Measured Size Calculator
Two tape measurements, no letters guessed. This calculator holds the body as the constant and writes it out in every major size run, then shows you what the +4 rule still sitting inside most brand calculators would have handed you instead — normally a band two or three sizes too loose, and a cup letter that reads several steps too small to compensate.
Units
Tape level and snug directly under the bust, arms down, breathing out. Snug means it will not slide, not that it hurts.
Tape level across the fullest point, no compression. Then bend forward and measure again — take the larger of the two.