Bra Size Calculator
Measure your underbust and full bust circumference to calculate your bra size in US, UK, EU, and Australian sizing. Includes step-by-step measuring instructions and guidance on fit.
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Enter your values above to see the results.
Tips & Notes
- ✓For the most accurate underbust measurement, pull the tape snugly around your ribcage just below the bra band position — not so tight it digs in, but firm enough to have no slack.
- ✓For the full bust measurement, stand straight with arms down and measure at the fullest point, keeping the tape parallel to the floor. Slight curve of the back can shift the tape and affect the reading.
- ✓The traditional +4/+5 method often results in a too-large band — modern fitting (using the snug underbust measurement directly and rounding to the nearest even number) frequently produces a better-fitting band.
- ✓Sister sizes let you adjust fit without changing cup volume: if 34C fits in cup but the band is loose, try 32D. If the band is too tight, try 36B. The cup volume is identical in all three.
- ✓Bra sizing varies significantly between brands and styles — use your calculated size as a starting point and expect to try adjacent sizes. The correct fit means: band lies horizontal, cups contain all breast tissue without spillage, underwire (if present) lies flat against the chest.
Common Mistakes
- ✗Measuring the underbust while fully exhaling or inhaling deeply — measure at the end of a normal relaxed exhale for the most consistent band size measurement.
- ✗Measuring the full bust over clothing or with arms raised — measure in a non-padded bra or no bra, with arms relaxed at sides, for accurate cup size calculation.
- ✗Assuming the same size across all brands — a 34C in one brand may fit like a 32D in another due to different construction standards. Use calculated size as a starting point only.
- ✗Not trying the bra on the loosest hook — the bra should fit correctly on the loosest hook when new, allowing you to tighten as the band stretches with wear over time.
- ✗Dismissing sister sizing as a significantly different size — going from 34C to 32D is not "going up a cup size" in terms of cup volume. It is the same cup volume in a smaller, more supportive band.
Bra Size Calculator Overview
Bra sizing is a two-part measurement system: the band (number) reflects ribcage circumference, and the cup (letter) reflects the volume difference between bust and band. Getting both measurements right is essential — a correct cup in the wrong band size will not fit properly.
Band and cup size calculation method:
Band size calculation (US/UK method): Underbust measurement (inches): If even: band size = underbust + 4 If odd: band size = underbust + 5 Modern fit method (often more accurate): round to nearest even number directly EU band size = underbust circumference in cm, rounded to nearest 5 Cup size calculation: Bust difference = full bust (inches) − band size 0 inch → AA, 0.5 → A, 1 → A, 2 → B, 3 → C, 4 → D, 5 → DD/E, 6 → DDD/F, 7 → G
EX: Underbust = 31 inches, full bust = 36 inches Traditional method: odd underbust → band = 31 + 5 = 36 Bust difference = 36 − 36 = 0 → AA cup Result: 36AA — but this often produces a too-loose band. Modern fitting method: round 31 to 32 → band = 32 Bust difference = 36 − 32 = 4 → D cup Result: 32D — likely a better fit for many people. The modern method often gives smaller band, larger cup — this is intentional and reflects how bras are actually constructed.
Sister sizing — equivalent cup volume across band sizes:
Sister sizing (equivalent volume in different band sizes): A bra sister size has the same cup volume but different band size. Going up one band size → go down one cup letter (same volume, looser band) Going down one band size → go up one cup letter (same volume, tighter band) Sister size examples for 34C: 32D (tighter band, same cup volume), 36B (looser band, same cup volume)
EX: Person measures as 34C but the band feels too loose: Sister size down: 32D — same cup volume, 2-inch smaller band The "D" appears to be a larger cup, but in a smaller band, the volume is identical. Many people misread sister sizing as a significant cup size change when it is not. This is why going from 34C to 32D feels like a correct fit rather than "going up a cup."
