Free Bra Size Calculator
Two measurements. Six sizing systems. Your size in seconds, with sister size alternatives for a better fit. No signup, no data stored.
A bra size calculator takes two body measurements, underbust (band) and bust circumference, and converts the difference into a bra size. In the US system, each inch of difference between bust and band equals one cup size: 1 inch = A, 2 inches = B, 3 inches = C, and so on. A 2012 study in the journal Ergonomics found that about 80% of the 262 women studied were wearing the wrong bra size, most often a band too large and a cup too small. Our calculator covers six international sizing systems (US, UK, EU, FR, AU, JP) and shows sister sizes so you have options if a band feels off.
Bra Size Chart
Cup size is the difference between your bust and underbust measurements. This chart shows how that difference translates across US, UK, and EU systems. Note that US and UK cup letters diverge after D.
| Bust-Band Difference | US Cup | UK Cup | EU Cup |
|---|---|---|---|
| 1 inch / 2.5 cm | A | A | A |
| 2 inches / 5 cm | B | B | B |
| 3 inches / 7.5 cm | C | C | C |
| 4 inches / 10 cm | D | D | D |
| 5 inches / 12.5 cm | DD/E | DD | E |
| 6 inches / 15 cm | DDD/F | E | F |
| 7 inches / 17.5 cm | G | F | G |
| 8 inches / 20 cm | H | FF | H |
| 9 inches / 22.5 cm | I | G | I |
| 10 inches / 25 cm | J | GG | J |
International Bra Size Conversion Chart
Band sizes translate differently across regions. US and UK use inches, EU uses centimeters, FR adds 15 to the EU band, AU uses dress sizing, and JP puts the cup letter first. Here are some common conversions:
| US | UK | EU | FR | AU | JP |
|---|---|---|---|---|---|
| 32A | 32A | 70A | 85A | 10A | A70 |
| 32B | 32B | 70B | 85B | 10B | B70 |
| 34B | 34B | 75B | 90B | 12B | B75 |
| 34C | 34C | 75C | 90C | 12C | C75 |
| 34D | 34D | 75D | 90D | 12D | D75 |
| 36C | 36C | 80C | 95C | 14C | C80 |
| 36D | 36D | 80D | 95D | 14D | D80 |
| 36DD | 36DD | 80E | 95E | 14DD | E80 |
| 38C | 38C | 85C | 100C | 16C | C85 |
| 38D | 38D | 85D | 100D | 16D | D85 |
Full conversion is calculated automatically when you use the tool above. This table covers the most common sizes. For sizes outside this range, use the calculator.
Sister Sizes Explained
Sister sizes are bra sizes with the same cup volume but different band lengths. They are your backup plan when a bra almost fits but the band is too tight or too loose.
Here is how it works: go down a band size and up a cup letter, or go up a band size and down a cup letter. The cup holds the same amount of breast tissue either way.
| Tighter Band | Your Size | Looser Band |
|---|---|---|
| 28D | 30C | 32B |
| 30DD | 32D | 34C |
| 32DD | 34D | 36C |
| 34DD | 36D | 38C |
| 36DD | 38D | 40C |
| 32DDD | 34DD | 36D |
When to use sister sizes: If you find a bra where the cup fits well but the band digs in, try the next band size up with one cup letter down. If the band slides around but the cup is fine, go the other direction. The calculator above shows your sister sizes automatically.
How to Measure Your Bra Size at Home
All you need is a flexible tape measure and two minutes. If you do not have a tape measure, a piece of string and a ruler work fine.
Get a soft measuring tape
Use a flexible fabric measuring tape. If you do not have one, wrap a piece of string around yourself and measure the string against a ruler.
Wear an unpadded bra or no bra
Padded or molded cups add volume that throws off the measurement. A thin, unlined bra or no bra gives the most accurate numbers.
Measure your underbust
Wrap the tape snugly around your ribcage, directly under your breasts. Keep it level and parallel to the floor. Write down the number.
Measure your bust
Wrap the tape around the fullest part of your bust. Keep the tape level but relaxed, not pulled tight. It should rest on your skin without digging in.
Note both numbers
Write down the underbust and bust measurements in inches or centimeters. Double-check that you used the same unit for both.
