The Ultimate Bra Fitting Bible: Sizes, Shapes & Sister Sizing Explained
Bras have a bad reputation. If you look forward to flinging off your bra the moment you step through the door, it’s possible that the issue isn’t the bra itself—it’s the fit. A well-fitting bra can be so comfortable that you hardly notice you’re wearing it. On the flip side, an ill-fitting bra that gapes, slips, rides up your back, or digs into your ribs is one of the most irritating feelings in the world.
Here’s a startling statistic: 38% of women are unsure if they are wearing the correct bra size—and research shows that when women do get professionally fitted, many walk away with a completely different size than what they’ve been wearing for years.
The consequences go beyond discomfort. Bras that are too tight can damage breast tissue and even cause breathing difficulties, while inadequate support can lead to neck, shoulder, and back pain, pinched nerves, poor posture, and premature sagging.
This guide is your complete resource for everything bra fitting. Whether you’re measuring for the first time, confused by international sizing, or wondering why your go-to bra suddenly feels off, you’ll find answers here. Let’s fix your fit—for good.
Table of Contents
- How to Measure Your Bra Size at Home
- Bra Size Calculator & Chart
- US vs UK vs EU Bra Size Conversion
- Sister Sizing Explained
- Signs Your Bra Doesn’t Fit
- Band Size vs Cup Size: What Matters More
- Frequently Asked Questions
1. How to Measure Your Bra Size at Home
Measuring your bra size at home is easier than you think. All you need is a soft, non-stretchy measuring tape, a mirror, and an unlined, non-padded bra (or a fitted t-shirt).
Step 1: Measure Your Band Size
Wrap the measuring tape snugly around your ribcage, directly under your bust—right where the bra band normally sits. Keep the tape level all the way around; it should not ride up in the back.
Pro Tip: Take a couple of deep breaths and make sure the band isn’t too snug—you should be able to comfortably fit two fingers beneath the tape measure.
Rounding Rule: If you get an odd number (e.g., 33 inches) or a measurement between sizes, round up to the nearest even number. For example: 31 inches → 32 band, 33 inches → 34 band, 35 inches → 36 band.
This number is your band size.
Step 2: Measure Your Bust
Run the measuring tape around your back, just under your shoulder blades, and up around the fullest part of your bust. The tape should just skim the bra in front—do not compress the breast tissue.
Step 3: Calculate Your Cup Size
Here’s the simple math: subtract your band size from your bust measurement.
| Difference (inches) | Cup Size |
|---|---|
| 0″ | AA |
| 1″ | A |
| 2″ | B |
| 3″ | C |
| 4″ | D |
| 5″ | DD/E |
| 6″ | DDD/F |
| 7″ | G |
| 8″ | H |
Example: If your bust measures 37 inches and your band size is 34 inches, the difference is 3 inches → 34C.
No measuring tape? No problem! Use a piece of string or ribbon (non-stretchy!), measure yourself with it, then measure the length of the string with a ruler.
2. Bra Size Calculator & Chart
Once you have your measurements, use this chart to find your size quickly.
Standard US Bra Size Chart
| Band Size | AA | A | B | C | D | DD/E | DDD/F | G |
|---|---|---|---|---|---|---|---|---|
| 28 | 28AA | 28A | 28B | 28C | 28D | 28DD | 28DDD | 28G |
| 30 | 30AA | 30A | 30B | 30C | 30D | 30DD | 30DDD | 30G |
| 32 | 32AA | 32A | 32B | 32C | 32D | 32DD | 32DDD | 32G |
| 34 | 34AA | 34A | 34B | 34C | 34D | 34DD | 34DDD | 34G |
| 36 | 36AA | 36A | 36B | 36C | 36D | 36DD | 36DDD | 36G |
| 38 | 38AA | 38A | 38B | 38C | 38D | 38DD | 38DDD | 38G |
| 40 | 40AA | 40A | 40B | 40C | 40D | 40DD | 40DDD | 40G |
| 42 | 42AA | 42A | 42B | 42C | 42D | 42DD | 42DDD | 42G |
| 44 | 44AA | 44A | 44B | 44C | 44D | 44DD | 44DDD | 44G |
A note on accuracy: The accuracy of your calculated size depends entirely on the precision of your measurements. For the most reliable results, measure to the nearest ¼ inch.
3. US vs UK vs EU Bra Size Conversion
Bra sizing varies dramatically across regions. A 34D in the US is not the same as a 34D in the UK or EU. Here’s how they compare.
