Bra Size Calculator
Estimate your band and cup size from two simple tape-measure measurements — with US, UK, EU, France, and Australia/NZ size conversions and sister sizes.
📏 Bra Size Calculator
What Is a Bra Size? A Complete Guide
A bra size is made up of two parts: a band number (like 32, 34, or 36) that describes the firm, supportive measurement around the ribcage, and a cup letter (like A, B, C, D) that describes the volume of space needed to hold the bust comfortably. Together, they form a size like "34C" — the number and letter aren't independent of each other, which is exactly why a simple bust measurement alone can't tell you your size; cup volume only makes sense relative to a specific band.
Research on bra fit consistently finds that a large share of people are wearing a size that doesn't actually match their measurements — often because they were sized once, years ago, and never re-measured despite the fact that band and cup size can shift with weight changes, hormonal changes, pregnancy and breastfeeding, and simply the natural wear of an older bra's elastic. Getting the band and cup calculation right is the first step toward a genuinely comfortable, well-supported fit.
Band Size vs. Cup Size — What Each One Actually Means
| Component | What It Measures | Where It's Measured | Role in Fit |
|---|---|---|---|
| Band Size | Firm circumference around the ribcage | Directly under the bust | Provides most of the support; should feel snug but not restrictive |
| Cup Size | Volume difference between bust and band | Around the fullest part of the bust | Determines how much space the cup needs to hold without gaping or spilling |
An important detail that trips a lot of people up: the same cup letter does not mean the same cup volume across different bands. A 32D and a 38D hold very different amounts of volume — the D simply describes the difference between that particular band and bust measurement, not an absolute size. This is why cup letters can only be compared meaningfully alongside their band number.
How to Measure for a Bra Size
Accurate measurements are the foundation of an accurate size estimate. Use a soft, flexible measuring tape (not a rigid one), wear a well-fitting, non-padded bra if possible, and stand naturally in front of a mirror or ask someone to help keep the tape level.
Measure the Band (Underbust)
Wrap the tape firmly and snugly around your ribcage, directly underneath the bust, keeping it level and parallel to the floor all the way around your back. It should feel snug — you should be able to breathe comfortably but not slide a hand underneath easily. Measure while exhaling normally, and round to the nearest whole inch (or half centimeter).
Measure the Bust
Wrap the tape loosely around the fullest part of the bust — typically at nipple level — keeping the tape level with the floor and not pulled tight or compressing the tissue. Stand with your arms relaxed at your sides. Round to the nearest whole inch (or half centimeter).
Calculate the Difference
Subtract the band measurement from the bust measurement. The resulting difference in inches determines the cup letter, following the standard difference-to-cup chart used throughout this calculator.
Round the Band to the Nearest Standard Size
Bra bands are traditionally manufactured in even numbers (28, 30, 32, 34...), so a raw measurement is rounded up to the next even number if it's odd. Many modern brands now also offer odd-numbered bands for a finer fit.
The Bra Size Formula — How Cup Size Is Calculated
The standard sizing method used by this calculator (and widely used across the industry as a measurement-based starting point) follows a simple rule: every one inch of difference between the bust and band measurement equals one cup letter.
Band measurement: 32 in. Bust measurement: 37 in. Difference = 37 − 32 = 5 inches → cup size DD/E. Estimated size: 32DD (US).
Cup Difference Chart
| Bust − Band Difference | US Cup | UK Cup | EU/FR Cup |
|---|---|---|---|
| 0" | AA | AA | A |
| 1" | A | A | B |
| 2" | B | B | C |
| 3" | C | C | D |
| 4" | D | D | E |
| 5" | DD/E | E | F |
| 6" | DDD/F | F | G |
| 7" | G | FF | H |
| 8" | H | G | I |
International Bra Size Conversion Chart
Band and cup naming conventions differ noticeably between regions. The table below shows how a selection of common US sizes translate across the UK, EU, France, and Australia/New Zealand systems.
| US | UK | EU / France | Australia / NZ |
|---|---|---|---|
| 32B | 32B | 70C | 32B |
| 34C | 34C | 75D | 34C |
| 36D | 36D | 80E | 36D |
| 38DD/E | 38E | 85F | 38DD |
| 40DDD/F | 40F | 90G | 40E |
Band Size Reference (Underbust Measurement)
| Underbust (in) | US/UK Band | EU/FR Band (cm) |
|---|---|---|
| 27–28 | 28 | 60 |
| 29–30 | 30 | 65 |
| 31–32 | 32 | 70 |
| 33–34 | 34 | 75 |
| 35–36 | 36 | 80 |
| 37–38 | 38 | 85 |
| 39–40 | 40 | 90 |
| 41–42 | 42 | 95 |
| 43–44 | 44 | 100 |
Sister Sizing — Finding an Equivalent Fit in a Different Band
Sister sizes are different band-and-cup combinations that hold approximately the same cup volume. Moving one band size up while moving one cup letter down (or vice versa) keeps the cup volume roughly constant, which is an extremely useful trick when a particular size is out of stock, when a brand runs differently than expected, or when a calculated size doesn't quite feel right at the first fit check.
34C, 36B, and 32D are all sister sizes — each holds roughly the same cup volume, just distributed across a different band size. If a 34C band feels loose but the cup fits well, a 32D (smaller band, one cup letter up) is a natural next size to try.
Sister sizing is why it's worth trying a couple of adjacent sizes at a fitting rather than assuming a single calculated size is final — cup volume, breast shape, and brand cut all influence which specific combination feels genuinely comfortable.
