Sewing Cup Size vs Bra Cup Size
They are not the same thing. Enter your three measurements below to see both sizes — and find out whether you need a Full Bust Adjustment for your next sewing project.
Sewing Cup Size vs Bra Cup Size Calculator
Enter three measurements to see why your sewing cup size is almost never the same as your bra cup size.
Wrap tape under your armpits, above your bust. This is the key measurement for sewing patterns.
Measure around the fullest part of your bust, usually at nipple level. Keep the tape parallel to the floor.
Measure directly under your bust, around your ribcage. Pull snug.
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Why Sewing Cup Size and Bra Cup Size Are Different
This is one of the most common points of confusion when starting to sew your own clothes. Both systems use cup letters (A, B, C, D, etc.), but they are calculated from completely different body measurements.
Bra Cup Size = Full Bust − Underbust
A bra supports from below. The band sits around your ribcage, so the underbust measurement is the reference point. The cup letter tells you how many inches of projection your breasts have relative to your ribcage. A 1-inch difference is an A cup, 2 inches a B, and so on.
Sewing Cup Size = Full Bust − High Bust
A garment hangs from your shoulders. The high bust measurement (around your chest, under your armpits, above your breasts) determines how the shoulders, neckline, and armholes fit. The sewing cup letter tells you how much extra room your bust needs relative to your upper frame.
Real-World Example
Someone with a 30-inch underbust and a 40-inch full bust wears a 30J bra (10-inch difference). But if their high bust measures 36 inches, their sewing cup size is only D (40 − 36 = 4 inches). That's a six-cup-letter difference between the two systems — and it's completely normal.
What Is a Full Bust Adjustment (FBA)?
Most commercial sewing patterns are drafted for a B sewing cup (a 2-inch difference between full bust and high bust). If your sewing cup is significantly larger than B, you should choose your pattern size based on your high bust measurement (so the shoulders fit), then perform a Full Bust Adjustment to add room at the bust line without changing the shoulder fit. If your sewing cup is smaller than B, you need a Small Bust Adjustment (SBA) instead.
How to Take the Three Measurements
1. High Bust (Upper Chest)
Wrap the measuring tape around your upper chest, directly under your armpits and above your breast tissue. Keep the tape parallel to the floor. This measurement is used to choose your sewing pattern size and is the reference point for sewing cup calculations.
2. Full Bust (Fullest Point)
Measure around the fullest part of your bust, usually at nipple level. Wear your everyday bra while measuring. Keep the tape level all the way around and do not compress breast tissue. This measurement is used in both calculations but referenced against different anchor points.
3. Underbust (Ribcage)
Measure directly under your bust, around your ribcage where a bra band would sit. Pull the tape snug (not loose). Exhale normally — don't pull it tight like a corset. This measurement is used only for the bra size calculation and is not relevant to sewing patterns.