International Bra Size Conversion Chart
| US Size | UK Size | EU Size | FR/BE Size | AUS/NZ Size |
|---|---|---|---|---|
| 28A | 28A | 60A | 75A | 6A |
| 30A | 30A | 65A | 80A | 8A |
| 32A | 32A | 70A | 85A | 10A |
| 34A | 34A | 75A | 90A | 12A |
| 36A | 36A | 80A | 95A | 14A |
| 32B | 32B | 70B | 85B | 10B |
| 34B | 34B | 75B | 90B | 12B |
| 36B | 36B | 80B | 95B | 14B |
| 32C | 32C | 70C | 85C | 10C |
| 34C | 34C | 75C | 90C | 12C |
| 36C | 36C | 80C | 95C | 14C |
| 32D | 32D | 70D | 85D | 10D |
| 34D | 34D | 75D | 90D | 12D |
| 36D | 36D | 80D | 95D | 14D |
| 32DD | 32DD | 70E | 85E | 10DD |
| 34DD | 34DD | 75E | 90E | 12DD |
| 32DDD/F | 32E | 70F | 85F | 10E |
| 34DDD/F | 34E | 75F | 90F | 12E |
| 32G | 32F | 70G | 85G | 10F |
| 34G | 34F | 75G | 90G | 12F |
Key Differences to Remember:
- UK sizing uses double letters beyond DD (E, F, FF, G, GG, etc.) and is considered more consistent for larger cup sizes.
- EU sizing uses centimeters for band sizes (e.g., 70, 75, 80) and uses single-letter cup progressions (A, B, C, D, E, F, G).
- French/Belgian sizing adds 15 to the EU band size (e.g., EU 70 = FR 85).
- Always use the same size system when converting—don’t mix US and UK cup letters.
4. Sister Sizing Explained
Sister sizing might be the most useful fitting concept you’ve never heard of.
What Is Sister Sizing?
Sister sizes are alternative bra sizes that have the same cup volume but different band and cup sizes. The cup volume stays the same—only the band (and the cup letter) changes.
The Rule:
- Go up a band size → go down a cup size
- Go down a band size → go up a cup size
Sister Size Chart
| Your Size | Sister Size Down (Tighter Band) | Sister Size Up (Looser Band) |
|---|---|---|
| 32A | 30B | 34AA |
| 32B | 30C | 34A |
| 32C | 30D | 34B |
| 32D | 30DD | 34C |
| 34A | 32B | 36AA |
| 34B | 32C | 36A |
| 34C | 32D | 36B |
| 34D | 32DD | 36C |
| 36A | 34B | 38AA |
| 36B | 34C | 38A |
| 36C | 34D | 38B |
| 36D | 34DD | 38C |
| 38C | 36D | 40B |
| 38D | 36DD | 40C |
| 40D | 38DD | 42C |
Example: If you wear a 34D, your sister sizes are 32DD (smaller band, larger cup) and 36C (larger band, smaller cup). They all hold the same breast volume.
Why does this matter? Cup size doesn’t express a fixed volume—it’s the difference between your underbust and bust measurements. This is why someone with a 75D may actually have smaller breasts than someone with a 90C.
When to Try a Sister Size
Try sister sizing when:
- Your band feels too tight or too loose in your usual size
- Your cups aren’t fitting quite right
- You’re trying a different brand or style (sizing varies across brands!)
- Your go-to size is out of stock
- You have asymmetrical breasts (one slightly bigger than the other)
- You’re between sizes
Important Warning: If you have to fasten your bra on the tightest hook in your usual size, your sister size (one band size up, one cup size down) may be too big. Remember: the band—not the cups—provides the majority of the support.
5. Signs Your Bra Doesn’t Fit
Most bras that feel uncomfortable don’t fit correctly. The discomfort is diagnostic, not inevitable.
The Band
| Problem | What It Means | Fix |
|---|---|---|
| Band rides up at the back | Band is too large | Go down one band size (and up one cup size to maintain volume) |
| Band feels too tight | Band is too small | Go up one band size (and down one cup size) |
| You’re on the tightest hook and it still feels loose | Band is too large | Size down in the band |
| Band doesn’t stay level when you raise your arms | Band is too loose | Size down |
The Band Rule: The band does 70–80% of the work in a well-fitted bra. Start on the loosest hook when a bra is new. As the band stretches with wear, move to tighter hooks.
The Cups
| Problem | What It Means | Fix |
|---|---|---|
| Spilling over the top, sides, or under the arm | Cup is too small | Go up one cup size |
| Gaping or puckering at the front | Cup is too large or wrong shape | Go down one cup size or try a different style |
| Center gore floats away from sternum | Cups are too small or wrong breast root width | Go up one cup size or try a style with narrower/wider wires |
| Cup wrinkles or creases | Cup is too big | Go down one cup size |
The Underwire
| Problem | What It Means | Fix |
|---|---|---|
| Underwire digs into breast tissue | Cup is too small or wire shape is wrong | Go up one cup size or try a different style |
| Underwire digs at center front | Cup is too small | Go up one cup size |
| Underwire digs at sides/underarm | Cup is too small or wire width is too narrow | Try a wider-set or plunge style |
| Underwire doesn’t sit in the breast crease | Wrong fit or style | Adjust size or try a different shape |
The Underwire Rule: The underwire should sit flat against your ribcage and encircle your breast tissue completely. No part of the wire should rest on breast tissue itself.
