Measure your underbust and bust once, then use our calculator to estimate your band, cup, and sister sizes in US, UK, EU, and AU formats.
Enter your measurements to see results
You only need a soft measuring tape, a mirror, and two numbers: your underbust measurement and your bust measurement. The more carefully you measure, the more useful your starting size will be.
Wrap the tape snugly around your rib cage directly under your bust. Keep the tape level and parallel to the floor.
Measure around the fullest part of your bust while standing straight. A non-padded bra usually gives the most consistent bust measurement.
Choose US, UK, EU, or AU sizing. The calculator applies the cup progression and band tables that match the system you select.
Use the main result as your starting point, then check the sister sizes if the band feels too tight or too loose in a specific bra.
Take your measurements at the end of the day if you notice normal swelling. Exhale gently before measuring your rib cage so your band size is firm but realistic. If your breasts are noticeably different in size, base your bra fit on the larger side and then fine-tune with strap adjustment or removable padding.
If you are using centimeters, measure to the nearest half centimeter. If you are using inches, measure to the nearest quarter inch when possible. Small differences in a bust measurement can change the cup size, especially when you are close to a boundary.
A bra size calculator gives the best result when the numbers reflect your current body. Remeasure after weight change, pregnancy, breastfeeding, hormone shifts, strength training, or when your bras start to ride up, dig in, gape, or spill.
Many people benefit from checking their size every 6 to 12 months. That habit helps you catch small changes before you keep buying bras that feel almost right but never quite fit.
Your result has two parts: a band size and a cup size. The band size is the number, such as 32, 34, or 36 in US sizing. It reflects your rib cage measurement and supplies most of the bra's support. The cup size is the letter, such as B, C, D, or DD. It reflects the difference between your bust and band measurements, not breast volume by itself.
That last point matters. A 32D and a 38D do not have the same cup volume because the band size changes the scale of the cup. This is why sister sizes exist. A 32D is closer in cup volume to 34C or 30DD than it is to 38D. If you understand that relationship, shopping becomes much easier because you know how to adjust when a style runs tight or loose.
The calculator also shows the cup difference used to estimate the cup letter. In many US systems, a 1-inch difference points to an A cup, 2 inches to B, 3 inches to C, 4 inches to D, and 5 inches to DD. UK sizing follows a similar pattern at first, then uses a different letter progression after D. EU sizing is usually shown with centimeter-based band sizes such as 70, 75, or 80.
The band should feel snug on the loosest hook when the bra is new. It should stay level around your body and not ride up your back when you raise your arms.
The cup should fully contain your breast tissue without wrinkling, cutting in, or leaving empty space near the top or sides of the cup.
If the cup volume feels right but the band does not, move to a sister size. Go up one band and down one cup, or down one band and up one cup.
Accurate tape placement, choosing the right sizing system, and checking real fit signs all matter. A calculator gives a strong starting size, but breast shape, cup construction, and brand grading still affect the final fit.
If you want to calculate bra size manually, start with two numbers. First, measure your underbust or frame size. Second, measure your bust at the fullest point. Then use your preferred regional sizing system to convert those measurements into a band number and a cup letter.
In the simplest US method, the cup size comes from the difference between the bust measurement and the band measurement. For example, if your band is 32 inches and your bust is 36 inches, the difference is 4 inches. In standard US cup mapping, that difference points to a D cup, so the starting result is 32D. If the difference were 5 inches, the result would usually be 32DD. UK sizing keeps the same logic for the early cups, then changes the labels after D. EU sizing usually converts the band to a centimeter-based number such as 70, 75, or 80 and then maps the difference in centimeters to a cup.
Suppose your snug underbust measurement is 31.5 inches and your bust measurement is 36.5 inches. The calculator rounds the band to the nearest supported band size, which gives you 32 in US sizing. Next, it compares the bust and band measurements. The difference is about 5 inches, so the starting cup is DD. Your recommended US size is 32DD.
Now imagine the cups feel good but the band feels too tight in one specific bra. Your sister size would usually be 34D. If the band feels too loose instead, you could test 30DDD. The calculator does not replace trying on bras, but it tells you which nearby sizes are most logical to test instead of guessing randomly.
Here is the same idea in EU sizing. If your underbust is about 75 cm and your bust is 90 cm, the difference is 15 cm. That maps to a B cup in many common EU charts, so your starting size is 75B.
1 inch: A cup in many US charts.
2 inches: B cup.
