30I Bra Size Guide: Measurements, Fit, Sister Sizes & Conversions

Last updated: August 2026

The 30I bra size pairs a 30 band with an I cup. This guide breaks down exactly what that means in real measurements, how to tell whether 30I is truly your size, which sister sizes to reach for, and how 30I converts around the world.

Band 30I cupUnderbust 29–30″Bust 39–40″

What a 30I bra actually means

Read a bra size as two parts. The 30 is the band, and it comes straight from your underbust — the snug measurement around your ribcage just beneath the bust. The I is the cup, and it comes from how much bigger your bust is than that underbust: about 9–10 inches more for an I cup. The cup letter is a difference, not a fixed volume — which is why the very same cup looks different on a narrow band than on a wide one.

30I measurements

If your own numbers land inside those ranges, 30I is your calculated size. If you are close but not quite, the sister sizes below will usually have you covered.

How to measure yourself for a 30I

You need a soft tape measure and, ideally, a non-padded bra. Stand up straight and breathe normally — measuring too tight is the most common reason people end up in the wrong size.

• Underbust: wrap the tape snugly around your ribcage directly under the bust, level all the way round. For a 30 band you are looking for roughly 29–30 inches (74–76 cm).
• Bust: measure loosely around the fullest part, tape parallel to the floor. For 30I that is about 39–40 inches (99–102 cm).
• Subtract: bust minus underbust gives you the cup. A gap of about 9–10 inches is the I cup that makes 30I.

How to check the fit is actually right

Numbers get you to the right size on paper; these hands-on checks tell you whether it fits in real life. Do them in front of a mirror, wearing the bra on the loosest hook:

• Band: it should sit level all the way around, in line with the front. Slide two fingers under it at the back — snug, not loose, no riding up when you raise your arms. The band does roughly 80% of the support work, so this matters most.
• Cups: do a quick “scoop and swoop” — lean forward, settle everything into the cup, then stand up. There should be no spillage over the top or sides, and no wrinkling or gaping at the edge. Spillage means size up a cup; gaping means size down a cup.
• Centre gore: the little bridge between the cups should lie flat against your breastbone. If it floats off the body, the cup is usually too small.
• Straps: you should fit a finger under them comfortably. If they dig in or carry most of the weight, the band is too loose — tighten it or size the band down.

30I in international sizing

So a US 30I is written 30G in the UK, 65I across much of the EU and 80I in France. The band number and the cup letter can both change between systems, which is why the same bra can look like two different sizes depending on where it was made.

Sister sizes of 30I

Sister sizes hold the same cup volume as 30I but on a different band — the natural sizes to try when the band feels tight or loose. Here is the 30I sister family:

Common fit problems with a 30I — and how to fix them

• Spillage over the top or sides: if a 30I overflows, the cup is too small for your bust — move up to 30J or try its sister size.
• Straps digging in: in fuller cups the band should carry the weight. If straps bite, the band is too loose — size the band down and the cup up.
• Centre gore floating: common in fuller cups when the cup is a touch small; the bridge should tack flat to the breastbone.
• Band riding up at the back: 30 is a narrow band and can creep up if it is even slightly loose. It should sit level with the front; if it rides up, go down a band and up a cup (a sister size).

Best bra styles for a 30I

For a 30I, look to full-cup styles with side support, three-part cups, wide comfort straps and firm bands from brands that specialise in fuller busts. On a narrow band, seek-out brands that cut small bands generously so the wing still feels supportive. Whatever the style, get the band right first — it is the foundation every other part of the fit sits on.

Where to find 30I bras

30I falls outside the core high-street band, so everyday shops may carry fewer options. Specialist fuller-bust or extended-size retailers will have far more choice, and knowing your sister sizes widens what you can wear when a specific size is out of stock.

Is 30I big or small?

30I sits in the very full range for cups on a smaller band. Whether that reads as ‘big’ or ‘small’ depends entirely on the band it sits on: the cup volume of 30I is identical to every sister size in the table above, even though the letters differ. That is the single most useful thing to understand about bra sizing — the letter alone tells you very little without the band.

Related sizes and comparisons

• 30H vs 30I — one cup smaller on the same band.
• 30I vs 30J — one cup larger on the same band.
• 28J guide — a sister size of 30I.
• 32H guide — a sister size of 30I.