Bra Size Calculator
Estimate a US bra size from underbust and bust measurements. Get a practical band, cup, cup difference, and simple sister-size suggestions based on common sizing charts.
Perfect Body Tape Measure - 80 Inch Automatic Telescopic Tape Measure - Retractable Measuring Tape for Body: Waist, Hip, Bust, Arms, and More (White - 80 inch)
Retractable body tape measure for taking bust and underbust measurements.
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A bra size is a practical label that combines two measurements: the band and the cup. The band is the number, such as 32, 34, or 36 in US sizing, and it describes the part of the bra that wraps around the ribcage. The cup is the letter, such as B, C, D, or DD, and it describes the difference between the bust measurement and the band measurement. Together, those two pieces create a starting size such as 34C or 36DD.
This calculator estimates a US bra size from an underbust measurement and a bust measurement. It uses a common chart-based approach: round the underbust to the nearest even band number, subtract that band from the bust measurement, and match the difference to a cup label. The result is useful when you are shopping online, comparing labels, or narrowing down which sizes to try first.
Treat the answer as an estimate rather than a final rule. Bra sizing is not fully standardized across every brand, market, or style. A smooth T-shirt bra, a balconette, a sports bra, and an unlined lace style can all feel different even when the tag shows the same size. The best size is the one that feels supportive, comfortable, and practical for the way you plan to wear it.
The two measurements should be taken with the same tape and in the same unit system. If you choose metric, enter both measurements in centimeters. If you choose imperial, enter both measurements in inches. The calculator converts centimeters to inches internally because common US cup charts are based on inch differences.
For the underbust measurement, place the tape around the ribcage where the bra band normally sits. Keep the tape level all the way around, pull it snugly, and breathe normally. It should not dig in, but it should be close enough to represent a supportive band. For the bust measurement, measure around the fullest part of the bust while keeping the tape level and relaxed. If a particular bra style changes your shape significantly, measuring in a light, non-padded bra can give a more useful starting point.
If you measure more than once and get slightly different numbers, use the average or the measurement that best matches how snugly you like a band to feel. Small changes of half an inch can affect the cup estimate, so consistency matters more than trying to force a perfect number.
This calculator uses a transparent nearest-even-band method for the band. A measured underbust of 32 inches estimates to a 32 band. A measured underbust near 33 inches estimates to a 34 band because US bands are usually even numbers. This is different from some older methods that add four or five inches to the underbust before choosing the band. The nearest-even approach is easy to audit because the displayed band comes directly from the measurement.
After the band is selected, the calculator subtracts the band from the bust measurement. The result is the cup difference. A difference near 1 inch maps to A, 2 inches maps to B, 3 inches maps to C, 4 inches maps to D, 5 inches maps to DD, 6 inches maps to DDD, 7 inches maps to G, and the sequence continues through larger cup labels. Since real measurements rarely land on exact whole inches, the cup difference is rounded to the nearest whole-inch chart step.
| 0 inches | AA |
| 1 inch | A |
| 2 inches | B |
| 3 inches | C |
| 4 inches | D |
| 5+ inches | DD, DDD, G, H, and beyond |
For example, an underbust of 31.5 inches rounds to a 32 band. If the bust measurement is 36 inches, the difference is 4 inches, so the estimate is 32D. If the same person prefers a looser band, a sister size such as 34C may be worth trying, but the calculator keeps the main result tied to the measured band method.
Sister sizes are neighboring sizes that keep roughly similar cup volume while changing the band. The most common one-step pattern is simple: when the band goes down by one band size, the cup goes up by one letter step; when the band goes up by one band size, the cup goes down by one letter step. In US sizing, one band step is usually 2 inches, so 34C has sister sizes around 32D and 36B.
Sister sizes are helpful because band comfort is personal. Some people prefer a firmer band for support, while others prefer more ease. A calculator cannot know how a fabric stretches, where the straps sit, or how a cup is shaped, so it provides straightforward one-step options rather than a long chain of possibilities.