3 inches: C cup.
4 inches: D cup.
5 inches: DD in US sizing or DD in UK sizing.
6 inches: DDD in US sizing or E in UK sizing.
If your underbust is 30 inches and your bust is 34 inches, a common starting point is 30D. This is useful when you want smooth cups under thin tops. If the band feels a touch firm in a molded cup bra, test the sister size 32C before jumping to a totally different cup range.
If your underbust is 32 inches and your bust is 38 inches, the difference is 6 inches. In UK sizing, that often points to 32E. This matters because a US 32DDD and a UK 32E can represent a similar size even though the cup label looks different.
If your rib cage measures about 75 cm and your bust measures 90 cm, many EU charts place you near 75B. This is helpful when an online store lists only continental sizes. Check whether the brand runs shallow or deep in the cup before ordering multiple sizes.
Say your underbust is 33 inches and your bust is 38 inches. A practical starting point is 34DD in US sizing because the nearest supported band is 34 and the difference is about 5 inches. If the band stretches easily, also test 32DDD for a firmer fit.
Use your calculated bra size as a baseline, then check the brand's activity chart. A sports bra should hold you more firmly than an everyday bra, but you should still breathe deeply and move your arms without sharp pressure from the band or straps.
If your body has changed recently, treat the calculator result as a new baseline and remeasure more than once over a few weeks. Band size may settle faster than cup size, and soft-cup styles or bras with extra adjustment can make that transition easier.
When you order a bra online for the first time from a new brand, try your calculated size plus one sister size. That small comparison set tells you quickly whether the brand runs tight, shallow, wide, or tall in the cup.
A common content gap on calculator pages is what to do after you get the number. Two people can enter the same bust measurement and underbust measurement and still prefer different bras because breast shape affects the final fit. The calculator gives you an efficient starting size, but shape explains why one bra in that size feels perfect while another feels off.
If your cups wrinkle near the top, the bra may be too closed on top, too tall for your shape, or slightly large in the cup. If you spill out at the center or sides, the cups may be too small or the wire may be too narrow. If the center gore floats away from your chest, the cups may not be deep enough. If the underwire sits on breast tissue under your arm, you may need a larger cup, a wider wire, or a different style.
Breast shape can also change which style feels best. Full-on-top breasts often prefer stretch lace or more open upper cups. Full-on- bottom shapes may like balconette or demi styles that lift from below. Wide-set breasts often need wider gores or side support. Close-set breasts often do better with lower center gores. The right size is important, but style choice is what turns a good calculation into a comfortable bra.
Riding up in back, sliding during the day, or forcing the straps to do all the work usually points to a loose band.
Gaping, overflow, quad-boob, collapsing fabric, or wires on tissue usually point to a cup issue, not just a strap issue.
If the cup volume feels correct but the tension around your rib cage feels wrong, sister sizing is the fastest adjustment to test.
Strapless, nursing, and high-impact sports bras often need a firmer band or a different construction than your favorite everyday bra.
Quick answers to the bra sizing questions people ask most before they buy.
Measure your underbust snugly right below the breasts and your bust around the fullest point. Keep the measuring tape level, use a non-padded bra if possible, and write down both numbers before entering them into the calculator.
Cup size comes from the difference between your bust and band measurements. In many US charts, 1 inch is A, 2 inches is B, 3 inches is C, 4 inches is D, and 5 inches is DD.
Sister sizes are different labels with similar cup volume. If you move up one band size, go down one cup. If you move down one band size, go up one cup. For example, 34C, 32D, and 36B are common sister sizes.
A band that rides up is usually too loose. The band should sit level around your torso and provide most of the support. Start by testing a smaller band and the matching sister size.
Remeasure every 6 to 12 months, or sooner after weight change, pregnancy, breastfeeding, major training changes, or any time your current bras start to feel wrong.
Start with the nearest band size and then compare the neighboring sister size. Your final choice may depend on how stretchy the band is, how tall the cups are, and how snug you want the fit.
The band numbers can look similar, but the cup sequence changes after D. US sizing often uses DD and DDD, while UK sizing continues DD, E, F, FF, G, and GG.
Yes. Shape affects wire width, cup depth, gore height, and where the fullness sits in the cup. That is why the same measured size can feel different in different bra styles.
A sports bra should feel firmer because it controls movement, but it should still let you breathe deeply and move without pain. Full coverage and a stable band matter more than chasing a loose, lounge-style feel.
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