What is the 'Add 4' rule and why is it considered incorrect?

The 'Add 4' rule is an old measuring technique where you add 4 or 5 inches to your actual underbust measurement to find your band size. This was invented when bra fabrics were non-stretchy and rigid. With modern elastic materials, adding 4 inches results in a band that is far too loose to provide support. In a correctly fitted bra, the band should be firm enough to stay level and provide most of the lift, allowing the shoulder straps to sit comfortably without digging in. Most modern fit experts recommend using your actual underbust measurement as your band size.

How should a correctly fitting bra actually feel?

A perfect fit involves three checks. First, the band should be level all the way around your back and snug enough that you can only fit two fingers underneath it. Second, the 'gore' (the flat part between the cups) should rest flat against your breastbone, not hovering in the air. Third, there should be no 'spillage' over the top of the cups (which means the cup is too small) and no 'gapping' or wrinkling in the fabric (which means the cup is too large). If you can't find a perfect fit in your calculated size, try a 'Sister Size'.

What are 'Sister Sizes' and how do I use them?

Sister Sizes are pairs of bra sizes where the cup volume is identical, even though the band and cup letters are different. For example, a 34C has the exact same cup volume as a 32D and a 36B. If you try a 34C and the cups feel perfect but the band is painfully tight, you should try your 'Sister Size Up'—which is a 36B. This gives you more room in the band while keeping the cup capacity the same. If the 34C band is too loose, you would try your 'Sister Size Down', the 32D.

How often should I re-measure my bra size?

Professional fitters recommend measuring yourself at least once a year, or whenever you experience a weight change of 5-10 pounds. Factors like exercise, diet, hormonal changes, pregnancy, and aging all significantly alter breast tissue density and ribcage shape. Additionally, bras physically stretch out over time; a bra that fit perfectly six months ago may now be too loose in the band due to the breakdown of the elastic fibers (elastane), indicating it's time for a replacement.

Bra Size Calculator

Find your perfect bra size using bust and band measurements. Supports US sizing system.

Complete User Guide

Using the Bra Size Calculator allows you to find a comfortable, supportive starting point for your lingerie, moving away from the common "plus 4" errors. Follow these precise measurement steps:

Step 1: Measure your Band Size. Wrap a soft measuring tape snugly around your ribcage, directly under your bust. Keep the tape level and firm. If you get an odd number, round to the nearest even number for most brands.

Step 2: Enter the Band Measurement into the first input field.

Step 3: Measure your Bust Size. Wrap the tape around the fullest part of your bust. Do not pull the tape tight; it should rest gently against your skin without indenting the tissue.

Step 4: Enter the Bust Measurement into the second input field.

Step 5: Click "Calculate."

Step 6: Review your Result. The calculator will determine your Band size and your Cup letter (e.g., 34C). It will also provide "Sister Sizes"—alternative sizes that have the same cup volume but different band widths—which are essential for fine-tuning the fit between different brands.

The Mathematical Formula

Cup = Bust - Band; Band = Round(Underbust)

The Bra Size Calculator uses the standard professional subtraction method to determine cup volume relative to the ribcage.

The mathematical logic is: 1. Band Size = The rounded even number of your underbust measurement (e.g., 31" underbust becomes a 32 band). 2. Cup Size Index = (Bust Measurement - Band Size).

The difference in inches determines the cup letter: - 1 inch difference = A Cup - 2 inch difference = B Cup - 3 inch difference = C Cup - 4 inch difference = D Cup - 5 inch difference = DD (or E in some systems) - 6 inch difference = DDD (or F)

Example: A woman measures 30 inches under her bust and 33 inches around the fullest part. Band = 30. Difference = 3 inches. Result = 30C. Note: We avoid the outdated "add 4 inches to the band" rule, as it results in a band that is too loose to provide any actual vertical support.

About Bra Size Calculator

The Bra Size Calculator is designed to fix the "fit crisis" in the lingerie industry, where an estimated 80% of women are wearing the wrong bra size. For decades, many retailers used an "Add 4" rule to simplify their inventory, which often forced women into bands that were too large and cups that were too small. Proper support should come 80% from the band and only 20% from the shoulder straps. A band that is too loose causes the weight to hang entirely on the shoulders, leading to chronic back pain and posture issues. By using direct measurements and modern sizing logic, this tool helps you find a size that balances the weight correctly across your ribcage, significantly improving daily comfort and physical health.

Frequently Asked Questions

Related Tools

Unit Converter

Calculator

Verified Precise

Secure

100% Free

Precise