How to Calculate Bra Size: Measurements, Chart & Tips
Learn how to calculate bra size with band and bust measurements, cup size math, a size chart, sister sizes, fit checks, and common measuring mistakes.
Bra sizing can feel confusing because the label is not one measurement. It combines a band size, a cup size, and a brandโs fit system. Knowing how to calculate bra size gives you a better starting point before trying bras on, especially if your current size pinches, gaps, rides up, or shifts during the day. The calculation uses two measurements: the ribcage under the bust and the fullest part of the bust. From there, the difference between those numbers estimates cup size.
What Is Bra Size?
A bra size has two parts:
- Band size: the number, such as 34 or 36
- Cup size: the letter, such as B, C, or D
For example, in 34C, 34 is the band size and C is the cup size. The cup letter is based on the difference between bust measurement and band measurement. It is not an absolute volume by itself, which is why a 32D and a 38D do not fit the same.
How to Calculate Bra Size
You need a soft measuring tape and a mirror.
- Measure snugly around your ribcage directly under your bust.
- Round to the nearest whole number.
- Measure around the fullest part of your bust without compressing tissue.
- Subtract band measurement from bust measurement.
- Match the difference to a cup size chart.
Use our free Bra Size Calculator to enter your measurements and get a starting size instantly.
Bra Size Formula
Cup Difference = Bust Measurement - Band Measurement
| Difference | Estimated Cup |
|---|---|
| 1 inch | A |
| 2 inches | B |
| 3 inches | C |
| 4 inches | D |
| 5 inches | DD/E |
| 6 inches | DDD/F |
| 7 inches | G |
| 8 inches | H |
Sizing varies by country and brand, so treat this as a starting point, not a final verdict.
Step-by-Step Example
Suppose your underbust measurement is 33 inches and your bust measurement is 37 inches.
Step 1 - Estimate band size: 33 inches rounds to a 34 band in many US sizing methods.
Step 2 - Find the difference: 37 - 34 = 3 inches
Step 3 - Convert difference to cup size: 3 inches = C cup
Your starting size is 34C.
How to Check the Fit
A calculated size gets you close, but fit checks matter:
| Fit Area | Good Fit | Possible Problem |
|---|---|---|
| Band | Level and snug on the loosest hook | Riding up means the band may be too large |
| Cups | Smooth without gaps or overflow | Gapping may mean too large; spilling may mean too small |
| Center gore | Lies close to the chest | Floating can mean cup or band mismatch |
| Straps | Stay in place without digging | Digging may mean the band is not supporting enough |
| Underwire | Sits around breast tissue | Sitting on tissue means cup shape or size is off |
The band should do most of the support. If tightening straps is the only way to feel supported, try a smaller band or different style.
Common Bra Size Mistakes
Measuring over a padded bra can inflate the bust measurement. Use an unpadded bra or measure braless if comfortable.
Assuming cup letters are fixed leads to confusion. Cup volume changes with band size, which is why sister sizes exist.
Ignoring breast shape can make the โrightโ size feel wrong. Fullness, spacing, projection, and root width all affect fit.
Wearing an old stretched band can make your size seem larger than it is. Elastic relaxes over time.
Sister Sizes
Sister sizes have similar cup volume with different band lengths. If the cup feels close but the band is wrong, sister sizing can help.
| Current Size | Tighter Band Sister Size | Looser Band Sister Size |
|---|---|---|
| 34C | 32D | 36B |
| 36D | 34DD/E | 38C |
| 32B | 30C | 34A |
Use sister sizes carefully. They are useful for small adjustments, but moving too far can change support and shape.
Frequently Asked Questions
How to calculate bra size at home?
Measure snugly under your bust for band size, then measure around the fullest part of your bust. Subtract band from bust to estimate cup size. For example, a 34-inch band and 37-inch bust gives a 3-inch difference, which is usually a C cup.
How tight should the bra band be?
The band should feel snug but not painful. It should sit level around your body and stay in place when you move. A new bra should usually fit on the loosest hook so you can tighten it later as the elastic stretches.
Why do two bras in the same size fit differently?
Brands use different patterns, fabrics, wire widths, and cup shapes. Your calculated size is a starting point, but style matters. A balconette, plunge, wireless, and sports bra can all fit differently even when the label shows the same size.
What are sister sizes in bras?
Sister sizes are sizes with similar cup volume but different band lengths. For example, 34C is close in cup volume to 32D and 36B. Try a sister size when the cup volume feels right but the band is too tight or too loose.
How often should I measure bra size?
Measure whenever your weight changes, after pregnancy or breastfeeding, after major fitness changes, or when bras start feeling uncomfortable. Even without major changes, checking every 6 to 12 months can help because elastic stretches and body shape changes over time.
The Bottom Line
To calculate bra size, measure your underbust for band size, measure your bust at the fullest point, then use the difference to estimate cup size. The result is a starting point, and the best size is the one that feels supportive, stable, and comfortable in real life.
Use our free Bra Size Calculator to get your starting size and compare sister sizes quickly.
How to Calculate: Step-by-Step Guide
Measure Your Band Size
Wrap a measuring tape snugly around your ribcage directly under your bust and round to the nearest whole number.
Measure Your Bust Size
Measure around the fullest part of your bust, keeping the tape level and not compressing the tissue.
Calculate the Difference
Subtract your band measurement from your bust measurement to find the difference in inches.
Determine Cup Size
Match the inch difference to a cup size (e.g., 1 inch = A, 2 inches = B, 3 inches = C).
Perform a Fit Check
Try on a bra in your calculated size and check that the band is level, the cups don't gap, and the gore sits flat.