Bra Size Grading: From a 34B Sample to a 40DD Production Run
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- Issue Time
- Oct 30,2026
Summary
Why a graded 40DD is not a scaled-up 34B: what changes in cup depth, wire length, band tension and strap width, plus the size chart and tolerance data a factory needs before it can grade your style.
Bra grading is not uniform scaling. Starting from a base size — usually 34B or 75B — each step up in band adds about 5 cm to the underbust and a fixed amount of wire length, while each cup step adds roughly 2.5 cm to the bust circumference and changes cup depth rather than cup width. A 40DD shares almost no pattern dimensions with a scaled 34B: the band is stiffer, the wire is longer and wider, the strap is wider and the hook count usually changes.
If a factory grades by scaling the pattern by a percentage, the larger sizes will look right on a hanger and fit wrong on a body. That is the single most common reason a size run fails at the top end.
What grading actually changes
- Underbust band — increases in fixed steps along the band, not proportionally to the bust.
- Cup depth — the volume grows mostly forward, so the cup gets deeper, not just wider.
- Wire — length and curvature change; you cannot bend a 34B wire into a 40DD shape.
- Band height and elasticity — a 40 band carries more tension, so the powernet grade or the number of hook rows changes.
- Strap width and ring/slider size — 8 mm straps on a 40DD cut into the shoulder; 12–15 mm is normal at the top of the range.
- Cup construction — 3-part cups often become necessary above DD to keep shape without excessive stretch.
| Size | Underbust (cm) | Bust (cm) | What changes vs. previous step |
|---|---|---|---|
| 34B / 75B | 73–77 | 86–88 | Base size |
| 36B / 80B | 78–82 | 91–93 | Band +5 cm, cup depth +2.5 cm |
| 38C / 85C | 83–87 | 96–98 | Band +5 cm, cup +1 step |
| 40D / 90D | 88–92 | 101–103 | Wire longer, band tension up |
| 40DD / 90E | 88–92 | 104–106 | Cup depth +2.5 cm, 3-part cup advised |
| 42DD / 95E | 93–97 | 109–111 | Strap width up, hook rows +1 |
The table is a starting point, not a standard. Every brand has its own grade rules and its own fit model. A factory that grades 2.5 cm per cup when your brand grades 2.0 cm will produce a size run that fits your chart on paper and not on your customers.
Why a 40DD is not a scaled 34B
Scaling multiplies every dimension by the same factor. Bodies do not work that way. Going from 34B to 40DD, the underbust grows about 15 cm while the bust grows about 18 cm — and almost all of that extra bust volume has to go somewhere the body allows, which is forward and slightly down, not sideways.
In practice this means the cup gets deeper, the cradle gets taller, the wing gets wider, and the centre front has to sit flat against the sternum without lifting. On a scaled pattern the centre front gapes, the wire digs into the ribcage, and the band rides up at the back — three complaints that all come from the same grading shortcut.
Sizing systems: where the conversions break
| System | Base label | Notes |
|---|---|---|
| EU / EN 13402 | 75B | Band in cm, often the same as CN sizing |
| UK | 34B | Cup letters run DD, E, F, FF — no double letters elsewhere |
| US | 34B | DDD instead of E in many brands; inconsistent across retailers |
| JP | B75 | Cup letter before band, and the fit runs smaller than EU |
| CN | 75B | Usually matches EU on the band, cups run slightly shallow |
If you sell in more than one market, decide which system the pattern is graded in and label the rest by conversion. Grading twice — once for EU and once for US — doubles the pattern set and the sample cost, and most brands do not need it until volume justifies it.
What to send a factory before it can grade your style
- Base size and base measurements — bust, underbust, cup depth, band height, strap length, all in cm.
- Your size chart for every size you intend to sell, not only the base.
- Grade rules if you have them. If you do not, say so — the factory will propose rules and you approve them in writing.
- Fit model measurements — the person the base size was fitted on. Fit is relative to a body, not to a chart.
- Tolerances — how much deviation you accept on bust, underbust and cup depth. ±1 cm is common; ±0.5 cm costs more and needs more QC time.
- Construction constraints — wire type, cup panels, hook rows, strap width per size.
How fit is checked before bulk
A workable sequence is: base size sample, fit session on a live model, corrections, then a graded set — usually the smallest, the base and the largest size — fitted again before bulk cutting. Skipping the graded set is how brands discover at the warehouse that the 40DD does not close.
- Fit the smallest and largest sizes, not just the middle. Extremes fail first.
- Wear-test for at least 4 hours before judging. A bra that feels fine after 10 minutes can still ride up after 3 hours.
- Wash one sample of each size before approval. Shrinkage moves the fit more than a millimetre of pattern error.
- Record the corrections in writing. Verbal fit comments are lost between the sample room and the line.
Realistic timing
For a style with an existing pattern library, sampling runs about 2–3 weeks per round, and two rounds is normal. Grading a full size run adds roughly a week. Bulk production after approval is typically 60–90 days depending on fabric lead time and line capacity. If someone quotes you 30 days for a graded, bonded, multi-colour lingerie program, ask what they are leaving out.
Have a size chart that needs grading?
Send the base size measurements and your full size chart. We will tell you which sizes need a different cup construction before anyone cuts fabric.