About Bra Size Calculator
Wearing the right bra size is about far more than aesthetics. A well-fitted bra distributes breast weight across the rib cage, reducing strain on the shoulders and upper back, improving posture, and preventing skin irritation and tissue stretching. Research suggests that as many as 80% of women are wearing the wrong bra size, often because they rely on outdated fitting myths—such as adding 4 or 5 inches to the band measurement—or because they focus only on cup letter without realizing that cup volume changes with band size. This Bra Size Calculator gives you a data-driven starting point. It uses two simple measurements—band (underbust) and bust (fullest part)—to calculate the difference in inches and map that difference to a cup size. For example, a 34-inch band and a 37-inch bust produce a 3-inch difference, which maps to a C cup and a 34C size. Importantly, a 36C is not the same as a 34C: as the band increases, the cup volume increases even though the letter stays the same. Because bras are the most intimate piece of daily support wear, fit matters. A band that rides up in the back, straps that dig into shoulders, cups that gap or overflow, or underwire that sits on breast tissue are all signs of a mismatch. Use this calculator to narrow your size, then try on bras in that size and neighboring "sister sizes" (e.g., 34C, 36B, 32D) to account for brand and style variation.
How It Works
The calculator takes your band measurement and your bust measurement, both in inches. It subtracts the band value from the bust value to find the difference, then converts that difference into a cup letter. Each whole inch of difference corresponds to one cup increment on the US sizing scale: a 1-inch difference is an A cup, 2 inches is a B cup, 3 inches is a C cup, and so on. The result is presented as your band number followed by the calculated cup letter, such as 36D. The band size is not modified by the tool, so accuracy depends on measuring the underbust snugly and level all the way around. If the difference falls outside the standard range, the result is clamped to the smallest or largest cup size supported.
Formula & Calculation Logic
The core logic is: Difference = bust size − band size Cup index = rounded difference − 1 The cup index selects from the sequence AA, A, B, C, D, DD/E, DDD/F, G, H, I, J. Rounding the difference and subtracting one aligns the 1-inch A-cup increment with the array index. The final bra size is the original band plus the selected cup letter. This approach assumes a US-style sizing convention; UK, European, and Australian systems use different cup increments and band conversions.
Step-by-Step Guide
- Step 1: Wrap a soft measuring tape around your ribcage directly under your bust, keeping it level and snug but not tight.
- Step 2: Measure around the fullest part of your bust, usually across the nipple line, with the tape level.
- Step 3: Round both measurements to the nearest whole inch.
- Step 4: Subtract the band measurement from the bust measurement to get the difference.
- Step 5: Match the difference to the cup scale and combine it with your band measurement to get your estimated bra size.
Example Calculations
- Scenario 1: A 32-inch band and a 35-inch bust give a 3-inch difference, resulting in a 32C.
- Scenario 2: A 38-inch band and a 42-inch bust give a 4-inch difference, resulting in a 38D.
- Scenario 3: A 30-inch band and a 33-inch bust give a 3-inch difference, resulting in a 30C, which has a smaller cup volume than a 34C.
Common Use Cases
- Shopping online when you cannot try bras on before buying.
- Tracking how size changes after weight loss, pregnancy, or surgery.
- Improving comfort and reducing shoulder or back strain during long workdays.
- Starting a professional bra fitting with an accurate baseline size.
Pro Tips
- Measure without a padded or underwire bra, or measure while braless, for the most accurate bust reading.
- Use a mirror or ask someone to help keep the measuring tape perfectly horizontal.
- Test the band fit by pulling it away from your body; it should stretch no more than about 2 inches.
- Explore sister sizes if your exact size feels tight or loose in a particular brand.
Common Mistakes to Avoid
- Adding extra inches to the band measurement, an outdated practice that skews cup size.
- Measuring over a thick or padded bra, which inflates the bust reading.
- Pulling the measuring tape too tight or too loose, producing inconsistent results.
- Ignoring band fit and assuming the same cup letter means the same volume across bands.
Why Use This Tool?
- Better support and weight distribution, which can reduce shoulder and back discomfort.
- A smoother silhouette under clothing.
- Less time wasted returning ill-fitting bras.
- A clearer starting point for professional fittings or online shopping.