What Is a Bra Size Calculator?
A bra size calculator helps turn basic body measurements into an estimated bra size. Instead of guessing your size from a previous purchase or clothing size, you can measure your body and use those measurements as a starting point.
This calculator is designed for measurements in inches and provides an estimated US bra size. You enter two measurements: the circumference around your ribcage underneath your breasts and the circumference around the fullest part of your bust.
The calculator then estimates the band size and compares the bust measurement with that band measurement to determine an estimated cup size. The result is useful when you are shopping for a new bra or when your usual size no longer seems to fit the way it used to.
How to Measure Your Bra Size in Inches
You only need a flexible measuring tape to get started. A cloth or tailor's measuring tape works well because it can follow the shape of your body without leaving large gaps.
Measure Your Underbust
Start by measuring directly underneath your breasts, around your ribcage. This is commonly called the underbust measurement. Keep the tape horizontal and snug against your body.
The tape should be firm enough to give you a useful measurement, but you should not pull it so tightly that it digs into your skin or changes the natural shape of your body.
Measure Your Fullest Bust
Next, measure around the fullest part of your bust. Keep your posture natural and your arms relaxed. The measuring tape should sit evenly around your body without pressing tightly into the breast tissue.
Check the tape in a mirror if possible. A tape that rises or falls significantly across your back can affect the measurement.
Enter Both Measurements
Enter both numbers in the calculator above. Use inches rather than centimeters because this calculator is specifically designed around inch-based US sizing.
How Bra Size Is Calculated
A bra size normally contains a number and a letter. The number is the band size, while the letter represents the cup size. These two parts work together, so the cup letter should not be interpreted by itself.
This calculator first estimates an even band size from the underbust measurement. It then calculates the difference between the fullest bust measurement and the estimated band size.
| Calculation | Meaning |
|---|---|
| Band size | Estimated from the snug underbust measurement. |
| Bust difference | Fullest bust measurement minus estimated band size. |
| Cup size | Estimated from the bust-to-band difference. |
For example, suppose your underbust measurement is 33 inches and your fullest bust measurement is 36 inches. The calculator can estimate a 34-inch band. The difference between 36 and 34 is 2 inches, producing an estimated B cup and a result of 34B.
These calculations provide a starting point rather than a guarantee of fit. Bra manufacturers can use different patterns, fabrics, cup shapes, and sizing systems.
US Bra Cup Size Difference Chart
The calculator uses a simplified US cup-size scale based on the difference between the bust measurement and estimated band size.
| Bust Difference | Estimated Cup |
|---|---|
| Less than 1 inch | AA |
| 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 |
| 9 inches | I |
| 10 inches | J |
Why Your Calculated Size May Not Fit Every Brand
Bra sizing is not perfectly standardized. A 34C from one manufacturer can feel different from a 34C from another manufacturer. The difference can come from the cup shape, fabric stretch, underwire design, band construction, padding, and overall pattern.
This is why a calculator should be used as a starting point. Once you have an estimated size, check the size chart provided by the brand or retailer you are buying from.
The style of the bra can matter as well. A wireless bra, sports bra, plunge bra, full-coverage bra, and structured underwire bra can all fit differently even when the label shows the same size.
What Are Sister Sizes?
Sister sizes are nearby bra sizes that have a similar cup volume but use different band numbers. They can be useful when the cup seems right but the band feels too loose or too tight.
As a general example, someone starting with 34C might compare nearby sizes such as 32D or 36B. These sizes are not identical, but the relationship illustrates how changing the band size usually requires changing the cup letter in the opposite direction.
Sister sizing is most useful after you have tried a bra and identified which part of the fit needs adjustment. It should not be used to force yourself into a size that is uncomfortable.
Common Signs of an Incorrect Bra Fit
Measurements are helpful, but the actual fit of a bra is equally important. A band that rides up your back can indicate that the band is too loose. A band that causes significant discomfort may be too tight or may not work well with that particular bra design.
Cup fit can provide another clue. Breast tissue spilling over the cup may indicate that the cup is too small or that the cup shape is not a good match. Noticeable empty space or persistent wrinkling can indicate that the cup is too large or has a different shape from what your body needs.
Straps should help support the bra without being tightened excessively. If you have to pull the straps very tight just to keep the bra in place, check the band and cup fit first.
Tips for Taking Better Measurements
- Use a flexible cloth measuring tape.
- Stand naturally while taking both measurements.
- Keep the tape level around your body.
- Do not twist the measuring tape.
- Keep the underbust tape snug but comfortable.
- Do not compress your bust when taking the fullest-bust measurement.
- Take each measurement more than once if the result seems unusual.
- Use the manufacturer's size chart before buying.
When Should You Measure Again?
It can be useful to remeasure when your usual bras no longer feel the way they did before. Body measurements can change, and even a modest change can affect how a bra fits.
You may also want to measure again when buying a different style. A sports bra and a structured underwire bra, for example, can have very different construction and support characteristics.
Frequently Asked Questions
How do I calculate my bra size in inches?
Measure your snug underbust and fullest bust in inches. Enter both measurements into the calculator to estimate your band and cup size.
What is an underbust measurement?
Your underbust is the measurement around your ribcage directly underneath your breasts. It is used to estimate your bra band size.
Where should I measure my bust?
Measure around the fullest part of your bust. Keep the tape level and avoid pulling it tightly enough to compress the breast tissue.
Is the calculated bra size guaranteed to fit?
No. The calculation is an estimate. Brand sizing, cup shape, materials, bra construction, and individual body shape can all affect the final fit.
What should I do if I am between sizes?
Recheck your measurements and compare the result with the manufacturer's size chart. You can also consider nearby sister sizes after trying the bra.
Can I enter centimeters?
This calculator is designed for inches. If your measuring tape uses centimeters, convert the measurements to inches before entering them.
Why can the same bra size fit differently between brands?
Brands can use different patterns, cup shapes, fabrics, band construction, and sizing conventions. A calculated size is therefore best used as a starting point.
Privacy and Calculator Limitations
The calculations on this page are performed directly in your browser. The calculator does not need to send your measurements to a server in order to calculate the estimated result.
This tool is intended for general sizing guidance. It is not a medical diagnostic tool and should not be used as a substitute for professional medical advice.
For the best shopping experience, use the calculated size as a starting point and check the current sizing information provided by the specific manufacturer.