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Modern UK sizing: the band is your snug underbust measurement (no adding inches), and each inch of difference between bust and band is one cup size. Measurements are a starting point; fit varies between brands and styles, so the fit checks matter more than the label.
What The Bra Size Calculator Works Out
- UK bra size: modern UK sizing uses two measurements. A snug underbust in inches, rounded to the nearest even number, is the band; each inch of difference between the fullest bust measurement and that band is one cup letter, running AA, A, B, C, D, DD, E, F, FF, G, GG, H and onward. A 76 cm underbust and 94 cm bust lands at 30F, a combination the old method would have called a 34D.
- Sister sizes: band down and cup up (or vice versa) keeps the same cup volume, so a gaping 34D often resolves to a 32DD or 30E. The usual wrong size is a too-big band with too-small cups, because the band, not the straps, provides most of the support.
- Band / cup difference: the working in inches. The old "add four inches" rule dates from an era of non-stretch fabrics and produces bands too loose to support.
- Fit check: the band should sit level front to back and feel firm on the loosest hook (the tighter hooks are for later, as it stretches with wear); the underwire or centre panel should lie flat against the sternum; cups should contain everything without gaping or overspill; and the wire should never sit on breast tissue. Straps fine-tune rather than support.
- Measurements entered in (advanced): enter inches directly if your tape reads that way.
- Leaning-forward bust (advanced): fitters often average the standing bust measurement with one taken bending forward at 90 degrees, which captures softer tissue that a standing measurement misses and usually adds a cup size.
- Tight underbust (advanced): averaging the snug and pulled-tight underbust readings is another fitter's trick for finding the band a stretchy bra will actually hold at.
- Band rounding (advanced): an odd underbust (31 in, say) sits between two bands. Rounding down gives a firmer band for softer tissue or heavier busts; rounding up suits anyone who finds bands dig in.
- US and EU sizes (advanced): UK letters differ from US and EU scales above D: a UK E is a US DDD/F and an EU F, while bands convert 30 to 32 UK = 65 to 70 EU in 5 cm steps. The sister size table shows two bands either side in all three systems.
- Caveats: measurements are the starting grid, not the finish line. Fit changes with weight, pregnancy, nursing and time, so remeasuring yearly and trying sister sizes across brands whose cuts differ beats loyalty to any number.