Bra Size Calculator

Buying a bra should be simple: walk in, grab your size, walk out. In practice, most people wear the wrong size for years because they were measured once with an outdated method or sized by a brand whose tags do not match the tape. A bra size calculator solves this with pure arithmetic: your underbust measurement gives the band, the difference between your bust and band gives the cup, and two minutes of measuring replaces years of guessing.

Enter your underbust and bust measurements in inches and the calculator returns your band size, cup size, complete bra size, and both sister sizes in the blue result box. It is the fastest route to a fitting room visit that actually works.

What Is Bra Size?

Bra size has two components. The band size is a number, such as 34, representing the inches around your ribcage under your bust, rounded to the nearest even number. The cup size is a letter, such as C, representing the difference in inches between your fullest bust measurement and your band size.

Each inch of difference equals one cup letter. A one-inch difference is an A cup, two inches a B, three a C, four a D, five a DD, six a DDD, and the scale continues through G, H, I, and beyond. This is why cup letters are meaningless without the band: a D cup on a 32 band holds far less volume than a D cup on a 38 band.

Understanding this two-part system is the key to everything else about bra fitting. Once you see size as band plus difference, size charts stop being mysterious and sister sizes become obvious.

The Two Measurements That Matter

Only two numbers drive the entire calculation. The underbust measurement is taken around your ribcage directly beneath your breasts, where a bra band sits. Keep the tape level around your back, pull it snug but not tight, and breathe normally. Round to the nearest half inch.

The bust measurement is taken around the fullest part of your chest, usually across the nipples. The tape should rest on the tissue without compressing it. Wear a thin, unpadded bra or nothing at all, keep the tape level, and round to the nearest half inch.

Take each measurement two or three times and use the most consistent value. Small errors here propagate directly into the result: a one-inch mistake in the bust measurement shifts the cup by a full letter, so careful measuring is worth the extra minute.

How Band Size Is Determined

Bands are manufactured in even-numbered sizes, so your underbust measurement is rounded to the nearest even number. An underbust of 32 inches gives a 32 band; 31 rounds to 32 as well, while 33 rounds to 34. This rounding is standard across the industry.

The band does the heavy lifting in a bra, providing roughly eighty percent of the support. That is why band accuracy matters more than most people realize. A band that is too loose rides up the back, shifts during the day, and dumps all the support work onto the shoulder straps, which then dig in painfully.

If you are between band sizes, the calculator’s rounding gives you the nearest standard size, and the sister sizes offer the alternative. Someone who finds the rounded band slightly snug can try the looser sister size; someone who wants maximum support can try the tighter one.

How Cup Size Is Determined

Cup size is pure subtraction: bust measurement minus band size, converted to a letter. The calculator rounds the difference to the nearest whole inch and maps it onto the standard scale: less than one inch is AA, one inch A, two B, three C, four D, five DD, six DDD, seven G, eight H, and so on.

This method replaced the old add-four-inches rule, which told wearers to add inches to the underbust before calculating. That rule made sense when bra fabrics had no stretch; today it systematically produces bands that are too big and cups that are too small. The modern subtraction method is what professional fitters use.

Note that cup progression is not perfectly standardized past DD across all brands, with US and UK scales diverging. The calculator uses the widely accepted US progression, which works as the correct starting point for American brands and converts easily for others.

How to Use This Calculator

Getting your size takes under a minute:

1. Measure your underbust in inches and enter it in the first box.
2. Measure your bust at the fullest point in inches and enter it in the second box.
3. Press Calculate.
4. Read the result box: your band size, cup size, full bra size, and two sister sizes.
5. Use Reset to clear the form and measure again or try different numbers.

Worked Example 1: A Common Fit

Suppose your underbust measures 30 inches and your bust measures 34 inches. Enter 30 and 34, then press Calculate.

The band is already an even number, so it stays 30 inches. The difference is 34 minus 30, which equals 4 inches. On the cup scale, four inches is a D cup. Your bra size is 30D.

