What a D+ Bra Must Be Built From: Construction Specs by Cup Size
Bras are graded mathematically outward from a single 34B fit block, so the letters scale while the hardware doesn't. Band width, hook rows, strap width, wire gauge, sling and seam count all have to increase with cup volume and in most ranges they simply don't. The three tables below give the minimum specification at each letter from D to K, in centimetres you can check with a tape.
By the Think Bra Editorial Team
Take a tape measure into the changing room and check four numbers before you check the mirror: back band width, hook rows, strap width, and cup seam count. If a garment misses the minimums in the tables below for the letter on its label, it will not hold you regardless of how it looks on the hanger, and no amount of adjustment will change that.
The reason it misses is structural rather than careless. A size run is graded outward from one fit block, conventionally a 34B, using a set of mathematical increments. Circumference increments scale reliably because they are arithmetic. Load does not, because load scales with volume, and volume grows far faster than the letter suggests. So the same range that puts a 3.2 cm band on a 34B puts a 3.8 cm band on a 34G and calls it a day.
Every number below is a minimum, not a target. Meeting it does not make a bra good. Missing it makes the bra unable to do the job at that letter, which is a more useful thing to be able to test.
The 34B block and what it does above a D
Grading is the process of generating a whole size run from one sample size. The pattern for the block is expanded and contracted by fixed increments at fixed points, and for band and cup circumference that works because those are one-dimensional measurements.
Cup volume isn't. Move from a 34D to a 34G and the cup has to contain roughly double the mass, but the wire is a fraction longer, the band is a few millimetres wider and the straps are frequently identical. The load path has been asked to carry twice as much through hardware that grew by ten per cent.
You can see the consequence on the garment. A 34G from a range whose top size is a G will usually have exactly the same hook configuration as the 34B, and the elastic will fail in the band first — stretched permanently within a few months, riding up at the back, and the wearer concludes that their body is the problem. Specialist ranges that grade to a K regrade the hardware at every step, which is most of what you are paying the difference for.
Band, hooks, straps and back geometry
Band width is measured across the back panel at its widest point, not including the hook tape. Hook rows are the number of hooks stacked vertically; columns are the number of positions horizontally. Three columns is the standard and anything with two columns above a D has been built down to a price.
Cup (UK) Back band width Hook rows × columns Strap width Back geometry required D 3.2–3.8 cm 2 × 3 1.3–1.6 cm Straight strap acceptable DD 3.8–4.5 cm 3 × 3 1.6–1.9 cm Straight strap acceptable E 4.5–5.1 cm 3 × 3 1.9 cm Straight strap; leotard back preferred F 5.1–5.7 cm 3 × 3 1.9–2.2 cm Leotard or U-back preferred FF 5.7–6.4 cm 3 × 3 2.2 cm Leotard or U-back preferred G 6.4–7.0 cm 3 × 3 minimum, 4 × 3 better 2.2–2.5 cm U-back required GG 6.4–7.6 cm 4 × 3 2.5 cm U-back required H 7.0–8.3 cm 4 × 3 2.5–3.2 cm U-back required, cushioned strap HH 7.6–8.9 cm 4 × 3 2.5–3.2 cm U-back required, cushioned strap J 7.6–9.0 cm 4 × 3 3.2 cm U-back required, cushioned strap K 8.9–10.2 cm 4 × 3, some 5 × 3 3.2–3.8 cm U-back required, cushioned strap, reinforced attachment Measure the back band of any bra you already own and find the row it belongs to. If that row is two or more letters below the label, you have found why the band rides up — and the specification, not the size, is the thing to shop against next time. A U-back or leotard back is where the strap attachment points sit closer to the spine and lower down the band, instead of at the outer edges near the armhole. It matters above an F because straps set wide at the top of a band lever the band upward at the sides every time the strap takes load. Moving the attachment inward converts that lever into a straight downward pull the band can resist.
Cushioned straps are not a luxury at an H. Pressure equals load divided by area, and a 2.5 cm strap under sustained load produces a groove in the trapezius that most wearers can still see an hour after taking the bra off. Widening the strap and padding it is the cheapest available intervention and a lot of ranges skip it.
Wire, sling, seams and gore
Wire gauge is quoted by class rather than in millimetres because makers publish it inconsistently and a stated thickness tells you nothing about the steel's temper. There is a better test and it takes three seconds: hold the wire by its two tips and press the middle of the arc with your thumb. A wire specified for a G and above should not visibly deflect under firm one-handed pressure. If it flexes like a hair grip, it is a D-cup wire in a G-cup garment.
Wire chord is the straight-line distance between the two tips with the wire laid flat. Compare it to your own root width, measured from sternum to under-arm along the chest wall. They should agree within about a centimetre.
