There doesn’t appear to be a widely used calculator designed specifically for men with broad rib cages or wider breast roots. Most online calculators can still give you a useful starting size, but they treat the chest as a simple circumference and don’t account well for breast-root width, shallow projection, or a larger pectoral/rib-cage area.
A reasonable option is the Title Nine calculator, because it asks for three measurements—including around the chest above the breasts and beneath the breasts—rather than relying only on the usual bust and underbust numbers.
titlenine.com
Calculator.net is another general-purpose option, but its result should be treated as approximate.
Calculator.net
For your shape, measure as follows:
Underbust: Measure firmly—but not tightly—directly below the breast tissue. This is your starting band measurement.
Full bust: Measure around the fullest part of the breasts, keeping the tape level and relaxed.
Upper chest: Measure around the torso above the breast tissue, just under the armpits. This helps identify whether your chest is broad relative to the breast volume.
Calculate the initial cup size from the difference between bust and underbust. In the common US system, each inch of difference is roughly one cup step, but this is only an estimate.
Calculator.net
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Because wider-rooted breasts often need more cup width but not necessarily more cup depth, prioritize bras with:
wider, shorter underwires;
full-coverage or side-support cups;
stretch lace or flexible cup fabric;
wireless or longline designs if standard underwires feel too narrow;
brands that offer extended band sizes and detailed garment measurements.
Fit is more informative than the calculator’s letter. If the cups cut into tissue at the sides, try a larger cup in the same band. If the cup has empty space at the center but the sides are contained, try a shallower or wider cup rather than simply sizing down. If the band rides up, it is likely too large; if it causes painful compression, it may be too small or the bra may have an overly rigid band.
The closest practical approach is to use a normal calculator for a starting point, then compare sister sizes and—especially—different wire shapes. Bra sizing varies substantially between brands and styles, so the label alone cannot determine whether it will fit.