Find Your Dome Size
There are 6 dome sizes. Each one is either Narrow or Wide, and assigned based on the underbust circumference of the user being below or above 73 cm:
Narrow (N)
Underbust < 73 cm
Wide (W)
Underbust > 73 cm
You can measure your underbust by measuring around your rib cage below your breasts. If you don't have a flexible measuring tape, you can use a ribbon and a ruler.
Additionally, each dome is either S, M, or L and assigned based on cup size. Victoria's Secret has a good calculator to find cup size (click here):
Narrow
S = AA Cup
M = A Cup
L = B Cup
Wide
S = A Cup
M = B Cup
L = C Cup