Bra Size Calculator

You need two measurements and a soft tape. First measure your band: wrap the tape snugly around your ribcage directly under your bust, keeping it level and parallel to the floor. Then measure your bust: wrap the tape around the fullest part, usually across the nipples, without pulling it tight. Enter both numbers into this bra size calculator and it works out your band and cup for you. Measure in a non-padded bra or without one, stand relaxed, and breathe out normally so the band figure is not squeezed too small.

A sister size is a different band and cup that holds the same cup volume. Go down one band and up one cup, or up one band and down one cup, and the cup capacity stays the same. So 34C, 32D, and 36B are all sisters. This matters because the band you calculate may not be the one that fits best in a given brand, and knowing the sisters lets you swap bands without losing cup volume. If a 34C band digs in, try the 36B; if it rides up, try the 32D. This tool prints your sister sizes automatically.

The old +4 method told you to add four or five inches to your underbust measurement, but modern band elastic is far stretchier than it was decades ago, so most fitters now measure the band directly. This calculator uses your snug underbust measurement rounded to the nearest even number, which is the method fitting communities recommend today. Adding four inches usually gives a band that is too loose to support you, since the band, not the straps, does most of the lifting. If a brand's own chart still uses the +4 method, follow the chart for that brand and then check the fit.

The three systems letter the cups differently once you go past a D. US brands double the letters, so the sizes run D, DD, DDD, then G. UK sizing adds single letters, running D, DD, E, F, FF, G. EU sizing uses one clean letter per cup, running D, E, F, G with no doubling, and it labels the band in centimetres instead of inches. The cup volume is the same, only the label changes, which is exactly why an import bra can feel like a surprise. The calculator shows all three so you can shop across regions with confidence.

Fix the band first, because it carries the support. If the band is comfortable but the cup gaps or overflows, stay on that band and change only the cup letter up or down. If the cup looks right in the mirror but the band rides up your back or digs in, keep the cup and move to a sister size with a different band. The size this tool gives you is a starting point; the mirror and how the bra feels after a few minutes are the real test. Aim for a band that sits level and snug on the loosest hook.

Remeasure any time your body changes: weight loss or gain, pregnancy and nursing, a new workout routine, or hormonal shifts can all move your size. Even without a change, checking once or twice a year is sensible, because bands wear out and stretch over time, which quietly nudges you into the wrong size. It only takes a minute with a tape measure, and wearing the correct size makes a bra far more comfortable and supportive. Keep the tape snug but not tight for the truest reading.

Cup shape, wire width, gore height, and how much the band stretches all vary from one maker to the next, so a 34C in one label can feel like a 34B or 34D in another. Sizing charts are a guide, not a guarantee, which is why sister sizes and trying a couple of options are so useful. Use the size this calculator gives you as your anchor size, then try it and one sister size on either side in each brand. Over time you will learn which brands run large or small for your shape.