if you have worn a pair of bifocal lenses, you can provide your segment height to us. You can measure it yourself. The process is easy and requires no special equipment. Measuring segment height on your own is especially simple if you already own a pair of bifocals. Simple put on your glasses and, using a ruler that features millimeter markings, measure (or have someone measure for you) the distance between the bottom of one of the lens and the top of your bottom eyelid. This measurement is the segment height and you can now enter it in the appropriate spot when ordering prescription bifocals online.
If you do not know your segment height, there is a formula to figure it out your segment height, that is 0.618*lens height+2mm.