Bra Size Calculator
Enter your snug underbust and fullest bust measurements to estimate your starting bra size, review the calculation breakdown, and explore volume-preserving sister sizes.
Your Starting Size
| Underbust Measurement | — |
|---|---|
| Bust Measurement | — |
| Calculated Band Size | — |
| Cup Difference (Bust − Band) | — |
Try These Sister Sizes
Sister sizes share approximately the same cup volume with a firmer or roomier band. If your starting size feels slightly loose or tight in the ribcage, test these sister sizes:
Bra sizing varies between brands and styles. This calculator provides a starting size, not a guarantee of fit.
Does Your Bra Actually Fit?
A calculated size is always your starting baseline. When trying on a new bra, use these six practical fit clues to evaluate how the garment interacts with your body:
The band is likely too loose and failing to provide anchor support. Try a sister size with a smaller band (e.g., move from 34D to 32DD).
Recheck your underbust measurement. If cups fit well but the ribcage is constricted, test a sister size with a larger band (e.g., move from 34D to 36C).
Breast tissue escaping at the top or sides indicates the cup is too small, or the cup neckline is too shallow for your breast fullness. Move up one cup letter.
Gaping can mean the cup is too large, but often indicates a shape mismatch (such as a rigid molded cup on softer breast tissue). Consider a balconette or unlined cut.
Adjust strap sliders first. If slipping persists, the band may be riding up, allowing straps to loosen, or the bra style may have wide-set straps.
Underwires should sit flush against the ribcage behind all breast tissue. If the wire rests on tissue, you need a wider wire or a larger cup volume.
Note: These fit checkpoints are general editorial guidelines. Individual breast shape, tissue density, and bra construction each play a significant role in comfort.
How the Starting Size Calculation Works
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1
Measure your snug underbust
Your ribcage measurement forms the structural foundation of the bra, determining your numeric band size.
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2
Measure your fullest bust
Taking your bust measurement around the fullest projection captures total breast tissue circumference.
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3
Internal unit conversion
When centimeters are selected, measurements are converted to inches ($cm / 2.54$) while preserving internal decimal precision.
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4
Determine starting band size
We round your snug underbust to the nearest manufactured even-numbered band size (28 through 52), avoiding arbitrary legacy +4 additions.
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5
Calculate starting cup letter
The difference between your bust measurement and calculated band size maps directly to standard US cup letters (1" = A, 2" = B, 3" = C, 4" = D, 5" = DD, etc.).
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6
Generate volume-preserving sister sizes
We calculate adjacent band-and-cup combinations that hold identical cup volume, giving you practical options for different brand fits.
Frequently Asked Questions
To calculate your bra size, take two measurements with a flexible tape: your snug underbust directly beneath your breasts and your bust around the fullest point. Your underbust rounded to the nearest even number indicates your band size (e.g., 33 inches rounds to a 34 band). Subtracting your band size from your bust measurement gives your cup difference: each 1-inch difference represents one cup letter (1 inch is an A cup, 2 inches is a B cup, 3 inches is a C cup, and 4 inches is a D cup).
Sister sizes are bra sizes that share the exact same cup volume despite having different band sizes. When you decrease your band size by one step (2 inches), you must increase your cup by one letter to maintain the same breast capacity (for example, a 34D sister sizes down to a 32DD). Conversely, increasing the band by one step requires decreasing the cup by one letter (34D sister sizes up to a 36C). Sister sizing allows you to adjust band firmness across brands without losing cup fit.
Bra sizes vary between brands because manufacturers use different proprietary fit models, pattern grading rules, and fabrics. A lace unlined bra with high elastane content stretches differently than a stiff molded foam T-shirt bra. Furthermore, breast shape (shallow, projected, full-on-top, or full-on-bottom) changes how breast tissue fills a specific cup style. For full-bust support dynamics and construction differences across brands, explore our in-depth guide to bras for large breasts.
The "+4" method was introduced decades ago when bra bands were made from rigid, non-elastic woven fabrics (such as cotton and rayon) that required extra inches for breathing ease. Modern lingerie fabrics contain flexible elastane and spandex engineered to stretch snugly around the ribcage. Adding 4 inches puts women into bands that are too loose to provide support and cups that are too small, forcing the shoulder straps to carry weight they were never designed to hold.
A bra size calculator provides an accurate mathematical starting point based on your tape measurements. However, no two-dimensional measurement can fully account for individual breast tissue density, root width, projection, or personal comfort preference. For the best result, use your calculated size as a starting point, check the fit using our Fit Check guidelines, and adjust with sister sizes as needed. If you prefer everyday support without an underwire, see our tested recommendations for the best wireless bras for large busts.