How to Measure the Power & Diameter of Contact Lenses

 Contact lens prescriptions have different sizes and measurements. A contact lens has more components and information for correcting vision than a spectacle lens. During an eye exam for contact lenses, an eye doctor takes more measurements of the eye and uses this information to order individually designed lenses for each eye. The power and diameter of a contact lens are determined during a contact lens fitting and are sometimes changed during follow-up appointments or later as the eye and prescription change. Measuring a contact lens power and diameter is done in a few steps.

Difficulty:Moderately ChallengingRuler with millimeters or PD StickLensometerTowel1
Place the contact lens on the of the objective lens-bearing seat of a lensometer to measure the power of the lens. The front of the contact should be facing up, toward you.
2
Observe the power cross, lines running horizontally and vertically inside the eyepiece or reticule of the lensometer. Adjust the power on the handwheel on the side of the lensometer until the cross is clearly in focus. Note the number on the power wheel; this is the power of the contact lens.
3
Lay the contact lens on a clean 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 over the lens, but not on it, and measuring the widest point across the contact. Diameters of contacts are written in millimeters and average about 14 mm, according to Contact Lens Spectrum.
Contact lens prescriptions contain other necessary prescription information, such as a base curve, which helps the lens to sit properly on your eye. Toric contact lenses include additional power information to correct for astigmatism. An experienced optician or eye care practitioner will measure a complicated lens power for little or no charge.