Statute miles to runway visual range
Memorization.  This is the FAA definition:

Vis / RVR
1 SM / 5000
3/4 SM / 4000
1/2 SM / 2400
1/4 SM / 1600


This and a few more are spelled out in the Airman's Information Manual, TBL 5−4−1, RVR Value Conversions