Enter into the calculator
Type your two measurements into the calculator above, pick your unit, and hit Calculate. You will get your size in six international systems plus sister sizes.
Tip: Measure at roughly the same time of day. Breast tissue can swell slightly during your menstrual cycle, after exercise, or in hot weather, which can shift the result by half a cup size.
What Is a Bra Size Calculator?
A bra size calculator is a measurement-based tool that converts the relationship between your underbust and bust circumference into standardized bra sizes. It replaces the guesswork of trying on bras at random with a calculated starting point based on your actual proportions.
Works well for
- ✓Finding a starting size when you have not been measured recently
- ✓Shopping internationally and needing to convert between systems
- ✓Figuring out sister sizes when a bra almost fits but not quite
- ✓Ordering online when you cannot try before buying
Less suited for
- ✗Maternity or nursing bra sizing (size fluctuates too much)
- ✗Post-surgical compression garments (different sizing system)
- ✗Medical assessments (consult a healthcare provider)
- ✗Adolescents whose bodies are still developing
Keep in mind
- If your breasts are noticeably asymmetric, fit to the larger side and use a pad or insert on the other.
- Athletic compression bras size differently. Use this as a baseline and adjust for your sport.
- Weight changes, hormonal shifts, and aging all move the target. Re-measure periodically.
How Our Calculator Works
The calculation, step by step
- Take two measurements: underbust (band) and bust circumference.
- Round the underbust to the nearest even number for US/UK band size.
- Subtract the band from the bust. Each inch of difference is one US cup size.
- Apply regional conversion formulas for EU (centimeters), FR (EU + 15), AU (dress-based bands), and JP (cup before band).
- Generate sister sizes by shifting one band size in each direction.
What we assume
- You measured at the correct anatomical positions.
- The tape was level and parallel to the floor.
- You stood in a relaxed, upright posture.
- Measurements reflect your normal body state.
What we simplify
- We use standard conversion formulas; some brands deviate.
- We do not account for breast shape, projection, or root width.
- We assume symmetric sizing. For asymmetry, fit to the larger side.
About accuracy
Any calculator gives you a starting point, not a guarantee. Bra sizing varies between brands, and two bras labeled the same size can fit very differently depending on the cut, fabric, and style. Use the result to narrow your options, then adjust from there.
Common Bra Fitting Problems and How to Fix Them
If something feels off, the fix is usually a one-step adjustment, not a completely different size.
Band rides up in the back
The band is too loose. Go down one band size and up one cup size (36C becomes 34D). A properly fitting band stays level all the way around your torso.
Cups wrinkle or gap
The cup is too large, or the shape is wrong for you. Try a smaller cup first. If only the top gaps, a demi or balconette cut may work better than going down a size.
Breast tissue spills over the cup
The cup is too small. Go up one cup size. For side spillage, look for bras with wider side panels or a full-coverage style.
Straps dig in or slip off
Straps should carry only about 10-20% of the support. If they dig in, the band is probably too loose and forcing the straps to compensate. Tighten the band first. For slipping straps, a racerback converter clip can help.
Underwire pokes or sits on breast tissue
The cup is too small, or the wire shape does not match your breast shape. Go up one cup size. The underwire should sit flat against your ribcage and fully enclose all breast tissue without pressing into it.
Center gore does not lie flat
The center piece between the cups should touch your sternum. If it lifts away, the cups are likely too small or too shallow for your projection. Try going up a cup size or switching to a plunge or balconette style.
Life Events That Change Your Bra Size
Your bra size is not a fixed number. These common events shift your measurements, sometimes by more than you would expect.
Weight changes
Gaining or losing around 5 kg (about 10 lbs) typically changes both band and cup size. Breast tissue contains fat, so it responds to weight fluctuations the way the rest of your body does.
Pregnancy and breastfeeding
Breast size can increase by 1-3 cup sizes during pregnancy and shift again during breastfeeding. Re-measure each trimester and again after weaning.
Hormonal changes
Starting or stopping birth control, perimenopause, and menopause all affect breast tissue density. Monthly cycle fluctuations of up to half a cup size are normal too.
