How to measure your bra size at home accurately

The standard advice for measuring bra size has not changed much in decades: measure the underbust, add a number, measure the bust, subtract, look up a chart. The problem is that this method was designed for a sizing system that most brands no longer follow consistently. This guide explains how to measure accurately and, more usefully, how to interpret the result.

What the measurements actually tell you ¶

The underbust measurement tells you the band size. In the Japanese system, this is measured in centimetres and rounded to the nearest standard band size: 65, 70, 75, 80, 85, and so on. In the European system the same measurement is used directly. In the UK and US systems, a number was historically added to the underbust measurement to arrive at the band size, though most modern brands have moved away from this addition.

The difference between the bust measurement and the underbust measurement gives the cup size. Each centimetre of difference corresponds to one cup size in most systems, though the starting point differs: in the Japanese system, an AA cup starts at a 7.5 cm difference, while in the UK system an A cup starts at a 2.5 cm difference. This is one reason why the same person can be a different cup letter in different systems.

Why the result is a starting point, not an answer ¶

Sizing is not standardised across brands. Two bras labelled 75C from different manufacturers can differ by a full cup volume in practice. This is not a quality issue; it is a consequence of the fact that there is no binding international standard for lingerie sizing.

The measurement gives you a starting point for a fitting session, not a final answer. The useful habit is to note which brands run large or small relative to their stated size, and to use that information when ordering. The size conversion reference on this site includes a short list of brands with known sizing tendencies, updated when readers flag changes.

Sister sizing as a practical tool ¶

If a style does not come in your measured size, sister sizing is the most useful adjustment. Moving up one band size and down one cup size keeps the cup volume the same while loosening the band. Moving down one band size and up one cup size keeps the cup volume the same while tightening the band.

The important thing to know is that changing the band size changes the fit, not just the size. A looser band sits differently and distributes support differently. For some styles and some bodies this is a workable trade. For others it introduces a new problem. The fit guide covers the three most common sister-sizing scenarios in detail.

Measuring is the beginning of the process. The more useful skill is learning to read how a bra fits on the body, which the fit guide covers in its troubleshooting section.