True Bra Size Calculator

An accurate tool to find your correct bra size and improve your comfort and fit.

What is a True Bra Size Calculator?

A true bra size calculator is a specialized tool designed to help you determine your most accurate bra size based on specific body measurements. Unlike generalized clothing sizes, bra sizes depend on two critical measurements: the circumference of your ribcage just below the bust (the band size) and the circumference of the fullest part of your bust (the bust size). Getting these right is crucial, as studies show that a vast majority of women wear the wrong bra size, leading to discomfort, poor support, and even health issues like back and neck pain. This true bra size calculator uses a modern fitting formula to give you a reliable starting point for finding a comfortable and supportive bra.

The True Bra Size Formula and Explanation

The calculation involves two main steps: determining your band size and then your cup size. The relationship between these two figures gives you your final bra size (e.g., 34C).

1. Band Size: This is based on your underbust measurement. Modern methods suggest that the underbust measurement, rounded to the nearest even number, is your band size. For example, a 31-inch underbust would round up to a 32 band.
2. Cup Size: This is determined by the difference between your bust measurement and your calculated band size. Each inch of difference corresponds to a letter.

The formula for cup difference is: Cup Difference = Bust Measurement - Band Size

Variables Table

Description of variables used in the true bra size calculator.

Variable	Meaning	Unit	Typical Range
Underbust Measurement	The measurement around the ribcage directly beneath the breasts.	inches or cm	26 – 50 in / 66 – 127 cm
Bust Measurement	The measurement around the fullest part of the breasts.	inches or cm	28 – 60 in / 71 – 152 cm
Band Size	The numeric part of your bra size, derived from the underbust.	(Varies)	28 – 52
Cup Size	The letter part of your bra size, representing breast volume relative to the band.	(Letter)	AA – K

Practical Examples

Example 1: Using Inches

Let’s say a person has the following measurements:

• Inputs:
  • Underbust Measurement: 31 inches
  • Bust Measurement: 35 inches
  • Unit: Inches
• Calculation:
  1. The underbust (31″) is rounded up to the nearest even number, making the Band Size 32.
  2. The cup difference is Bust (35″) – Band (32″) = 3 inches.
  3. A 3-inch difference corresponds to a C cup.
• Result: The calculated true bra size is 32C.

Example 2: Using Centimeters

Now, let’s consider measurements in centimeters:

• Inputs:
  • Underbust Measurement: 85 cm
  • Bust Measurement: 102 cm
  • Unit: Centimeters
• Calculation:
  1. The inputs are first converted to inches (1 inch = 2.54 cm).
  2. Underbust: 85 cm / 2.54 = 33.46 inches. This rounds up to a Band Size of 34.
  3. Bust: 102 cm / 2.54 = 40.16 inches.
  4. The cup difference is Bust (40.16″) – Band (34″) = 6.16 inches.
  5. A 6-inch difference corresponds to a DDD/F cup.
• Result: The calculated true bra size is 34F. For help with your fit, check out our bra fitting guide.

How to Use This True Bra Size Calculator

1. Select Your Unit: Start by choosing whether you’ll enter your measurements in inches or centimeters from the dropdown menu.
2. Measure Your Underbust: Wrap a soft measuring tape snugly around your ribcage, directly underneath your breasts. Ensure the tape is level all the way around. Enter this number into the “Underbust Measurement” field.
3. Measure Your Bust: Now, wrap the tape around the fullest part of your bust, typically over the nipples. The tape should be level and not too tight. Enter this value into the “Bust Measurement” field.
4. View Your Results: The calculator will instantly display your calculated bra size. The primary result is your starting point.
5. Consider Sister Sizes: The calculator also provides “sister sizes.” These are alternate sizes where the cup volume is the same. For example, if your band feels too tight, you could try the sister size with a larger band and smaller cup (e.g., if you are a 34C, try a 36B). Learning about your sister size chart can be very helpful.

Key Factors That Affect Bra Fit

Your calculated size is a great starting point, but several other factors can influence the perfect fit:

• Brand and Style Variation: Just like with shoes, bra sizing can vary significantly between different brands and even different styles (e.g., plunge, t-shirt, balconette) from the same brand.
• Breast Shape: Breasts come in many shapes (e.g., round, teardrop, slender, east-west). Some bra styles are better suited for certain shapes. A full guide on understanding breast shape can help you choose better.
• The “Scoop and Swoop”: After putting on a bra, you should lean forward and “scoop” your breast tissue from your sides into the cups to ensure everything is properly contained and supported.
• Band Tightness: The band provides about 80% of a bra’s support. It should be snug and level around your torso. You should be able to fit only two fingers underneath it. If it rides up, it’s too loose.
• Cup Fit: There should be no gaping at the top of the cups, nor should your breasts be spilling out (the “quad-boob” effect). The underwire should sit flat against your sternum and fully encase your breast tissue without digging in.
• Strap Adjustment: Straps should be adjusted to be comfortable but secure, providing the final 20% of support. They shouldn’t dig into your shoulders.

Frequently Asked Questions (FAQ)

How often should I measure my bra size?

It’s a good idea to remeasure yourself every 6 to 12 months. Body changes due to weight fluctuations, pregnancy, hormonal shifts, or exercise can all affect your bra size.

What are sister sizes and why are they important?

Sister sizes are related bra sizes with the same cup volume. To find a sister size, you go up one band size and down one cup size, or vice versa. For example, the sister sizes for a 34C are 32D (tighter band) and 36B (looser band). They are useful if a bra fits well in the cup but not the band.

My breasts are two different sizes. What should I do?

This is very common! Always fit your bra to the larger breast. For the smaller breast, you can either tighten the strap slightly or add a small insert (a “cookie”) to fill the gap and create a more symmetrical look.

Why does the underwire dig into my chest?

This can happen for a few reasons: the band may be too tight, or the cup size is too small, causing the wire to sit on breast tissue instead of your ribcage. It could also be that the shape of the underwire isn’t right for your body.

Why do my bra straps keep slipping off?

Slipping straps are often a sign that your bra band is too loose and riding up your back, which loosens the straps. First, try tightening the straps. If that doesn’t work, you may need a smaller band size (and likely a larger cup size to compensate, per sister sizing).

What if I’m between two sizes?

If you’re between sizes, it’s often best to try both. Given the inconsistencies between brands, you might be one size in one brand and a different size in another. Trying on is always the best way to confirm your fit.

Does cup size stay the same across different band sizes?

No, and this is a common misconception. Cup volume is relative to the band size. A 32D cup is smaller than a 36D cup. This is why sister sizing is so important to understand. You can learn about different styles on our page about types of bras.

Is there a difference between an underwire and a wireless bra fit?

Yes. While your core measurements remain the same, the fit and feel will be different. Underwire bras provide more structure and lift, while wireless bras prioritize comfort and flexibility. You might find you prefer a slightly different size or style depending on whether you choose an underwire vs wireless bra.