Signs Your Bra Size Is Right (or Wrong)
A measurement-based calculation is an excellent starting point, but the true test of a size is how it actually feels and looks once worn. Use these checkpoints to confirm or adjust your calculated size.
Signs of a Good Fit
- The band sits level and horizontal all the way around, front and back — not riding up in the back
- The band feels snug on the loosest hook, leaving room to move to tighter hooks as it stretches with wear over time
- The center gore (the piece between the cups) lies flat against the breastbone without gapping
- The cups fully contain breast tissue with no gaping, spilling over the top, or bulging at the sides
- Straps stay in place on their own without digging in or constantly needing adjustment
Common Signs of an Incorrect Fit
- Band too big: Rides up in the back, straps do most of the work and dig into the shoulders, overall support feels weak
- Band too small: Bulging or spillage above and below the band, visible indentation marks, feels restrictive when breathing
- Cup too small: Breast tissue spills over the top or sides, underwire doesn't sit flat against the ribcage
- Cup too big: Fabric wrinkles or gaps at the top of the cup, underwire sits away from the body instead of flush against it
Real-World Bra Size Calculation Examples
Seeing the formula applied to a few different measurement combinations makes it easier to trust the math behind your own result. Each example below follows the same two-step process: round the band, then subtract it from the bust measurement to find the cup difference.
Example 1 — A Smaller Band, Larger Cup Difference
Band measurement: 29 in (rounds up to an even 30). Bust measurement: 35 in. Difference = 35 − 30 = 5 inches.
- Estimated size: 30DD/E (US), which is 30E (UK) and 65F (EU/FR)
- Nearby sister sizes to also try: 28G and 32D
Example 2 — A Common Mid-Range Size
Band measurement: 33 in (rounds up to an even 34). Bust measurement: 37 in. Difference = 37 − 34 = 3 inches.
- Estimated size: 34C (US/UK), which is 75D (EU/FR)
- Nearby sister sizes to also try: 32D and 36B
Example 3 — Measurements Taken in Centimeters
Band measurement: 81 cm (≈31.9 in, rounds to an even 32). Bust measurement: 96 cm (≈37.8 in, rounds to 38). Difference = 38 − 32 = 6 inches.
- Estimated size: 32DDD/F (US), which is 32F (UK) and 70G (EU/FR)
- This example shows why the calculator converts centimeter input to inches internally before rounding, since the standard cup-difference chart is built around whole-inch increments
Example 4 — A Larger Band Size
Band measurement: 41 in (rounds up to an even 42). Bust measurement: 45 in. Difference = 45 − 42 = 3 inches.
- Estimated size: 42C (US/UK), which is 95D (EU/FR)
- At larger band sizes, trying both the calculated size and its two nearest sister sizes is especially worthwhile, since fit sensitivity to breast shape tends to increase alongside band and cup size
Advanced Fit Considerations
Why Sizing Isn't Perfectly Standardized
Unlike some clothing categories with tightly regulated sizing standards, bra sizing varies meaningfully between manufacturers. Two brands can both label a bra "34C" and produce noticeably different fits, because cup shape, wire width, band elastic, and overall pattern design all differ by brand and even by specific style within the same brand. This is exactly why a calculated size should be treated as a starting point for trying on, rather than a guaranteed final size across every brand. Some manufacturers also use entirely different internal sizing blocks depending on whether a style is a everyday t-shirt bra, a sports bra, or a specialty bralette, which is part of why the same person can genuinely need three different labeled sizes across three different styles from the very same brand.
Breast Shape and Cup Fit
Two people with mathematically identical band and cup measurements can still need different bra styles, because breast shape — how tissue is distributed within a similar overall volume — affects which cup cut fits comfortably. Fuller-on-top, fuller-on-bottom, shallow, and asymmetric shapes (having one side naturally larger than the other, which is extremely common) all interact differently with a given cup design, which is part of why trying on a couple of styles at a similar calculated size is genuinely useful. A cup that gapes at the top on one shape might fit another shape of the exact same volume perfectly, simply because the tissue sits differently within the same overall space.
Why Band Size Matters More Than Most People Expect
Because the band — not the straps — provides the majority of a bra's support, a band that's too large is one of the most common fit mistakes and one of the most common causes of discomfort, even when the cup size feels approximately right. Many fit guides recommend sizing down in the band and correspondingly up in the cup (using sister sizing) as a very common correction from a person's previously assumed size. A properly snug band also means a bra doesn't need to be worn on its tightest hook right away — a fresh bra should close comfortably on the loosest of its two or three hook settings, leaving room to tighten as the elastic naturally relaxes with wash-and-wear cycles over its lifespan.
Measurement Changes Over Time
Body measurements are not static. Weight fluctuations, hormonal cycles, pregnancy and breastfeeding, and simply the natural stretching of a bra's elastic over months of wear can all shift the ideal size. Re-measuring every six to twelve months, or after any significant body change, helps keep a wardrobe fitting well rather than relying on a size determined once and never revisited. It's also worth re-measuring both numbers together rather than assuming only one has changed — a shift in the band measurement often changes the ideal cup letter as well, even if the bust measurement itself hasn't moved much.
When to Consider a Professional Fitting
A measurement-based calculator is a reliable and private starting point, but a professional, in-person fitting can be especially useful in a few specific situations: right after a major body change like pregnancy or significant weight change, when shopping for a specialty need like post-surgical or nursing bras, or when a calculated size and its sister sizes all consistently feel wrong despite careful re-measuring. A trained fitter can visually assess tissue distribution and shape in ways that two tape-measure numbers alone can't fully capture.
Frequently Asked Questions About Bra Sizing
Answers to the most commonly searched bra size questions — from the basic formula to international conversions.
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