the center gore: the fabric between the cups should lie flat against your sternum; if it floats off, the cup is usually too small.
• Match the style to your shape: a plunge, balconette, or full-coverage cut in the same labeled size can fit very differently, so pick the cut that suits you, not just the size.

Limitations and assumptions

The calculator is an accurate starting point, but it cannot account for everything:

• It assumes a careful tape measurement; a loose or tilted tape throws off both band and cup.
• It returns US sizing only and does not convert to UK, EU, or Asian charts.
• It cannot account for breast shape and projection - two people with identical numbers may need different cup depths or styles.
• It is a fitting guide, not medical advice. Sudden, one-sided size changes or other breast-health concerns should be discussed with a qualified healthcare professional.

Sources

• U.S. Office on Women's Health (OWH) - women's health and breast health information.
• National Cancer Institute (NCI / NIH) - breast changes and what to discuss with a clinician.
• National Institutes of Health (NIH) - general health guidance for when to consult a professional.