Fitness and muscle changes
Building chest and back muscles can change your band size. Long-term high-impact exercise may reduce breast volume as overall body fat decreases.
Aging
Ligament elasticity decreases over time, changing breast shape and position. Many women find that a different cup shape (full coverage instead of demi, for example) works better as they get older.
Surgery
Any breast surgery, whether augmentation, reduction, mastectomy, or reconstruction, means you need a complete re-measurement. Your surgeon can advise on timing.
Beyond the Numbers: Shape and Brand Differences
Breast shape matters as much as size
Two people with the same measurements can need completely different bra styles. Whether your breast tissue is projected, shallow, wide-set, or close-set affects how a bra fits even when the numbers match. If your calculated size feels off in a particular style, try a different cup shape (full coverage, demi, or plunge) before changing the size.
Brand sizing is not standardized
There is no universal sizing law for bras. A 34C from one brand might fit like a 32D from another. Some brands run large in the cup, others tight in the band. When trying a brand for the first time, ordering your calculated size plus one sister size saves a return trip.
Research and Sources
Evaluation of professional bra fitting criteria
Wood, Cameron, Fitzgerald. Ergonomics, 2012.
Found that 80% of 262 participants wore the wrong bra size. The most common error was a band too large and a cup too small.
Brassiere measurement
Wikipedia. Covers international sizing standards, band and cup calculation methods, and historical context.
EN 13402: European standard for clothing sizes
The basis for EU bra band sizing in centimeters. Defines body dimensions and size designation for clothing sold in the European market.
Our calculator uses formulas consistent with the international sizing conventions described in these sources. See our Sources and Methodology page for more detail on how DiscoverFashions tools are built.
Frequently Asked Questions
How do I calculate my bra size at home?
You need two measurements: your underbust (wrap a tape snugly around your ribcage, just below your breasts) and your bust (wrap the tape around the fullest point, without pulling tight). Enter both into the calculator above. It gives you sizes in US, UK, EU, FR, AU, and JP systems, plus sister size alternatives.
What are sister sizes in bras?
Sister sizes share the same cup volume but use different band lengths. If you wear 34C, your sister sizes are 32D (tighter band, one cup letter up) and 36B (looser band, one cup letter down). They are useful when a band feels too tight or too loose while the cup fits well.
Why does my bra size change between brands?
There is no universal manufacturing standard for bras. A 34C from one label may fit like a 32D or 36B from another, depending on the pattern, fabric stretch, and the brand's own size chart. Treat any calculated size as a starting point and try on when you can.
How do I convert US bra sizes to UK or EU?
US and UK band numbers are the same (32, 34, 36, etc.), but cup letters split after D. For EU sizing, bands are in centimeters, so a US 34D becomes EU 75D. French sizing adds 15 to the EU band, making it FR 90D. The calculator above handles all six conversions automatically.
What is the most common bra size in the US?
The most purchased size in the US is 34DD. However, a 2012 study published in the journal Ergonomics found that roughly 80% of participants were wearing the wrong size. When properly measured, many women discover their actual size differs from what they have been buying.
How tight should a bra band feel?
Snug enough that you can slide two fingers under it, but not so loose that it rides up your back. Fasten it on the loosest hook when the bra is new; as the elastic stretches over time, you can move to tighter hooks. The band carries about 80% of the support, not the straps.
How often should I re-measure my bra size?
Every 6 to 12 months, or after any noticeable body change: weight gain or loss of around 5 kg, pregnancy, breastfeeding, starting or stopping hormonal medication, or a significant shift in exercise routine. Your bra size is not fixed for life.
Is this bra size calculator really free?
Yes. No signup, no email gate, no paywall. The measurements are processed in your browser and never leave your device. Use it as many times as you need.
What is a bra size chart and how do I read one?
A bra size chart maps the difference between your bust and underbust measurements to a cup letter. Each inch (or 2 cm) of difference equals one cup size. A 1-inch difference is an A cup, 2 inches is a B, 3 inches is a C, and so on. Band size is your rounded underbust measurement.
Can I use this calculator for sports bras?
You can use it as a starting point. Sports bras often fit differently because they prioritize compression and support over shaping. For high-impact activities like running, many women size down in the band for extra support. Try your calculated size first, then adjust.
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