Accurate Bra Size Calculator

Find your perfect fit and say goodbye to uncomfortable bras. This tool helps you understand your measurements.

Calculator

This calculator uses the common “+4/+5 method” for band sizing. Your fit may vary by brand. See the article below for more details.

Your Measurements Visualized

Sister Size Chart

If a bra with your calculated size doesn’t feel right, you might have better luck with a “sister size.” These are sizes with the same cup volume. If you go down a band size, go up a cup size (e.g., 34D -> 32DD). If you go up a band size, go down a cup size (e.g., 34D -> 36C).

Your primary size is highlighted, with sister sizes along the same row.

Band 30	Band 32	Band 34	Band 36	Band 38

What is an Accurate Bra Size Calculator?

An accurate bra size calculator is a digital tool designed to estimate a person’s bra size based on their body measurements. By inputting the circumference of the rib cage (underbust) and the fullest part of the bust, the calculator applies a formula to determine a suggested band and cup size. The goal is to provide a starting point for finding a well-fitting bra, a task that is notoriously difficult. Studies show that a vast majority of women wear the wrong bra size, leading to discomfort, poor posture, and other issues. An accurate bra size calculator aims to demystify the process.

This tool is for anyone who wears a bra and wants to ensure they are getting the best support and comfort. Whether you’ve experienced weight changes, pregnancy, or simply haven’t been measured in years, using an accurate bra size calculator is a crucial first step. Common misconceptions are that bra sizes are static or that a “D cup” is always large. In reality, cup size is relative to band size, a concept this calculator helps to clarify.

Accurate Bra Size Calculator Formula and Mathematical Explanation

Most traditional bra sizing methods, including the one used in this accurate bra size calculator, involve two key steps.

1. Band Size Calculation: The underbust measurement is taken. Using the common “Plus Four/Five” method, 4 inches are added if the measurement is an even number, and 5 inches are added if it’s odd. The result is rounded to the nearest even number, which becomes the band size. For example, an underbust of 29″ (odd) + 5″ = 34″. An underbust of 30″ (even) + 4″ = 34″.
2. Cup Size Calculation: The cup size is determined by the difference between the bust measurement and the calculated band size. Each inch of difference corresponds to a letter in the alphabet. A 1-inch difference is an ‘A’ cup, 2 inches is a ‘B’ cup, 3 inches is a ‘C’ cup, and so on.

Variables in Bra Size Calculation

Variable	Meaning	Unit	Typical Range
Underbust (U)	Measurement around the ribcage directly under the breasts.	inches or cm	26 – 48 in
Bust (B)	Measurement around the fullest part of the bust.	inches or cm	28 – 55 in
Calculated Band Size	The resulting band size after applying the formula.	inches	30 – 52 (even numbers)
Cup Difference	Bust Measurement – Underbust Measurement	inches	0 – 10+ in

Practical Examples (Real-World Use Cases)

Example 1: Standard Measurements

Let’s say a person uses this accurate bra size calculator and enters the following measurements:

• Underbust: 31 inches (odd number)
• Bust: 37 inches

Calculation:

1. Band Size: 31″ + 5″ = 36″. The band size is 36.

2. Cup Difference: 37″ (Bust) – 31″ (Underbust) = 6 inches.

3. Cup Size: A 6-inch difference corresponds to an F cup in US sizing (A=1, B=2, C=3, D=4, DD/E=5, DDD/F=6).

Resulting Size: 36F

Example 2: Smaller Frame, Larger Bust

Another person uses the accurate bra size calculator with these inputs:

• Underbust: 28 inches (even number)
• Bust: 33 inches

Calculation:

1. Band Size: 28″ + 4″ = 32″. The band size is 32.

2. Cup Difference: 33″ (Bust) – 28″ (Underbust) = 5 inches.

3. Cup Size: A 5-inch difference corresponds to a DD or E cup.

Resulting Size: 32DD (or 32E)

How to Use This Accurate Bra Size Calculator

1. Measure Yourself: Using a soft measuring tape, take your underbust and bust measurements as described in the helper text. For best results, wear a non-padded, non-compressive bra.
2. Enter Your Data: Input your measurements into the “Underbust Measurement” and “Bust Measurement” fields.
3. Select Units: Choose whether your measurements are in inches or centimeters. The calculator will handle any necessary conversions.
4. View Your Results: The calculator automatically updates to show your primary estimated bra size, along with intermediate values like the calculated band and cup.
5. Explore Sister Sizes: The table below the main calculator shows your sister sizes. If your recommended size (e.g., 34D) feels too tight in the band, you could try its sister size with a larger band (36C).

Key Factors That Affect Bra Fit

Beyond the numbers, several factors influence how a bra fits. An accurate bra size calculator is just a starting point.

• Brand Differences: Sizing is not standardized across brands. A 34C from one company might fit differently than a 34C from another.
• Bra Style: A plunge, balconette, and full-cup bra in the same size will all fit and support differently. Your breast shape might be better suited to certain styles.
• Band Support: The band should provide 80% of the support. It should be snug and level across your back. If it rides up, the band is too loose.
• Cup Encapsulation: Your breasts should be fully contained within the cups with no spillage (quad-boob) or gaping. The center panel (gore) should lie flat against your sternum.
• Strap Tension: Straps should not be doing the heavy lifting. If they dig into your shoulders, your band is likely too loose and not providing enough support.
• Age and Stretch: Bras stretch out over time. A bra that fit perfectly a year ago may no longer be supportive. It’s recommended to be re-measured every 6-12 months.

Frequently Asked Questions (FAQ)

1. Why doesn’t this calculator use the “Underbust = Band Size” method?

While newer methods often equate the underbust measurement directly to the band size (e.g., 30″ underbust = 30 band), the “+4” method is a long-standing industry standard that many major brands still design around. This accurate bra size calculator uses the traditional method, but we encourage you to try bras using both sizing systems to see what works for your body.

2. What is a sister size?

Sister sizes are a set of bra sizes with the same cup volume. For example, a 32D, 34C, and 36B all have roughly the same cup capacity. Knowing your sister sizes is useful if a bra’s band fits well but the cup doesn’t, or vice versa.

3. How do I know if my bra fits correctly?

A well-fitting bra has a snug, level band, cups that fully contain your breast tissue without spillage or gaps, and straps that don’t dig in. The center gore should lie flat against your chest.

4. Can my bra size change?

Yes, absolutely. Factors like weight fluctuation, hormonal changes (pregnancy, menopause), and exercise can all affect your bra size. It’s a good idea to remeasure every 6 to 12 months.

5. The calculator gave me a size I’ve never seen in stores. What now?

This is a common issue, especially for those with smaller band sizes and larger cup sizes. It often means you’ve been wearing an incorrect sister size for convenience. We recommend searching for your calculated size at online specialty lingerie retailers who carry a wider range of sizes.

6. Why do my straps keep falling down?

This is usually due to a band that is too loose. When the band rides up your back, it changes the angle of the straps, causing them to slip. Try a smaller band size (and go up a cup size to maintain volume).

7. What does it mean if the underwire is digging in?

If the underwire is poking you on the sides, your cup might be too small, forcing breast tissue to push the wire outwards. If it’s digging in at the bottom, your band may be too tight, or the bra’s shape might not be right for your ribcage.

8. Is this accurate bra size calculator perfect?

No calculator is a substitute for trying on a bra. It’s a highly effective starting point designed to get you much closer to your true size than guessing. Always use the result from an accurate bra size calculator as a guide for your shopping.