Contacts vs. Glasses Prescriptions: Are they the Same?

Contacts vs. Glasses Prescriptions: Are they the Same?

Contact lens prescriptions and glasses prescriptions are not the same, although they do have many similarities. 

Many of the measurements and values that are needed for a valid prescription are consistent and you’ll still need to get an eye exam from a licensed eyecare professional to make a purchase.

Are Contacts Available in My Eyeglasses Prescription?

If you wear glasses, there is likely a contact lens that will come in your necessary prescription. That doesn’t mean that every contact lens brand will convert directly one-to-one to all glasses prescriptions, but it does mean that there is at least a contact lens in your power available.

Besides all of the fitting details such as base curve and diameter of the contact lens, a contact lens power usually does not convert directly from a glasses power.

Glasses to Contacts Conversion Example Table

Glasses Power	Contacts Power
-2.00	-2.00
-4.00	-4.00
-4.25	-4.00
-6.50	-6.00
-2.50-0.50X180	-2.75
-2.50 -1.50X075	-2.50 -1.25X080
4.50 -3.00X178	-4.25 -2.75X180
-6.00-0.50X065 +2.00ADD	-5.75 Mid ADD

As you can see, the initial contact lens prescription only sometimes converts directly from a glasses prescription, but it usually does not convert directly. 

Why Are Contacts Prescriptions Different From Eyeglasses?

There are several main reasons for this indirect conversion for prescription types.

1. Vertex distance: The distance from the back of an eyeglass to the cornea is further than a contact lens that rests directly on the eye.
2. Availability: Contacts do not come in all of the different powers that eyeglasses come in.
3. Convenience: Sometimes, to make wearing contacts more convenient, the doctor may make the lenses the same power in each eye because the powers convert very closely.

These conversions are also only the initial choice that a doctor may make when deciding upon your contact lens. The finalization of a contact lens fit is determined after a patient samples the contact lenses for a period of time, and both the doctor and patient agree that the fit is good. 

The doctor may make adjustments in lens design or power based upon patient feedback or the lens movements. The initial choice for a contact lens is often modified based on this fitting process.

Therefore, for a proper finalized contact lens prescription, you should only trust your experienced licensed eyecare professional with a proper contact lens fitting.

ADD Power Differences Between Contacts and Glasses

As seen in the table above, ADD power (additional power for presbyopia patients) is also typically different between prescription types.

Contacts ADD Power	Glasses ADD Power
LOW ADD	+0.75, +1.00, and +1.25
MED ADD	+1.50 and +1.75
HIGH ADD	+2.00, +2.25, and +2.50