Why a 34DD Isn't a 34DD: Bra Sizes by Brand Across 30 Size Runs

Worth being precise about these, because three of them get blamed on the fourth.

• ▸Base fit block. Almost every range in the category is graded outward mathematically from a single sample size, historically a 34B. Wire gauge, band width, hook count, sling structure and strap anchor points are scaled by formula from that block, even though the load on them grows far faster than the letters do. This is why a style that is excellent at C collapses at G.
• ▸Band rule. Direct underbust with nothing added is the reference. Some runs still draft against an underbust plus two inches, and a stubborn tail of US mass-market heritage charts still adds four or five. A run drafted under an added-inches rule will feel loose at your measured band and force you upward.
• ▸Cup step. UK and US runs step at one inch, 2.54 cm. A number of Polish and German runs step at roughly 2 cm. Both are internally coherent. Cross-converting by letter rather than by centimetre accumulates error the further up the run you go.
• ▸Cup alphabet. UK doubles (DD, FF, GG, HH, JJ) and skips I. US inserts DDD and often skips I as well. Continental runs go straight through with no doubles. Australian runs mostly keep DD and then run straight. Four alphabets, one physical progression.

There is a fifth variable that resists tabulation: cup depth versus spread at a given letter. Two runs can hold the same volume at 32F and distribute it completely differently — one deep and projected, one shallow and wide. That one is a shape question rather than a size one, and no conversion table will settle it. The registry flags it in the conversion column where a run is known to sit at one extreme.

Band ranges are the core published run and shift with seasonal collections, so treat them as the shape of the range rather than a guarantee. The lettering system, band rule and conversion columns are structural — those do not move season to season, and those are the ones to act on.

The registry deliberately does not grade any of these ranges as good or bad. A shallow run is not worse than a deep one — it is correct for a different distribution of tissue, and a wearer who suits it will find every deep run gapes at the top edge. What matters is knowing which one you are standing in before you order.

Two structural patterns are visible across the whole table. Ranges built specifically for cup letters above D almost universally draft against the direct underbust, because a loose band at that volume fails immediately and visibly. Ranges whose deepest coordinate is DDD are the ones still carrying add-inches heritage, because at those volumes a loose band fails slowly enough that nobody complains.

You can reverse-engineer a run's band rule from one garment. Fasten it on the loosest hook and compare the band's relaxed circumference against your snug underbust measurement.

A cup step is not a universal quantity. Where a UK or US run adds 2.54 cm of difference per letter, several continental runs add roughly 2 cm. At the shallow end nobody notices. By the middle of the run the two systems are counting different numbers of letters for the same body.

The table shows the drift in ordinal positions — how many letters up the run a given centimetre difference lands, under each step size.

This is the single strongest argument for recording your measurements in centimetres rather than as a size. A difference of 22.9 cm is a fact about your body that does not change when you cross a border. UK G is a claim about one alphabet.

Almost every range in the category is drafted at one sample size and scaled outward by formula. Historically that sample is a 34B. Every other coordinate in the run is a mathematical projection from it.

Volume grows fast along that projection. Working from the model on the sister-size page, a 34B cup holds something like 380 ml and a 34G around 1,330 — roughly three and a half times as much, hanging off a band of similar width, on hooks of the same gauge, over a wire drawn from the same stock.

Grading rules do increase wire gauge, band width and hook count as the run climbs, but they increase them by pattern increments rather than by load. The result is predictable and shows up in the same places: the band width chosen for a 34B rolls at 34GG, the two-hook back that is ample at C has no adjustment margin at G, and the sling that is decorative at DD is structural at H and often is not there at all.

This is the practical reason a wearer can love a brand at DD and find the same style unwearable two years later at G. The brand did not change. The garment was extrapolated past the point where its construction still matched the load, and no amount of size conversion recovers that. At that point the question stops being which size and starts being which construction.

• ▸Start from a garment that works, not from a size you have always bought. Note the brand, the style, the size and the system it was labelled in.
• ▸Convert that size into the reference standard using the brand's row in the registry. If the band rule is direct, no band correction is needed.
• ▸Convert out of the standard into the destination run using its row. Apply the band rule first, then the alphabet.
• ▸If the destination run uses a different cup step, work the conversion in centimetres of difference rather than in letters.
• ▸Check the destination run actually contains your coordinate. A range stopping at DDD has nothing for a 20 cm difference, and no correction reaches it.
• ▸Order your converted size plus both one-band sister sizes. Whichever fails tells you which direction that run sits from the standard, and you only pay that tuition once per brand.
• ▸Write the result down — brand, style, size, system, and whether it worked. Over a couple of years that record beats every chart on this site.

A cup letter tells you where a garment sits inside one manufacturer's run. It tells you nothing about your body that your tape measure has not already told you more precisely.

That reframing solves a surprising amount. It means a size going up is not a change in you, it is a change in whose grading you are standing in. It means the range that stops at DDD has not judged you, it has simply chosen not to draft that far. And it means the useful record to keep is a set of centimetre measurements and a short list of brand-and-size pairs that worked, rather than a single number you think of as your size.

The tape is the constant. Everything printed on a tag is a variable, and the registry above is a list of how far each one has wandered.