Bra Size Calculator

Find your accurate bra size for perfect comfort and support.

Your Measurements

Sister Size Chart

If your size feels a bit off, a sister size might be a better fit. These sizes have the same cup volume.

Go Down a Band Size (Tighter Band, Larger Cup)	Your Calculated Size	Go Up a Band Size (Looser Band, Smaller Cup)
—	—	—

A bra sister size chart helps you find alternative sizes.

What is a Bra Size Calculator?

A bra size calculator is a digital tool designed to help you determine your most accurate bra size based on your body measurements. Finding the right fit is crucial not just for comfort, but also for posture, support, and confidence. An ill-fitting bra can cause a range of issues, from shoulder pain and skin irritation to a less-than-flattering silhouette under clothing. This is why using a precise bra size calculator is the first and most important step in any lingerie purchase.

Anyone who wears a bra should use a bra size calculator periodically, as our bodies change over time due to factors like weight fluctuations, hormonal shifts, and aging. Common misconceptions often lead people to wear the wrong size for years. For instance, many believe that cup size is static across all band sizes (e.g., a C cup is the same on a 32 band and a 38 band), which is incorrect. A reliable bra size calculator demystifies this process, providing a clear, data-driven starting point for finding the perfect fit.

Bra Size Calculator Formula and Mathematical Explanation

The calculation for bra size involves two key measurements: the band size (underbust) and the bust size. The bra size calculator uses a modern formula for accuracy. Here’s a step-by-step breakdown:

1. Determine Band Size: The band measurement is taken snugly around the ribcage, directly under the bust. The bra size calculator then rounds this raw measurement to the nearest even number. For example, if your measurement is 29.5 inches, your band size is 30. If it’s 31 inches, your band size becomes 32. This ensures the band is snug enough to provide the majority of the support.
2. Calculate Cup Volume: The cup size is determined by the difference between your bust measurement (taken at the fullest part) and your calculated band size.
3. Determine Cup Letter: Each inch of difference corresponds to a cup letter. A 1-inch difference is an A cup, a 2-inch difference is a B cup, a 3-inch difference is a C cup, and so on.

The final result combines the band number and the cup letter (e.g., 34C). Understanding this logic is key to using a bra size calculator effectively.

Variables in the Bra Size Calculation

Variable	Meaning	Unit	Typical Range
Underbust Measurement	The circumference of the ribcage directly under the breasts.	in / cm	26 – 48 in
Bust Measurement	The circumference over the fullest part of the bust.	in / cm	28 – 55 in
Band Size	The numerical part of the bra size, rounded to an even number.	Numeric (e.g., 32, 34)	28 – 50
Cup Difference	Bust Measurement – Band Size.	in / cm	1 – 10 in
Cup Size	The letter part of the bra size, determined by the cup difference.	Letter (e.g., A, B, C)	A – H

Practical Examples (Real-World Use Cases)

Example 1: Standard Measurement

• Inputs:
  • Underbust (Band) Measurement: 31 inches
  • Bust Measurement: 35 inches
• Calculation:
  1. The bra size calculator rounds the 31-inch underbust up to the nearest even number, resulting in a Band Size of 32.
  2. The cup difference is calculated: 35 inches (Bust) – 32 (Band Size) = 3 inches.
  3. A 3-inch difference corresponds to a C cup.
• Result: The calculated bra size is 32C.

Example 2: In-Between Sizes

• Inputs:
  • Underbust (Band) Measurement: 34.5 inches
  • Bust Measurement: 41 inches
• Calculation:
  1. The bra size calculator rounds the 34.5-inch underbust up to the nearest even number: 36.
  2. The cup difference is: 41 inches (Bust) – 36 (Band Size) = 5 inches.
  3. A 5-inch difference corresponds to a DD (or E) cup.
• Result: The bra size calculator recommends a size of 36DD. In this case, exploring sister sizes like 34DDD or 38D using a bra fitting guide might also be beneficial.

How to Use This Bra Size Calculator

1. Select Your Unit: Start by choosing your preferred unit of measurement, either inches or centimeters. The bra size calculator will adapt its formula accordingly.
2. Measure Your Band: Using a soft measuring tape, measure around your ribcage directly beneath your bust. The tape should be level and snug, but not digging in. Enter this number into the “Band Measurement” field.
3. Measure Your Bust: Measure around the fullest part of your bust. Ensure the tape is level across your back and not too tight. Enter this value into the “Bust Measurement” field.
4. Read Your Results: The bra size calculator will instantly display your calculated bra size, including the band number and cup letter. It will also show intermediate values like your final band size and cup difference.
5. Check the Chart and Table: The dynamic bar chart helps you visualize the measurements, while the sister size table provides alternatives if your calculated size doesn’t feel quite right. Finding the correct bra fit is about more than just numbers.

Key Factors That Affect Bra Fit

While a bra size calculator provides a crucial starting point, several other factors influence the perfect fit:

• Brand and Style Variation: Sizing is not universal. A 34C from one brand might fit differently than a 34C from another. Similarly, a plunge bra will fit differently than a full-coverage style in the same size.
• Band Snugness: The band should provide about 80% of the support. It must be level across your back and snug enough that you can only fit two fingers underneath. If it rides up, you need a smaller band size.
• Cup Fit: Your breasts should be fully encased within the cups with no spillage at the top, sides, or bottom. Wrinkles or gaps in the cup mean it’s too large.
• Gore (Center Panel): The center panel of the bra (the part between the cups) should lie flat against your sternum without gaping or digging in.
• Straps: Straps should only provide about 20% of the support and should not dig into your shoulders. If they do, your band is likely too loose. Learning how to measure bra size correctly is key.
• Sister Sizing: Understanding sister sizes gives you flexibility. If your band is too tight but the cup fits, you can go up a band size and down a cup size (e.g., from 32D to 34C). This is a critical part of finding your ideal fit beyond the initial bra size calculator result.

Frequently Asked Questions (FAQ)

1. How often should I use a bra size calculator?

It’s recommended to remeasure yourself and use a bra size calculator every 6 to 12 months, or whenever you experience significant body changes like weight loss/gain, pregnancy, or post-nursing.

2. What if my measurements are between two sizes?

If you’re in between sizes, it’s often best to try both. Also, consult the sister size chart. For example, if a 34C is too tight in the band, its sister size 36B might be perfect.

3. Is the “+4” method for band measurement accurate?

The “+4/+5” method is an older, more traditional approach that can often result in a band size that is too large and a cup size that is too small. This bra size calculator uses the modern method of rounding to the nearest even number for better accuracy.

4. Why does my bra band ride up my back?

This is a classic sign that your band size is too big and isn’t providing adequate support. The support is being transferred to the straps, causing the back to ride up. Try a smaller band size and a larger cup size (e.g., switch from 36B to 34C).

5. What are sister sizes?

Sister sizes are a set of bra sizes with the same cup volume. For example, a 32D, 34C, and 36B are all sister sizes. Knowing this can help you find a better fit if a particular size isn’t quite right. Our bra size chart can help.

6. Can I be a different size in different bra brands?

Absolutely. Sizing standards vary between manufacturers and even between different styles from the same brand. Always treat your calculated size as a starting point and adjust as needed.

7. Does cup size D mean “large”?

Not necessarily. Cup size is relative to the band size. A 30D has a much smaller cup volume than a 40D. It’s a common misconception that certain letters automatically mean a large bust. A good bra size calculator helps clarify this relationship.

8. What if the center panel (gore) of my bra doesn’t touch my chest?

If the gore is floating away from your sternum, it’s a sign that your cups are too small. Your breast tissue is pushing the bra away from your body. Try going up a cup size.