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No. Contact lenses and glasses do not use the same prescription, even when they correct the same eyes. A glasses prescription and a contact lens prescription are two separate documents with different numbers and different measurements. The powers can look similar on a mild prescription, but they are set for lenses that sit at different distances from your eye, so they are not interchangeable.
If you have tried to order contacts using your glasses prescription, you have probably hit a wall. Here is why the two are not the same and what sets them apart.
Why Are the Two Prescriptions Different?
The main reason is distance. Glasses rest about 12 millimeters in front of your eyes, held there by the frame. Contact lenses sit directly on the surface of your eye. That difference in position changes how much power each lens needs to focus light onto your retina.
A lens that sits closer to the eye does not need as much power to do the same job. For nearsighted prescriptions, the contact lens power ends up slightly lower than the glasses power. For farsighted prescriptions, it ends up slightly higher. Eye doctors account for this using a calculation called vertex distance.
Vertex distance is the gap between the back of a glasses lens and the front of your eye, usually about 12 millimeters. A contact lens sits at zero, which is why the power often needs adjusting on stronger prescriptions.
How Much Do the Numbers Change?
For a lot of people, they barely change at all. If your prescription is on the milder side, the adjustment is so small it rounds away, and your glasses and contacts can show the same number.
The change becomes real once your prescription gets stronger. The cutoff most eye doctors use is around 4.00 diopters. Below that, little to no conversion is needed. Above it, the two prescriptions pull apart, and the stronger the prescription, the bigger the gap. A -10.00 glasses prescription, for example, works out to about -8.93 on a contact lens.
Glasses Power vs Contact Lens Power
This table shows how a few sample glasses powers convert to contact lens power once the lens position is accounted for.
| Glasses Power | Contact Lens Power | Noticeable Difference? |
|---|---|---|
| -1.00 | -1.00 | No |
| -3.00 | -2.87 | Barely |
| -5.00 | -4.75 | Slight |
| -8.00 | -7.27 | Yes |
| -10.00 | -8.93 | Yes |
| +5.00 | +5.37 | Slight |
| +8.00 | +8.93 | Yes |
These are sample conversions based on a 12 millimeter vertex distance. Your real contact lens power is set by your eye doctor at a fitting.
What Does a Contact Lens Prescription Include That Glasses Do Not?
A contact lens prescription has to describe a lens that physically sits on your eye, so it carries fit details a glasses prescription never needs.
- Base curve – The curve of the back of the lens, matched to the shape of your cornea.
- Diameter – How wide the lens is and how much of your eye it covers.
- Lens name – The exact lens you were fitted for, since each one fits differently.
- Power – The corrected power for a lens sitting on the eye.
A glasses prescription leaves all of these out. Instead it lists your lens power, any astigmatism correction, and your pupillary distance so the lenses can be centered in your frames. The two documents serve different products and cannot be swapped.
Can You Buy Contacts With a Glasses Prescription?
No. A glasses prescription does not contain the fit measurements a contact lens needs, and the power is set for the wrong distance. To buy contacts, you need a contact lens prescription that comes from a fitting.
During a fitting, your eye doctor checks how a specific lens sits and moves on your eye and confirms the right power, base curve, and diameter. Only then do you get a contact lens prescription. After that, you can fill it wherever you want.
Under the FTC Contact Lens Rule, your eye doctor must give you a copy of your contact lens prescription after your fitting, whether you ask for it or not. You are free to fill it at any retailer you choose.
How To Tell Your Two Prescriptions Apart
If you are looking at a prescription and are not sure which one it is, check a few things. Start with the sphere number, the main power figure. Then look for a base curve and diameter. If those are present, you are holding a contact lens prescription. If they are missing, it is a glasses prescription.
You can also compare the power directly. On a mild prescription the two will look almost identical. On a stronger one, the contact lens number will be a little lower for minus powers and a little higher for plus powers. The expiration date is worth checking too, since you need a current prescription to order.
Have a current prescription? You can order your multifocal contact lenses and more at 1-800-GET-LENS.
Frequently Asked Questions
Are glasses and contact prescriptions ever the same?
Can I use my glasses prescription to order contacts online?
Why is my contact lens power lower than my glasses?
Is a stronger prescription number bad for my eyes?
How frequently do I need a new contact lens prescription?
References
American Academy of Ophthalmology. Contact Lens Prescriptions. AAO.org. 2023.
US Food and Drug Administration. Contact Lenses. FDA.gov. 2023.
American Optometric Association. Prescribing Contact Lenses. AOA.org. 2023.
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