everyday life

Bra Size Calculator

Getting the right bra size requires just two measurements: your underbust and full bust. The difference between these numbers determines your cup size, while your underbust measurement tells you the band size. This calculator converts your measurements into US, UK, and European sizing systems, eliminating confusion when shopping across brands or internationally. Proper fit matters—an ill-fitting bra compromises both comfort and support.

Last updated: April 23, 2026

Creators Jasmine J. Mah, BSc

Reviewers Steven Wooding and Tibor Pál

8,931 people find this calculator helpful

How to Measure for Your Bra Size

Accurate measurements are the foundation of finding your correct bra size. Measure while braless or wearing a non-padded bra to avoid artificial padding that skews results.

Underbust measurement: Wrap a soft measuring tape directly under your bust, keeping it level and parallel to the ground. The tape should sit snugly but not dig into skin. Exhale fully before noting the measurement. This number determines your band size.
Bust measurement: Measure around the fullest part of your chest with the tape held loosely—it should glide freely as you breathe. Don't pull tight or allow slack. This measurement combined with your underbust reveals your cup size.

Take both measurements twice to verify accuracy. If they differ, use the larger figure. Record to the nearest half-inch or centimetre; rounding up is acceptable if you fall between sizes.

Bra Size Calculation Formula

Your bra size consists of a band number (derived from underbust) and a cup letter (derived from the bust-to-underbust difference). The formulas below show how these measurements translate into sizing across different systems.

Cup Size Difference = Bust Measurement − Underbust Measurement

Band Size = Underbust Measurement (rounded to nearest whole inch or cm)

Cup Letter = Lookup(Cup Size Difference) in regional cup size chart

Example: Underbust 34 inches, Bust 36 inches

Difference = 36 − 34 = 2 inches → Cup B

Result: US size 34B, UK size 34B, EU size 75B

Underbust Measurement — Distance around your chest directly beneath the bust, measured snugly but not compressively
Bust Measurement — Distance around the fullest part of your bust, measured loosely to allow natural breathing
Cup Size Difference — Numeric difference in inches or centimetres between bust and underbust measurements, translating to letter sizes A through K

Understanding Cup Sizes and Sister Sizes

Cup size isn't absolute across band sizes. A 32D and 34C contain the same cup volume despite different letter designations—these are called sister sizes. Manufacturers create a limited number of cup moulds, then pair them with various band widths and assign different letters accordingly.

This practical approach means:

If your calculated size is unavailable, sister sizes offer identical cup volume with different band tightness
A band that feels too snug can be upsized one band and downsized one cup letter (e.g., 32D → 34C) for the same cup fit
Conversely, a band that feels loose can be downsized one band while upsizing the cup (e.g., 34C → 32D)

Cup sizing differs between US, UK, and European systems. The UK system uses DD and E interchangeably, while US brands distinguish between DD, DDD, and DDDD. European sizing follows its own progression. Always check regional charts when shopping internationally.

International Sizing Systems

Bra sizes vary by country due to different measurement standards and manufacturing conventions. The three main systems are:

US sizing: Band sizes in inches (32, 34, 36, 38, etc.), cups from AA to K. Common in North America and increasingly worldwide.
UK/Australian sizing: Band sizes in inches with identical numbering to US, but cup sizes differ (AA, A, B, C, D, DD, E, F, FF, G, GG). Widely used in Commonwealth countries.
European sizing: Band sizes in centimetres (65, 70, 75, 80, 85, etc.), representing underbust measurement plus 15 cm. Cup progression uses A through L with double letters appearing later in the range.

Conversion between systems requires knowing both your band and cup in one system, then consulting conversion charts. Band numbers don't translate directly between inches and centimetres; a 34 inch band is approximately 75 cm in European sizing.

Common Fitting Mistakes and How to Avoid Them

Even with correct measurements, poor fitting technique compromises comfort and support.

Fastening the band incorrectly — Always fasten your bra on the loosest hook first, allowing room to tighten it as the elastic relaxes over time. Put on the bra by leaning forward to scoop breast tissue into the cups before fastening, ensuring full contact and support. The band should sit level around your torso—if it rides up in the back, the band is too large or the cups are too small.
Ignoring band migration signals — If the band rides upward during the day or feels uncomfortably tight within an hour of putting it on, the size needs adjustment. A band riding up indicates insufficient support; tightening the straps won't fix this fundamental issue. Conversely, if you can pull the band more than two finger-widths away from your ribs, the band is too loose.
Assuming the same size across brands — Bra sizing lacks standardization; fit varies between manufacturers and styles within the same brand. Always try on multiple sizes, including sister sizes, when shopping. Padded, balconette, and full-coverage styles fit differently even in identical measurements. What works in one brand may require adjustment in another.
Neglecting strap adjustment — Straps sit at the edge of your shoulder, not the side of your neck. If straps slip or create shoulder marks, adjust them inward on the back of the bra or size down the band. Conversely, straps shouldn't dig into shoulders or pull the entire bra upward during movement.

Frequently Asked Questions

Why do my measurements keep changing when I re-measure?

Breast tissue responds to hormonal cycles, body weight changes, and even time of day. Measure consistently at the same point in your cycle for the most reliable baseline. Morning measurements tend to be more accurate than evening, when swelling from activity or standing increases. If measurements vary by more than half an inch, take the average of multiple attempts. Soft measuring tapes can stretch over time, so replace yours annually to maintain accuracy.

What does it mean if no bra in my calculated size feels comfortable?

Your measurements may be accurate, but breast shape, root width, and projection vary individually—factors that numbers alone cannot capture. Some people have shallow breast tissue that fits better in larger cups with smaller bands (using sister sizes), while others have projected shapes requiring specialized cuts. Try sister sizes first, then consider that certain styles (balconette versus full-coverage) accommodate different shapes. Don't force fit; visit specialty lingerie shops where staff can assess fit visually beyond calculations.

Can I measure my bra size without removing my current bra?

No. Existing bras compress and shape your chest unnaturally, preventing accurate measurement. Padding, underwires, and elastic all distort the true dimensions needed for sizing. Measure braless in front of a mirror to ensure the tape lies level and straight. If you're self-conscious, measure over a thin vest or t-shirt without a bra, but be consistent—always use the same layer if remeasuring.

How often should I recalculate my bra size?

Major life events warrant recalculation: after pregnancy, significant weight loss or gain, hormonal changes, or if your current bra consistently feels uncomfortable. Most people remain the same size for several years if weight is stable. However, bra elastic gradually loosens, so if your comfortable size shifts over time, bras may have stretched rather than your body changing. Replace well-worn bras and re-measure before assuming you've sized up.

Are European sizes really different, or just converted?

European sizing uses centimetres instead of inches, so a 75 cm band doesn't simply convert from 34 inches—it's slightly different (75 cm ≈ 29.5 inches underbust). The cup progression also differs; an EU 75C is not identical to a US 34C. Always verify cup size separately when converting, as cup letters don't align directly across systems. Use a conversion chart that accounts for both band and cup differences, not just one or the other.

What if one breast is larger than the other?

Most people have slight asymmetry; significant differences are still common. Measure each side separately, then calculate based on the larger measurement to ensure adequate cup coverage. Some bras accommodate this through stretchy fabrics and padding that mould to individual shape. If asymmetry is pronounced, specialized lingerie brands offer customized fitting options. Never size down to fit the smaller side, as this creates inadequate support and coverage for the larger side.

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