Sister Sizes for a Larger Bust

The cup ladder you are moving along

Sister sizing means stepping one place along the UK cup ladder. In the upper range, that ladder is:

E, F, FF, G, GG, H, HH, J, JJ, K, KK

Every entry is a full size. One step up from F is FF, and not G. One step down from H is GG. There is no I. Most sister-sizing mistakes at larger cups come from skipping a double letter and landing a whole cup away from where you meant to be.

Sister size chart for larger cups

Each row holds one cup volume.

For any size not shown, the sister size calculator covers bands 28 to 48 and cups to KK.

Why direction matters more at larger sizes

A fuller bust is heavier, and the band is what carries that weight. With a small bust, a slightly loose band costs little. With a G cup, the same slack lets the cups drop, shifts the load to the straps, and tilts the wires forward.

• Sister sizing down (firmer band) is usually safe and often an improvement, provided you can still breathe comfortably.
• Sister sizing up (looser band) should be a last resort. Do it only when the band is painful on the loosest hook and the cups pass every check.

The stock problem

Each step along a row of the chart changes where you can shop. High-street ranges tend to thin out above a G cup, and bands of 28 and 30 are scarce outside specialists. So a 32GG may be findable where a 30H is not, and it is tempting to buy what is on the rail.

If you are a 30H, a 32GG will hold the right volume on a band that is too loose, and a 34G will be looser still. Wearing one occasionally is fine. As your everyday size, it works against you. Full-bust brands and specialist retailers stock these sizes as standard: see where to buy bras in unusual sizes.

Wires change more noticeably

At larger cup volumes the underwire is long, and its width changes visibly from one band to the next. After a sister-size move, check two things:

• At the side, the wire should end on your ribcage behind the breast tissue. If it has moved onto tissue, the cup is now too narrow.
• Under the arm, the wire should not reach into your armpit. If it does, the cup is now too wide or too tall.

If either happens, the volume may be right while the shape is wrong, and a different style in your original size will serve you better than another size change.

Check the label system

UK, US and European letters are furthest apart in this range. A UK G is a US I and usually a European I or J. If a chart shows DDD, or an I cup, it is not a UK chart, and applying it to a UK size will send you to the wrong place. Convert first with the international chart.

Frequently asked questions

What is the sister size of a 34G?

In UK sizing, 32GG on a firmer band and 36FF on a looser one. The cup above G is GG and the cup below is FF.

What is the sister size of a 36GG?

34H on a firmer band and 38G on a looser one.

Should I sister size up to find my size in a shop that stops at a G?

It is rarely worth it. Moving up a band to reach a stocked cup gives you a band too loose to support a fuller bust, which is where support matters most. A retailer that carries your real size is the better answer.