Cup size from bust difference — complete reference:
| Bust difference (in) | US/UK cup | EU cup | AU/NZ cup |
|---|---|---|---|
| Below 1 inch | AA or A | AA | AA |
| 1 inch | A | A | A |
| 2 inches | B | B | B |
| 3 inches | C | C | C |
| 4 inches | D | D | D |
| 5 inches | DD / E | E | DD |
| 6 inches | DDD / F | F | E |
| 7 inches | G | G | F |
Fit problem troubleshooting guide:
| Fit issue | Likely cause | Adjustment |
|---|---|---|
| Band rides up in back | Band too large | Go down 1–2 band sizes; adjust cup up accordingly |
| Straps dig in or slip off | Band providing insufficient support | Smaller band — straps should not bear weight |
| Cup spillage or quadraboob | Cup too small | Go up 1 cup size |
| Cup gapes or wrinkles | Cup too large or wrong shape | Go down 1 cup; try different cup shape |
| Center gore floats off chest | Cup too small or wrong style | Go up 1 cup size; try plunge style |
| General discomfort, correct size | Wrong bra construction for anatomy | Try different brands — construction varies significantly |
Sizing systems differ significantly between countries. A UK 34D equals a US 34D, but translates to an EU 75D and an AU/NZ 16D. EU sizing uses underbust in centimeters (rounded to nearest 5) for the band and the same cup letters. French and Italian sizing adds 15 to EU band sizes. Australian sizing uses a number-based size system (8–22) for bands rather than inches. When shopping internationally or ordering online, always verify the specific brand's sizing chart — construction and sizing conventions vary even within the same country.
Frequently Asked Questions
You need two measurements: underbust and full bust. For underbust: stand straight, exhale normally, and measure directly under your bust where the bra band sits — the tape should be snug but not compressing the skin. Note whether this number is odd or even for the traditional calculation method. For full bust: keep your arms down and measure at the fullest point of your bust, typically across the nipples, keeping the tape horizontal. If you are wearing a bra, use a non-padded bralette or soft cup bra for this measurement — a padded bra adds artificial volume that will inflate your cup size calculation. Take each measurement twice and average them.
Most people wear a band size that is too large because larger bands feel more comfortable when trying on bras — they offer less resistance. However, a loose band provides less support (it rides up), puts more strain on the straps, and causes the cup to sag. Modern fitting specialists consistently find that women who think they are a 36C or 38C are often better fitted as a 32F or 34E — the same cup volume, but in a band that actually stays in place and distributes support correctly. The cup letter looks larger but the physical cup is the same volume. This is why "full bust" retailers with extended cup sizes are growing rapidly — they are correctly sizing customers who were previously wearing incorrect sizes.
US and UK sizes share the same numerical band sizes (inches) and the same cup letter system through D, but diverge at larger cup sizes: what the US calls DDD, the UK calls F. This means a UK 34E equals a US 34DDD. EU sizes use centimeters for the band (the underbust measurement rounded to the nearest 5), so a 34-inch band (86 cm) corresponds to EU band 85. EU cup letters generally align with UK letters. Australian/New Zealand sizing uses a completely different band number system (dress-size based: 8, 10, 12, 14, etc.) where a 34-inch band roughly corresponds to an AU size 14. Cup letters are similar to UK. When shopping internationally, always check the brand's specific size chart rather than assuming conversions.
Common fit problems and what they indicate: the band riding up at the back means the band is too large — a correctly fitting band stays horizontal. Straps digging in or leaving marks means the band is too loose and the straps are compensating by bearing too much weight. Cup spillage (breast tissue overflowing at the top, sides, or underwire) means the cup is too small. Cups wrinkling or gaping means the cup is too large. The center gore (the bridge between cups) floating off the chest means the cups are too small. Underwire sitting on breast tissue rather than flat against the chest means the cup is too small or the underwire shape does not match breast placement. A correctly fitting bra sits comfortably, stays in place without adjustment, and supports without pressure points.
Yes — both band size and cup size can change with body weight changes. Band size tracks ribcage circumference, which changes with significant weight gain or loss (typically more than 5–7 kg). Cup size in terms of letter may change or stay the same depending on where weight is gained or lost. Some people store more breast tissue fat; others gain or lose proportionally across the body. Post-pregnancy and breastfeeding typically produce significant temporary size changes — band size may increase due to ribcage expansion, and cup size often increases significantly during lactation. Most people find it useful to re-measure after significant weight changes (more than 5 kg) or changes in life stage (pregnancy, post-breastfeeding, menopause).
Sizing principles are the same for both styles, but fit experience differs. Underwire bras rely on the wire lying flat against the ribcage and circling completely under the bust tissue — if the cup is too small, the wire sits on breast tissue rather than against the chest wall, causing discomfort. Underwire bras provide more defined shape and lift. Soft cup bras are more forgiving of cup size variations and more comfortable for casual wear, sleeping, or lower-impact activities. For very large cup sizes (D and above), underwire typically provides better support. For accurate sizing, measure in whatever style you prefer to wear and start with that calculation. If the underwire causes discomfort at the calculated size, verify cup size first (going up a cup often resolves this) before assuming the style is wrong.