Sister sizing does not mean every listed size will fit the same. Wires may be wider or narrower, cup height can change, and a molded cup may behave differently from stretch lace. Use sister sizes as a shortlist for fitting-room or online-order comparisons.
Once you have an estimated size, the next step is checking the fit in a real bra. Start with the band on the loosest hook if the bra is new; this leaves room to tighten it as the elastic relaxes over time. The band should feel secure and should stay roughly level around the body during normal movement. A band that rides up may be too loose, while a band that feels uncomfortable after a short time may be too firm or the wrong shape for that style.
Cup fit is about containment and comfort, not about meeting a single visual rule. The cup should hold the bust without obvious gaping or overflow in the position you plan to wear the bra. The center gore of an underwire bra may sit closer to the body in some styles than in others, and wire width can vary a lot across brands. Strap tension should fine-tune the fit rather than carry all of the support.
Comfort is a valid fit factor. If a chart result technically looks close but does not feel good, try an adjacent size, a sister size, or a different style. The calculator helps narrow the search, but the wearer decides what works.
Bra size systems developed through a mix of manufacturing practice, regional labeling traditions, and consumer fit expectations. That is why a US 34DD, a UK 34DD, and a European size label may not describe the exact same grading sequence. Even within one market, brands can choose different block patterns, wire shapes, cup depths, and fabric stretch. A calculator can apply a clear chart, but it cannot remove all variation from the real products behind the labels.
This calculator deliberately uses a simple method and explains it in plain terms. It does not evaluate posture, tissue distribution, asymmetry, projection, root width, preferred compression, sports support needs, or style-specific construction. Those details can matter when comparing a plunge bra with a full-cup bra, a wire-free bralette, a nursing style, or a high-impact sports bra.
Use the result as a first size to try, then let fit feedback guide the next step. If the band feels right but the cups do not, adjust the cup. If the cups feel close but the band does not, consider a sister size. If neither part feels close, remeasure and test a different starting size. This practical approach keeps the focus on comfort, support, and choice rather than treating any single chart as the only possible answer.
A bra size calculator gives an informed estimate based on common US sizing charts. The right final size can still vary by brand, style, fabric, and personal comfort preference.
The calculator converts your measurements to inches, rounds the underbust measurement to the nearest even band number, then compares the bust measurement with that band. The difference is matched to a common US cup chart where about 1 inch is A, 2 inches is B, 3 inches is C, 4 inches is D, 5 inches is DD, and so on. It is an estimate, not a guarantee, because brands and bra styles use slightly different grading rules.
US band sizes are usually even whole numbers, so a measurement like 31 inches may estimate to a 32 band while 34 inches stays a 34 band. Some older fitting methods add inches to the underbust, but this calculator uses a transparent nearest-even-band approach so you can see how the number was chosen. If you prefer a firmer or looser band, trying the adjacent band and a sister size can be useful.
Sister sizes are neighboring sizes that keep roughly similar cup volume while changing the band. For example, if 34C is close but the band feels too snug, 36B may feel similar in cup volume with a looser band; if the band feels too loose, 32D may be worth trying. Sister sizing is only a starting point because wire shape, cup height, strap placement, and fabric stretch can change the fit.
Use whichever unit is easier and enter both measurements in the unit selected by the toggle. The calculator converts centimeters to inches internally because the US cup chart is based on inch differences. If you switch units, re-enter the numbers in the selected unit so the estimate is based on the correct scale.
The cup estimate depends on the bust measurement being larger than the estimated band number. Recheck that the tape is level, that the underbust and bust entries are in the same unit system, and that the measuring tape is snug but not compressed. If your measurements are close together, an AA or custom fit may be more accurate than a simple chart estimate.
Bra size labels describe a chart position, but fit also depends on the bra's cut, cup spacing, wire width, gore height, strap design, fabric stretch, and the wearer's comfort preferences. Two bras with the same printed size can fit very differently across brands and styles. Treat the calculator result as a practical starting point for trying sizes, not as a final rule.
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