US Bra Size Calculator

An easy and accurate tool to find your perfect US bra size based on your measurements.

Your Calculated Size

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Enter your measurements above to see your result.

Band Size: —

Cup Letter: —

Difference: —

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What is a US Bra Size Calculator?

A us bra size calculator is a specialized tool designed to help you find your most accurate bra size based on two key body measurements: your underbust (band) and your fullest bust. Finding the right bra size is crucial for comfort, support, and health, yet studies suggest over 80% of women wear the wrong size. This can lead to issues like back pain, poor posture, and skin irritation. This calculator simplifies the process, removing guesswork and applying the standard US sizing formula to give you a reliable starting point for your next bra purchase.

This tool is for anyone who is unsure of their current bra size, has experienced changes in their body shape due to weight fluctuations, pregnancy, or aging, or simply wants to confirm they are wearing the best possible fit. A common misunderstanding is that cup sizes are static; for example, that a ‘C’ cup is the same across all band sizes. In reality, cup volume is relative to the band size. Our sister size chart can help you understand this concept, check out our guide on {related_keywords}.

US Bra Size Formula and Explanation

The US bra sizing system is based on two primary calculations: determining the band size and then finding the cup size. The formulas are straightforward:

1. Band Size Calculation: The band size is determined from the underbust measurement. The modern method, which this calculator uses, involves rounding the underbust measurement up to the next even number. For example, if you measure 31 inches, your band size is 32. If you measure 32 inches, your band size is 32.
2. Cup Size Calculation: The cup size is found by subtracting the final band size from the fullest bust measurement. Each inch of difference corresponds to a cup letter.

The core formula is: Cup Difference = Bust Measurement – Band Size.

Variables Table

Explanation of variables used in the calculation

Variable	Meaning	Unit (auto-inferred)	Typical Range
Underbust Measurement	The measurement around the ribcage directly beneath the breasts.	Inches or Centimeters	26 – 48 in
Bust Measurement	The measurement around the fullest part of the bust.	Inches or Centimeters	28 – 60 in
Band Size	The numerical part of a bra size, representing the ribcage frame.	Inches (Even Numbers)	28 – 50
Cup Difference	The difference in inches between the bust and the band size, which determines the cup letter.	Inches	0 – 15 in

Practical Examples

Example 1: A Common Size

Let’s walk through a calculation with some typical measurements.

• Inputs:
  • Underbust Measurement: 31 inches
  • Bust Measurement: 37 inches
  • Units: Inches
• Calculation:
  1. The underbust of 31 inches is rounded up to the next even number, making the Band Size 32.
  2. The cup difference is calculated: 37 inches (Bust) – 32 (Band Size) = 5 inches.
  3. A 5-inch difference corresponds to a DD (or E) cup.
• Result: 32DD

Example 2: Using Centimeters

The calculator handles unit conversions automatically. Here’s how it works if you measure in centimeters.

• Inputs:
  • Underbust Measurement: 85 cm
  • Bust Measurement: 102 cm
  • Units: Centimeters
• Calculation (Internal Conversion to Inches):
  1. Underbust: 85 cm / 2.54 = 33.46 inches. This is rounded up to the next even number, making the Band Size 34.
  2. Bust: 102 cm / 2.54 = 40.16 inches.
  3. The cup difference is calculated: 40.16 inches (Bust) – 34 (Band Size) = 6.16 inches.
  4. A 6-inch difference corresponds to an F (or DDD) cup.
• Result: 34F

For more examples, see our {related_keywords} page.

