Grading Rules for B/C Cup Sports Bra Production
Apparel grading assumes the body scales evenly, and for most garments that assumption is close enough. A bra is different because two dimensions that must move together, band circumference and cup volume, do not scale at the same rate. Get this wrong and the size run will fit one size perfectly and fail at both ends, which is exactly the pattern seen in returns data: the base size sells through and the extremes come back. A B/C cup medium impact block makes the problem easier than a full cup range, but only if the grade rule is written down deliberately.
Why Bra Grading Is Not Apparel Grading
In apparel a size step is typically a fixed width increment applied across the pattern. In a bra, the circumference step and the volume step are separate decisions. A 5 centimetre band step and a one-cup volume step are not the same thing, and blending them into a single proportional scale factor produces a size run where the cup is too small at the top and too shallow at the bottom.
Grading the Underband
Start with the circumference step, commonly 5 centimetres per full size, split across the pattern pieces. Because a bra band is tensioned, the elastic length is not simply the body measurement; it is the body measurement minus a reduction factor that depends on the modulus of the elastic you specified. State that reduction factor in the grade rule, and state it separately for the relaxed and stretched measurement.
Also grade the wing height. A taller wing at larger sizes stabilises the band and stops it rolling, but it must stay clear of the cut-out geometry.
Grading a Wireless Cup
Without a wire, cup volume is controlled by width, depth and the amount of ease in the fabric. A workable starting rule for a B/C block is roughly 0.6 to 0.8 centimetres of cup width and 0.5 to 0.7 centimetres of cup depth per size, adjusted for the stretch of the shell fabric. A four-way stretch shell with higher elastane content needs less geometric increase than a firm double-knit, because it contributes volume through stretch.
Grade the pad pocket with the cup, not independently. A pocket that stays the same size while the cup grows will pull the cup flat at the larger sizes and leave the pad floating at the smaller ones.
Grading the Racerback Strap Path
Strap length typically grades 6 to 10 millimetres per size, but the convergence point needs its own rule. If the convergence point is graded proportionally with the body length, it will sit lower on larger sizes and change the strap angle, which changes how much the strap can migrate during rowing or cycling. Many blocks grade the convergence point at a reduced rate, or hold it at a fixed distance from the neckline, to keep the strap angle consistent.
Where Cut-Outs Break the Grade
This is the single most common grading failure in cut-out back designs. A cut-out that is scaled proportionally will drift as the body length grows, and at the largest sizes it can land directly over the shoulder blade, which is the one place it must not be. The fix is to define the cut-out position relative to a landmark rather than as a percentage of the panel, and in some cases to hold the cut-out dimensions constant while grading only its position.
Verifying the Grade
Request a nested plot of all sizes before cutting, and check the cut-out and convergence points visually across the range. Then measure every size in the size set against the graded spec table. A grade that looks correct in size M and XL but jumps between L and XL usually indicates an inconsistent increment rather than a pattern error.
