This might help. Just bookmark it or put it on your desktop as a link. It takes about 2 seconds for it to calculate it for you!
Convert milliliter to oz - Conversion of Measurement Units
Or, you could just use the standard of 30mL = 1 oz., and go from there figuring the commonly sold eliquid quantities in your head:
10mL = 1/3 oz.
15mL = 1/2 oz.
30mL = 1 oz.
50mL = 5mL more than 1 1/2 oz.
100mL = 10mL more than 3 oz.
They've been saying the US was going to change to using the metric system since I was a kid...and I'm 63 now!
We're pretty much the last major country in the world, that's still a "hold out", and hasn't changed.