The sister sizes are 28DD for a tighter band and 32C for a looser one. All three hold the same cup volume. Many people who thought they were a 34B discover through this exact math that they are actually a 30D, which explains years of slipping straps and gaping cups.

Worked Example 2: Rounding in Action

Suppose your underbust measures 33 inches and your bust measures 38 inches. Enter 33 and 38, then press Calculate.

The band rounds from 33 to the nearest even number, 34 inches. The difference is 38 minus 34, which equals 4 inches, giving a D cup. Your bra size is 34D.

The sister sizes are 32DD and 36C. Notice that the rounding absorbed the odd measurement cleanly: you get a buyable size instead of a theoretical 33D that no store stocks. If the 34 band feels slightly loose in a particular brand, the 32DD sister size is your instant backup plan.

Sister Sizes and Why They Matter

Sister sizes keep the same cup volume while trading band length for cup letters. Drop one band size and go up one cup letter, or raise one band size and drop one cup letter, and the cups hold the same amount of tissue. The calculator shows both of yours because they are the most practical fitting tool you own.

Brands cut differently. A 34 band in one label can fit like a 32 in another thanks to variations in elastic strength, fabric, and pattern grading. When your calculated size feels off in a new brand, the answer is usually a sister size, not a re-measure. Trying the tighter or looser sister before abandoning the size saves fitting-room frustration.

Sister sizes also adapt to body changes. Hormonal cycles, weight shifts, and even bloating can make your usual band feel different week to week. Knowing that your 34D has sisters in 32DD and 36C means you can flex with your body instead of fighting it.

How to Tell If Your Current Bra Fits

Run through this checklist in front of a mirror. The band should sit level around your body, parallel to the floor, and you should only be able to slide two fingers underneath it. If it rides up in back, the band is too big.

The cups should contain all breast tissue with no spilling over the top or sides and no gaping or wrinkling. The center gore should tack flat against your sternum. Underwires should encircle the breast tissue and rest on the ribcage, never on the tissue itself.

Straps should stay up without digging. They provide only about twenty percent of support, so if you need them cranked tight to feel held, the band is doing too little and is probably too large. A well-fitting bra feels secure and forgettable, not something you cannot wait to remove.

US, UK, and EU Sizing: Converting Your True Size

Your true size is universal, but the tags are not. US sizing, which this calculator uses, runs A, B, C, D, DD, DDD, G, H, and onward. UK sizing matches the US up to DD, then diverges: where the US has DDD, the UK has E, followed by F, FF, G, GG, and so on, with doubled letters marking the half steps the US skips.

EU sizing works differently altogether. The band is your underbust in centimeters rather than inches, so a US 34 roughly becomes an EU 75, and cup letters follow their own progression. French and Spanish sizing add yet another wrinkle by adding 15 to the EU band number. A conversion chart is essential whenever you cross these borders.

The practical method is simple: calculate your true size once, then convert it with the target country’s chart before shopping. A US 32DDD becomes a UK 32E and roughly an EU 70F. Keep a conversion note on your phone next to your sister sizes, and international bra shopping becomes as straightforward as domestic.

Be aware that cup volume still follows the band within each system, so the sister-size principle applies everywhere. A UK 34E and a UK 32F are sisters just as surely as their US equivalents, because the underlying geometry of bodies does not change at national borders.

How Bra Sizes Change Over a Lifetime

Your true size is not permanent. Weight changes shift both measurements: gaining weight typically increases the band first, while the cup follows depending on where your body stores tissue. Losing weight reverses the pattern, often dropping band sizes faster than cup letters.

Pregnancy and breastfeeding cause some of the largest swings, with many people going up several cup sizes and at least one band size. A nursing bra bought in the third trimester may not fit two months postpartum, which is why fitters recommend waiting until breastfeeding is established before investing in an expensive nursing wardrobe.

