How do the numbers 20/? relate to my prescription?

My prescription for my glasses is about -13.5 in my left eye and -12.5 in my right eye.  Judging by how little I can see without them, I'm sure I'd be legally blind if I didn't have some kind of correction.  My question, then, is, in terms of 20/20, what would I be?  20/?  Seeing as a prescription around -4 matches up to something like 20/600, am I like, 20/five million?

[berrywoo]

FIrst off sorry if I got your user name wrong. I'm having trouble seeing clearly at the moment.
Generally, a prescriptions for glasses gives measurements in diopters usually ranging from 0 (plano) to +/- 20. Contacts are a little different.
The numbers given in a prescription do not correlate to a 20/? at all. Someone with your same Rx may only be able to see 20/50, while you're able to see 2020.

[berrywoo]

I forgot to add that legal blindness is always determined as the best vision in a corrected eye, which must be 20/200 or worse, and/or less than 20 degrees in terms of peripheral vision/visual field. Also, note that without correction many people cannot see the big E on an eye chart but often with correction they are able to see 20/20 or better. Also, in terms of measuring visual acuity, if a person fails to read an eye chart with correction, then generally vision is assessed with counting fingers at a close range, if that fails, then next, it is determined if a person can detect hand motion, and if that fails it is noted whether or not a patient can detect light perception.

[John C Hagan III, MD, FACS, FAAO]

Additional thoughts to what berrywoo says:

you can NEVER tell a person's vision from the glasses RX. It's like trying to tell how tall someone is by looking at his shoes.

To be legally blind your BEST CORRECTED vision at both distance and near would have to be 20/200 or worse in both eyes.

That term "I'm legally blind without my glasses" is used  incorrectly almost all the time.

JCH MD