HOW TO MEASURE BOTTOM CUP DEPTH
This is the vertical measurement of the breast from the middle of the nipple to the bottom of the breast. If you're wearing a good non-padded bra, you can do this while keeping the bra on. If not, measure without a bra on.
Start with bringing the the top of the measuring tape (at the zero mark) against the center of your nipple. Keep measuring tape in place with one hand and cup your other hand over your entire breast to lift it up. Hold the breast in a way that a good bra cup would when measuring. Use other hand to bring the tape straight down around the curve of your beast until you hit the point where your breast ends, commonly known as your wireline where your wires would normally sit at the breast root.