The Eyeline Back Vertex Distance (BVD) Gauge is an accurate low-cost device for measuring the distance between the back of a lens to the cornea in mm.  The set comes with a pouch and calculator.  The instructions for plus and minus powers are printed on the reverse side of the calculator. The gauge and calculator can be purchased separately i.e. one calculator and several gauges.

This measurement is very important when deciding on the lens power especially in swimming/ski goggles when the lenses are in the frame will more than 10/12 mm from the cornea, or when the lens powers are greater than ±6D

The advantage of this gauge is once the measurement is made, the distance set on the scale does not move when the measurement is completed.  It also compensates for 1 mm to account for the eyelid thickness.

Easy to use. Simply read the measurements and sanitize.