Bra Sizing Guide

Understanding your true bra size is one of the most overlooked skills in sewing and costuming. Most people are wearing the wrong size — and that affects how every garment fits across the bust. This guide explains how to measure yourself correctly and what those numbers actually mean.

Why Bra Size Matters for Sewing

Commercial sewing patterns are drafted to a specific cup size — almost always a B cup. If your cup size is larger, the pattern will not fit your bust correctly without alteration, no matter how accurately you cut the size matching your measurements. Understanding your cup size helps you know immediately whether you need a Full Bust Adjustment (FBA).

How to Measure Your Bra Size

Step 1 — Band Size

Measure around your ribcage directly under your bust. Keep the tape snug but not tight. Round to the nearest whole number. If the measurement is odd, round up. This is your band size (e.g., 34, 36, 38).

Step 2 — Bust Measurement

Measure around the fullest part of your bust, keeping the tape parallel to the floor. Measure while wearing a non-padded, well-fitting bra if possible.

Step 3 — Calculate Cup Size

Subtract your band size from your bust measurement. The difference determines your cup size:

Sister Sizes

Sister sizes are bra sizes that share the same cup volume but with different band sizes. A 34C, 36B, and 32D all have approximately the same cup volume — they are sister sizes of each other. This is important when fitting garments because the cup volume you need does not change just because the band size does.

What This Means for Pattern Selection

When choosing a pattern size, use your high bust measurement (measured across the chest above the bust, around the shoulder blades) rather than your full bust measurement. Then perform a Full Bust Adjustment to add the necessary cup volume. This gives you a far better starting fit than cutting the size matching your full bust.