How to Use This US Bra Size Calculator

Using our calculator is simple. Follow these steps for the most accurate results:

1. Select Your Units: Start by choosing whether you will enter your measurements in inches or centimeters from the dropdown menu.
2. Measure Your Underbust: While wearing a non-padded bra, wrap a soft measuring tape snugly around your ribcage, just below your bust. Ensure the tape is level all the way around and exhale for the tightest measurement. Enter this number into the “Underbust Measurement” field.
3. Measure Your Bust: Wrap the tape loosely around the fullest part of your bust. Again, make sure the tape is parallel to the floor. Enter this value into the “Bust Measurement” field.
4. Interpret the Results: The calculator will instantly display your calculated bra size, including the band size, cup letter, and the measurement difference used to determine the cup. The results update in real-time as you type.
5. Explore Sister Sizes: Once you have your size, the “Sister Size Chart” will populate. Sister sizes have the same cup volume but different band and cup letter combinations, which can be useful if a band feels too tight or loose. You can find more about this in our {related_keywords} guide.

Key Factors That Affect Bra Size

Your bra size is not static and can change multiple times throughout your life. Understanding the factors that influence it can help you know when it’s time to re-measure.

• Weight Fluctuations: Gaining or losing even 10 pounds can significantly alter your bra size. Since breasts are composed of fatty tissue, they are often one of the first places to show changes in weight.
• Hormonal Changes: The menstrual cycle can cause temporary swelling and tenderness, affecting breast volume. It’s often best to measure when your breasts are in their most “normal” state, typically after your period.
• Pregnancy and Breastfeeding: Hormones during pregnancy cause breasts to enlarge in preparation for milk production. Your size will likely change both during pregnancy and after you stop breastfeeding.
• Aging: As we age, skin loses elasticity, and hormonal shifts (like menopause) can change breast tissue composition, leading to changes in size and shape.
• Exercise: While exercise doesn’t change breast tissue itself, building pectoral muscles can make breasts appear perkier. Significant weight loss from exercise, however, will reduce breast size.
• Bra Style and Brand: Just like clothing, bra sizes can vary between different brands and even different styles (e.g., a plunge vs. a full-cup) from the same brand. Always use your calculated size as a starting point. To understand more, read our guide on {related_keywords}.

Frequently Asked Questions (FAQ)

1. How often should I measure my bra size?

It’s recommended to measure your bra size at least once a year, or anytime you experience significant life changes like weight loss/gain, pregnancy, or starting/stopping hormonal medications.

2. What’s more important: the band or the cup?

The band provides 80% of a bra’s support, so getting the band size right is critical. If the band is too loose, the straps will bear too much weight, leading to shoulder pain. A snug (but not tight) band is key.

3. My measurements are between sizes. What should I do?

If you’re between sizes, sister sizing is your best friend. If your calculated band is 33, for instance, you might try both a 32 and a 34 band. Remember, if you go down a band size, you need to go up a cup size to maintain the same cup volume (e.g., 34C -> 32D).

4. Why does the calculator add to my underbust measurement?

The traditional “+4” method is largely outdated. This calculator uses the modern “round up to the next even number” method, which provides a much snugger and more supportive fit, reflecting current lingerie industry standards.

5. Can I use this calculator for UK or EU sizes?

This calculator is specifically for US bra sizing. While some sizes overlap with the UK, cup sizing diverges after DD. EU sizing uses a completely different system based on centimeters. Always look for a region-specific calculator.

6. What are sister sizes?

Sister sizes are alternative sizes where the cup volume is the same. For example, a 34C, 32D, and 36B all have the same cup volume. Knowing these can help you find a better fit if your band is too tight or loose in your standard size. Our calculator provides a chart for this.

7. The calculated size seems very different from what I wear. Is it right?

It’s very common for the calculated size to be a surprise, especially if it’s been a while since you were last fitted. We recommend trying on a bra in your newly calculated size and its sister sizes to judge the fit for yourself. Look for signs of a good fit: the band is level, the cups don’t have spillage or gaps, and the center panel sits flat against your chest.

8. Does breast shape matter?

Absolutely. While this calculator gives you a size, your breast shape (e.g., round, shallow, projected) will determine which bra *styles* fit you best. A size 34C in a t-shirt bra might fit very differently from a 34C in a balconette bra. You may find our {related_keywords} guide helpful.