Cup (UK) Wire gauge class Wire shape Side sling or power bar Cup seams Gore height D Light — some deflection acceptable Standard height at the side Optional Moulded acceptable, two-part better 4–5 cm DD Light to standard Standard height Optional Two-part minimum 4–6 cm E Standard Raised side tail Recommended Two-part with a vertical seam 5–6 cm F Standard Raised side tail Required Two-part minimum, three-part better 5–7 cm FF Heavy Raised side tail, longer arc Required Three-part 6–7 cm G Heavy Raised side tail, longer arc Required, full-height Three-part 6–8 cm GG Heavy High side tail to the axilla Required, full-height Three-part with reinforced lower cup 6–8 cm H Extra-heavy High side tail to the axilla Required, full-height, stitched to the band Three-part with reinforced lower cup 7–9 cm HH Extra-heavy High side tail, deep front curve Required, full-height, stitched to the band Three-part with reinforced lower cup 7–9 cm J Extra-heavy High side tail, deep front curve Required, boned side seam preferred Three-part, four-part in the deepest cuts 7–10 cm K Extra-heavy or double wire High side tail, deep front curve Required, boned side seam Four-part 8–10 cm Use this to reject garments quickly. A seamless moulded cup at a G, or a bra above an F with no internal panel when you press the side of the cup from the inside, is not built for the letter it is labelled with. The sling is the feature most people have never heard of and the one that changes the most. Reach inside the cup and press against the outer side wall: in a slinged bra you can feel a separate panel of firm fabric running from the side seam forward and slightly upward. It is doing the job of stopping tissue migrating toward the armpit when you lie down or lean forward, and above an F its absence shows up as a permanent full feeling under the arm.
Gore height is where plunges pay their structural bill. Every centimetre you take off the gore is a centimetre of wire arc removed from the front, and the load that arc was carrying transfers to the side. That is why a plunge at a G needs a heavier wire and a taller side tail than a full cup at the same letter, and why so few ranges bother to build one.
What the Hold Index weights at your cup size
The Hold Index runs 0–100 and the components don't carry the same weight at every letter. At a D, band width and tension dominate almost everything else. At a K, wire and sling between them account for nearly a third of the score, because at that volume the band on its own can no longer hold the apex position no matter how wide it is.
Feature D–DD E–F FF–G GG–H J–K Band width and tension 44 38 33 29 25 Wire gauge and shape 9 12 14 15 16 Side sling or power bar 3 6 9 12 15 Cup seam structure 13 13 13 13 13 Strap width and back geometry 13 13 13 13 13 Hook rows and columns 6 6 6 6 6 Elastic retention at 100 wears 12 12 12 12 12 Percentage of the Hold Index score each feature contributes at that cup band; each column sums to 100. Use it to decide what to spend on: below an E, put the money into the band; above a G, put it into the wire and the sling. The shift across the columns is the argument against buying a large cup from a range whose grading stops just above it. A range that tops out at a G has designed its G to be the extreme edge of a block built around much smaller volumes. A range that grades to a K has designed its G as a mid-run size with the hardware regraded twice on the way up. Same letter. Different engineering.
Five checks with a tape and two hands
- Band width. Tape across the back panel at its widest. Compare against the first table. This takes ten seconds and eliminates more unsuitable garments than any other single test.
- Wire deflection. Hold the wire tips through the fabric, press the arc with your thumb. Visible flex at an F or above means the wire is under-specified.
- Sling check. Reach inside the cup, press outward against the side wall from within. You are feeling for a distinct second layer, not for lining.
- Hook geometry. Four rows on a 3.8 cm band is decoration. Rows and width need to increase together or the band folds along the top edge under tension.
- Strap attachment. Look at where the back strap meets the band. Set at the outer edge near the armhole, it will lever the band up at the sides all day. Set inward toward the spine, it won't.
The band carries it; the straps stop it swinging
The working rule in specialist fitting is roughly eighty per cent of load through the band and twenty through the straps, and it holds up across the whole run. It is why the diagnosis for shoulder grooving is almost never "the straps are too narrow" and almost always "the band is too loose, so the straps have been promoted into carrying weight they were never meant to carry."
Straps do have a real job. They control forward and lateral movement, they set the apex position, and above a G they stop the cup from rocking when you turn. But they are stabilisers, and stabilisers fail badly when they are asked to be load-bearing.
Which is why the tightest hook is a retirement signal rather than a setting. A new bra should fit on the loosest hook, and the remaining hooks are the allowance for elastic aging over the garment's life. If a bra fits only on the tightest hook when it is new, the band is a size too big and the tables above are being defeated before you have worn it once.
What a correctly specified G costs you in style
Honesty is better than optimism here. Buying to this specification narrows your choices, and it narrows them in a specific direction: away from seamless, away from very low gores, away from anything sold as barely-there.
You keep balconettes, full cups, longlines, three-part cut-and-sew, structured racerbacks and a decent plunge if you shop the specialist ranges that build one. You lose the moulded seamless t-shirt bra above about an F, because moulding tooling at that depth distorts and the resulting cup shape fits a narrow subset of body shapes. You lose most of the strapless category above a GG unless you go longline and boned.
The trade is worth naming plainly: fewer options that work, versus many options that don't. The ranges that grade properly to a K — Elomi, Panache, Curvy Kate, Goddess and the Polish makers such as Ewa Michalak and Comexim among them — exist because that trade was worth making to enough people to sustain a business.