Bra Size Calculator
Find your correct bra size using the modern 6-measurement method (underbust loose/snug/tight, bust standing/leaning/lying). Includes sister sizes and explains why the traditional +4 method is outdated.
Bra Size
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The Formula
Modern bra sizing uses multiple bust measurements (standing, leaning, and lying) for greater accuracy, recognizing that breast tissue distributes differently in each position. The band size is the snug underbust measurement rounded to the nearest even number. The cup size is the difference between the average of the three bust measurements and the band measurement, mapped to cup letters (1 inch = A, 2 inches = B, 3 inches = C, etc.). Sister sizes provide alternative band/cup combinations that hold the same volume.
Variable Definitions
Band & Cup Size
Band is the underbust rounded to the nearest even. Cup is the difference between average bust and underbust mapped to a letter (1" = A, 2" = B, etc.). Together they form your size (e.g., 34C).
Average Bust
The arithmetic mean of the standing, leaning, and lying bust measurements. Using the average smooths out measurement variations and provides a more representative bust circumference.
Sister Size
An alternative size combination where the band changes by ±2 inches and the cup changes by ±1 step to maintain approximately the same cup volume. Useful when your recommended size is not available.
How to Use This Calculator
- 1
Select your preferred measurement unit: inches or centimeters.
- 2
Take all six measurements carefully using a soft, flexible measuring tape. Measure snug but not tight.
- 3
Enter each measurement in the corresponding input field — standing bust, leaning bust, lying bust, snug underbust, tight underbust.
- 4
View your calculated band size, cup size, full bra size, sister sizes, and a detailed measurement breakdown.
- 5
Check sister sizes — if your exact size is unavailable, these alternatives hold the same cup volume.
Modern bra sizing uses the snug underbust for band and average of 3 bust measurements for cup, plus sister sizes.
Understanding the Concept
Traditional bra fitting uses just two measurements (underbust and standing bust) with an outdated "+4" method that adds 4 inches to the band size — a practice from an era when bras were made of non-stretchy materials. Modern bras contain elastic fabrics that provide support through a snug band, not an artificially inflated one. Adding 4 inches puts most of the support on the straps instead of the band, leading to poor fit, shoulder pain, and inadequate support. This calculator follows the "A Bra That Fits" (ABTF) methodology, developed by the bra-fitting community, which uses six measurements to account for the fact that breast tissue distributes differently when standing, leaning forward at 90 degrees, and lying down. This multi-position approach gives a much more accurate starting point for finding well-fitting bras. Many people who use this method discover they have been wearing the wrong size for years — often a band that is too large and a cup that is too small. No single measurement tells the whole story, which is why the three-position bust averaging is central to this approach.
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