How Are Contacts Measured?

Replacemycontacts/ June 25, 2014/ Eye Health, Lens Care/

Do you know how it is that your eyes are measured, in order to select the best fitting contact lenses for your specific size and shape? If you’re like most people, that knowledge is likely left to your doctor, who administers care and writes the official prescription. Yet it is important for many reasons to understand this procedure which determines our unique measurements.191297778_9eedbec371_z

One size does not fit all

Have you ever worn a pair of ill-fitting shoes that squeezed your toes and gave you a nasty blister within the first hour of wearing them? Seeing as blisters can take days to heal, the discomfort you experienced likely wasn’t worth the brief amount of time spent wearing those shoes. It’s the same when it comes to contact lenses; a size too small or large, even by a tenth of a millimeter, can cause the same long-lasting irritation. In some cases, you could run the risk of endangering your eyes’ health.


Having your contact lenses fitted is much different than fitting a pair of eyeglasses. Contacts need to rest perfectly on the cornea of the eye. If they don’t, eye pain, tissue damage, or eye infection can occur.  When you visit the eye doctor for a contact lens fitting, your prescription will be based on the specific measurements of your eye, and location of your cornea. This is why your doctor will use various types of measurement instruments to correctly prescribe a comfortable and safe contact lens for you.

One of the most important measurements of the eye’s surface is eye curvature, for which your doctor will use an instrument called a keratometer. The keratometer analyzes the cornea’s light reflection, and from this measurement can calculate the curvature of your eye and the size of contact lens you need. If the curvature of the lens fits too flat or too steep, it actually can be harmful to your eye.

Soft-Contact-Lens_lgIf your contact lens prescription specifies a BC value, it is referring to the quantity that denotes the curve of your eye. It is essential to purchase the brand of contact lenses prescribed by your doctor, because just as a pair of jeans in one size from the Gap may not fit as well as a pair in the same size from J.Crew, the BC value of every contact brand is sized differently. Normal BC values run 8.0 to 10.0 in contact lens prescriptions. When we speak of a contact lens and its BC value, a prescription for a 9.7 in one brand, may fit like a BC value of 9.8 in a different contact lens brand.

Different needs require different lenses

Your doctor can help decide if it is safer for you to wear soft and pliable contacts or hard, more rigid lenses. Soft lenses tend to be the preferred choice in most contact wearers, but your lifestyle and schedule will be key factors in determining the best choice for you.

If you have an eye condition, you may be prescribed a specialty lens; for example, those that have astigmatism shop for specialty contact lenses like the Biofinity Toric or Proclear Toric XR. Your age also influences the type of material recommended for wear. As we get older, our ability to see nearsighted objects begins to dissipate: a common condition for those ages 40 and up, called presbyopia. In addition, middle-aged adults may experience a decrease or loss in flexibility of the natural lens in their eyes, which, like presbyopia, can be aided by monovision or multifocal contact lenses.

Understanding your contact lens prescription and ordering your contacts based on the specific instructions given by your eye doctor will help you to keep your vision safe and healthy. The guide in the table below is designed to help you understand the numbers and abbreviations on your contact lens prescription, so that you can be more active in looking after your eyes’ health and wellness.