Bust Size Calculator

Calculate your correct bra band and cup size instantly. Compare the Modern Snug Fit and Classic +4 methods, find sister sizes, and get custom fit tips.

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Step-by-Step Guide

Measure Bust & Band in 4 Easy Steps

Measure physical underbust and fullest bust to determine correct band and cup size fit.

1
Measure Underbust (Ribcage)

Wrap measuring tape snug around your ribcage directly beneath your breasts. Keep it flat and parallel to the floor.

Underbust Tape Band Ribcage
2
Measure Over Bust

Wrap the tape around the fullest part of your bust (across the nipples). Keep it loose and even, without indenting the skin.

Over Bust Fullest projection
3
Choose Fitting Method

Toggle between "Modern Snug Fit" (highly recommended for modern stretchy materials) and "Classic +4 Method" (the traditional industry formula).

Modern Snug Fit Classic +4 method
4
Review Perfect Sizes & Sister Sizes

Instantly obtain your band and cup sizes, read fit check troubleshooting pointers, and view your sister size options.

Bust & Band Sizes Sister sizing
Client-side processing
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Zero server uploads
Fitting Engine

Dual Sizing Mode Engine

Determine your exact underwear measurements by selecting from classic or modern sizing equations. Includes global conversions.

Dual Sizing Standard

Compares standard Classic +4 and Modern snug equations side-by-side.

Sister Sizes Explorer

Provides sister size matching cards to find alternative chest/band configurations.

Fitting Troubleshooting

Contains structured fit pointers (e.g. strap tension, underwire digging, cups overflow).

Multi-Unit Ready

Inputs support centimeter (cm) and inch measurements flawlessly.

FAQ

Common Questions

What is the "+4 rule" in bra fitting?
The "+4 rule" is a legacy manufacturing convention from the mid-20th century when brassieres were made of non-elastic materials (cotton/linen), necessitating adding 4 to 5 inches to the ribcage to ensure comfortable breathing room. With modern elastane/spandex, this rule often results in a band that is too loose and cups that are too small.
Why does my bra band ride up my back?
A band riding up in the back indicates it is too large. When the band is too loose, it cannot anchor itself securely around your ribcage. Instead, the weight of the breasts pulls the front down and forces the back of the band to pull upwards. Try dropping a band size and increasing a cup size.
What is the center "gore" and how should it sit?
The gore is the small fabric panel connecting the two cups at the front. In a correctly fitting underwire bra, the gore should lay completely flat against your breastbone. If it hovers or pulls away from the skin, the cups are likely too small, causing breasts to push the entire bra away from your frame.
What are the signs of cups that are too small?
Key indications: breast tissue spilling out over the top of the cups (creating a "double bust" line), breast tissue escaping from the underarms or side wire panels, or wires pressing directly onto sensitive breast tissue instead of framing it against the ribcage.
How often should I measure my bra size?
It is recommended to measure once a year, or after any significant body weight fluctuations, pregnancy, shifts in muscle mass from weightlifting, or hormonal changes. Over time, bra bands also stretch out, which changes how they support you.