Q: How to measure the power and diameter of contact lenses?
A: Position the contact on the with the objective lens-bearing seat of your lensometer to measure the power of the lens. Top on the contact needs to be facing up, toward you.
Observe the power cross, lines running horizontally and vertically in the eyepiece or reticule from the lensometer. Adjust the power on the handwheel assisting the lensometer before the cross is clearly in focus. Note the number within the power wheel; this is the power on the lens.
Lay the lens with a neat and sanitary surface. A soft, clean towel works well. Measure the diameter of a contact lens by laying a ruler with millimeters or an optical PD stick within the lens, but not onto it, and measuring the widest way the contact. Diameters of contacts are written in millimeters and average about 14 mm, according to Contact Spectrum.