Aging changes tissue density and elasticity, which alters how a given size fits even when the tape shows the same numbers. Menopause, hormonal therapies, and even long-term exercise routines reshape the bust gradually. The calculator’s math never changes, but your inputs will, so treat measuring as a recurring habit rather than a one-time event. An annual two-minute measure keeps every purchase accurate.

7 Tips for Better Bra Shopping

1. Measure before every major purchase. A quick re-measure beats assuming last year’s size still fits.
2. Start with the calculator’s size. Try it first in the fitting room, then adjust based on feel.
3. Bring your sister sizes. Save them on your phone for instant alternatives in unfamiliar brands.
4. Fasten on the loosest hook. New bras should fit on the loosest setting so you can tighten as the elastic relaxes.
5. Scoop and swoop. Lean forward and guide all tissue into the cups after putting the bra on.
6. Try the same size in multiple styles. Balconette, plunge, and full-coverage cuts fit the same size differently.
7. Replace stretched-out bras. When the band no longer grips on the tightest hook, its labeled size is fiction.

Frequently Asked Questions

1. How is bra size calculated?

Round your underbust measurement to the nearest even number for the band, then subtract the band from your bust measurement and convert each inch of difference to a cup letter: one inch is A, two is B, three is C, four is D, and so on.

2. What is the difference between band size and cup size?

The band is the number, based on your ribcage measurement, and provides most of the support. The cup is the letter, based on the bust-minus-band difference, and describes the cup volume relative to that band.

3. Why do I get a different size than the store told me?

Many stores still use the outdated add-inches method or steer customers toward the sizes they stock. The calculator uses the modern measurement method that professional fitters recommend.

4. What are sister sizes?

Sizes with the same cup volume but different bands, like 34D, 32DD, and 36C. Moving down a band means moving up a cup letter to keep the volume identical.

5. My underbust is an odd number. What band do I wear?

Round to the nearest even number, since bands are made in even sizes. An underbust of 31 becomes a 32 band; 33 becomes a 34 band.

6. How tight should the band be?

Snug and level around your ribcage. You should fit two fingers under it but not be able to pull it far from your body. It should stay put without the straps holding it up.

7. What does it mean if the gore does not touch my chest?

The center gore should lie flat against your sternum. If it floats, the cups are usually too small or the band is too loose, pushing the cups apart.

8. Should the calculator work for teenagers?

Yes, the math is the same at any age. Growing bodies change quickly, so re-measure every few months during growth spurts rather than relying on an old size.

9. Do I need a different size for sports bras?

Start with your calculated size. Many sports bras use standard band and cup sizing; compression styles that use S, M, L need the brand’s conversion chart instead.

10. Why does my cup size change when the band changes?

Cup letters are relative to the band. A D cup on a 32 band is much smaller in volume than a D cup on a 38 band, which is exactly why sister sizes adjust the letter when the band changes.

11. How often should bras be replaced?

Every six to twelve months with regular wear, or when the band stretches to the point that even the tightest hook feels loose. Rotating several bras extends each one’s life.

12. What is the scoop and swoop method?

After putting on a bra, lean forward and use your hands to guide all breast tissue from under your arms into the cups. It ensures nothing is trapped under the band or wires and improves fit immediately.

13. Can I use centimeters instead of inches?

This calculator takes inches. To convert, divide your centimeter measurement by 2.54 and round to the nearest half inch before entering it.

14. Why do straps keep falling down?

Usually because the band is too loose, forcing the straps to carry weight they cannot manage, or because the strap placement does not suit your shoulder shape. Fix the band size first.

15. Is professional fitting better than a calculator?

A good fitter adds judgment about shape and style that math cannot, but the calculator gives you the correct starting size, which protects you from bad fittings. Use both: calculate first, then get fitted.

CONCLUSION

Your bra size is not a mystery and not a matter of opinion; it is two measurements and a little arithmetic. Measure your underbust and bust, let the calculator do the math, and walk into your next fitting with your true size and both sister sizes in hand. The difference between guessing and knowing is the difference between enduring your bra and forgetting you are wearing one.