Bra Size Calculator
Over 80% of women wear the wrong bra size. Find your perfect fit today with our easy-to-use bra size calculator. Get accurate results for better comfort, support, and confidence.
Your Results
A visual comparison of your underbust and bust measurements.
Your Sister Size Chart
| Go Down a Band Size | Your Calculated Size | Go Up a Band Size |
|---|---|---|
| — | — | — |
Sister sizes have the same cup volume. If your band feels too tight or loose, you can try a sister size. Go down a band and up a cup, or vice versa.
What is a Bra Size Calculator?
A bra size calculator is an essential online tool designed to help you determine your most accurate bra size based on your body measurements. Given that a vast majority of individuals wear ill-fitting bras, a reliable calculator removes the guesswork and provides a solid starting point for finding comfortable and supportive lingerie. This tool is for anyone who has experienced bra fit issues like slipping straps, a riding-up band, or gaping cups. It’s especially useful if you’ve recently experienced bodily changes due to weight fluctuation, pregnancy, or hormonal shifts. A common misconception is that bra sizes are static, but in reality, your ideal size can change multiple times throughout your life. Using a bra size calculator regularly ensures you are always wearing the best size for your current shape.
Bra Size Calculator Formula and Mathematical Explanation
The calculation for bra size is a two-part process involving the band size and the cup size. While different brands may have slight variations, the standard method used by most bra size calculators is as follows:
- Determine Band Size: Measure your underbust (rib cage directly under the breasts). This measurement is then typically rounded. A common method is to add 4 or 5 inches to get an even number (e.g., 31 inches + 5 = 36 band). However, a more modern and accurate method, used by this bra size calculator, is to round the raw underbust measurement up to the next even number. For example, an underbust of 31 inches results in a 32 band size.
- Determine Cup Size: Measure the fullest part of your bust. The difference between your bust measurement and your calculated band size determines the cup letter. Each inch of difference corresponds to a cup size.
This simple subtraction is the core of any good bra size calculator.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Underbust (U) | The measurement around the rib cage just beneath the breasts. | inches/cm | 26 – 48 in |
| Bust (B) | The measurement around the fullest part of the bust. | inches/cm | 30 – 55 in |
| Band Size | The numerical part of the bra size, derived from the underbust. | inches | 28, 30, 32… |
| Cup Difference (D) | The difference calculated as D = Bust – Band Size. | inches | 1 – 10 in |
Practical Examples (Real-World Use Cases)
Example 1: Standard Measurement
Let’s say a user’s measurements are:
- Underbust: 31.5 inches
- Bust: 37 inches
The bra size calculator would perform the following steps:
- Calculate Band Size: Round 31.5 inches up to the next even number, which is 32.
- Calculate Cup Difference: Subtract the *original band measurement* from the bust measurement. Some systems subtract the calculated band size. A common method is 37 (Bust) – 32 (Calculated Band) = 5 inches.
- Determine Cup Letter: A 5-inch difference corresponds to a DD cup.
- Final Result: The calculated bra size is 32DD.
Example 2: A Smaller Frame
Another user provides these measurements:
- Underbust: 29 inches
- Bust: 33 inches
The bra size calculator logic is:
- Calculate Band Size: Round 29 inches up to the next even number, which is 30.
- Calculate Cup Difference: 33 (Bust) – 30 (Calculated Band) = 3 inches.
- Determine Cup Letter: A 3-inch difference corresponds to a C cup.
- Final Result: The calculated size is 30C. For more details, see this bra fitting guide.
How to Use This Bra Size Calculator
Using this bra size calculator is straightforward. Follow these steps for an accurate reading:
- Select Your Unit: Choose between inches and centimeters. Inches are standard for US/UK sizing.
- Measure Your Underbust: Use a soft measuring tape. Wrap it snugly around your ribcage, just below your bust line. Ensure it’s parallel to the floor. Enter this number into the “Underbust Measurement” field.
- Measure Your Bust: Wrap the tape around the fullest part of your bust, typically at nipple level. The tape should be comfortably loose and not compress your breast tissue. Enter this into the “Bust Measurement” field.
- Read Your Results: The calculator will instantly display your calculated band size, cup letter, and final bra size. It will also show key intermediate values and a sister size chart for more fitting options. Making an informed decision means understanding that your calculated size is a starting point. It’s always best to try on a bra to check the fit. If you’re unsure how to measure bra size correctly, watch a video tutorial.
Key Factors That Affect Bra Size Results
Your bra size isn’t just about two numbers; several factors can influence the fit and what size you should ultimately choose. A good bra size calculator gives you a baseline, but consider these points:
- Brand and Style: Sizing can vary significantly between brands and even between different styles from the same brand. A plunge bra may fit differently than a full-cup bra in the same size.
- Breast Shape: Are your breasts full on top, shallow, or wide-set? Different breast shapes fit better into certain bra styles (e.g., balconette vs. t-shirt bra). A calculator can’t see your shape, so check out a breast shape guide to learn more.
- Band Elasticity: The support of a bra comes primarily from the band. A new bra should fit snugly on the loosest hook. As the elastic stretches over time, you can move to the tighter hooks.
- Time of the Month: Hormonal fluctuations during your menstrual cycle can cause your breasts to swell, temporarily changing your cup size.
- Weight Changes: Gaining or losing even a few pounds can affect both your band and cup size, making a regular check with a bra size calculator a good habit.
- Pregnancy and Nursing: Motherhood brings significant and often permanent changes to breast size and shape, requiring frequent re-measuring.
Frequently Asked Questions (FAQ)
1. How often should I use a bra size calculator?
It’s recommended to measure yourself and use a bra size calculator every 6-12 months, or after any significant life event like pregnancy, weight change, or starting a new hormonal medication.
2. What if my measurements are between two sizes?
This is very common. The best approach is to try on both sizes. You might also benefit from exploring your “sister sizes.” Our calculator provides a sister size calculator chart to help you find alternatives.
3. My calculator result is very different from what I’m wearing. Is it wrong?
Not necessarily. Since over 80% of women wear the wrong size, it’s highly probable your calculated size is more accurate. The most common mistake is wearing a band that’s too large and a cup that’s too small. Trust the tape measure and give the new size a try.
4. Does the “add 4 inches” method work?
This is an older, outdated method from when bra materials had no stretch. Modern bras are made with elastic, so adding inches often results in a band size that is too loose and unsupportive. This bra size calculator uses the more modern “round up” method for a snugger, more accurate fit.
5. What is a sister size?
A sister size refers to a bra with the same cup volume but a different band and cup letter combination. For example, a 34C has the same cup volume as a 32D and a 36B. If your bra’s cups fit well but the band is too loose, you would go down a band size and up a cup size (to 32D).
6. Can I use a bra size calculator for sports bras?
While this bra size calculator gives a great starting point, sports bras often use S/M/L sizing. However, knowing your precise bra size helps you choose the correct S/M/L size based on the brand’s specific conversion chart and provides insight into different bra types explained.
7. Why is the band so important?
The band provides 80-90% of a bra’s support. If the band is too loose, it will ride up your back, and the weight of your breasts will be put on the shoulder straps, causing them to dig in. A snug band is the foundation of a good fit.
8. What about post-surgery or mastectomy fittings?
For specialized fittings after surgery, it’s always best to consult with a professional fitter. However, a bra size calculator can be a useful tool for monitoring changes at home. There are resources available for a post-mastectomy bra fitting.