The Straps
| Problem | What It Means | Fix |
|---|---|---|
| Straps dig in and leave marks | Band is likely too loose (straps are doing too much work) | Size down in the band first |
| Straps constantly fall down | Strap attachment points are too wide for your shoulders | Try a racerback or convertible style |
The Strap Rule: Straps contribute support, but they shouldn’t be doing the majority of the work. You should be able to slide two fingers comfortably underneath them.
6. Band Size vs Cup Size: What Matters More
Here’s a truth that surprises most women: the band matters more than the cup.
Why the Band Is the Foundation
The band provides 70–80% of a bra’s support. The cups simply hold the breast tissue; the band does the heavy lifting. A bra with a properly fitted band will:
- Stay in place all day without riding up
- Distribute weight evenly across your back and shoulders
- Prevent straps from digging in
- Provide actual lift and support
The Interrelationship Between Band and Cup
Band and cup sizes are interrelated. When you change your band size, the cup volume changes too.
Example: A 34C and a 32D hold the same cup volume. The 32D has a tighter band and a larger cup letter, but the cups hold the same amount of breast tissue.
The Traditional Method Problem: Research has found that the traditional method of bra fitting overestimates band size and underestimates cup size. As band size increases, the traditional method becomes more inaccurate.
The Takeaway
If your bra feels uncomfortable:
- Check the band first. Most fit issues start with the wrong band size.
- Sister size to adjust. If the band feels too loose, go down a band size and up a cup size. If it feels too tight, go up a band size and down a cup size.
- Don’t be afraid of the cup letter. A larger cup letter with a smaller band often provides better support.
7. Frequently Asked Questions
Q: How often should I get my bra size measured?
Breasts can fluctuate in size due to weight changes, pregnancy, nursing, menopause, and even the time of month. Experts recommend getting measured at least once a year—or any time you notice changes in how your bras fit.
Q: How often should I replace my bras?
With daily wear and regular washing, bras typically last 6–12 months. Signs it’s time to replace:
- The band feels loose even on the tightest hook
- The fabric is stretched, pilling, or worn
- The underwire is poking out
- The cups no longer contain your breast tissue properly
Q: What if I have asymmetrical breasts?
It’s completely normal to have one breast slightly larger than the other. Options:
- Fit to the larger breast—you can use a padding insert in the smaller cup
- Try sister sizing to find a better overall fit
- Look for bras with stretch lace that accommodates asymmetry
Q: Why does my underwire keep poking out?
Underwire pokes out when the bra is too small, too worn, or improperly washed. Prevention:
- Always hand-wash or use a lingerie bag in the washing machine
- Never put underwire bras in the dryer
- Make sure the cup size is correct—too small puts pressure on the wire
Q: Can a bad bra cause health problems?
Yes. Bras that are too tight can damage breast tissue and cause breathing difficulties. Inadequate support can lead to neck, shoulder, and back pain, pinched nerves, poor posture, and premature sagging. A properly fitted bra supports your body and prevents these issues.
Q: Do I really need to start on the loosest hook?
Yes. Bras stretch with wear. Starting on the loosest hook allows you to move to tighter hooks as the band stretches, extending the life of your bra. If you’re already on the tightest hook and the bra still feels loose, the band is too large.
Q: What’s the difference between US, UK, and EU sizing?
The main differences:
- US: Band sizes in even numbers (32, 34, 36); cups use A, B, C, D, DD, DDD, G
- UK: Band sizes in even numbers; cups use A, B, C, D, DD, E, F, FF, G, GG (more consistent for larger cups)
- EU: Band sizes in centimeters (65, 70, 75, 80); cups use A, B, C, D, E, F, G
Always check which sizing system a brand uses before ordering.
Conclusion
Finding the perfect bra isn’t magic—it’s measurement, knowledge, and a little patience. Armed with this guide, you now have everything you need to measure yourself accurately, understand international sizing, navigate sister sizing, and identify exactly what a well-fitted bra should feel like.
Your next steps:
- Measure yourself at home using the step-by-step guide above
- Check your current bras against the Perfect Fit Checklist
- Try a sister size if your usual size isn’t working
- Bookmark this guide—your size may change, and you’ll want to come back
Remember: Your body is not the problem. The bra is. With the right fit, you can finally experience the comfort and support you deserve.