The simplest way to find your glasses prescription is to ask your eye doctor’s office. They are legally required to give you a copy after your exam, and most offices can email or fax it to you with a quick phone call. If you’ve lost your copy, your doctor’s office keeps it on file and can provide a duplicate. But there are several other ways to track down or determine your prescription depending on your situation.
Your Eye Doctor Must Give You a Copy
Under the Federal Trade Commission’s Eyeglass Rule, your eye care provider must hand you a written copy of your prescription at the end of your exam, whether or not you ask for it. You don’t have to buy glasses from that office, and they can’t charge you extra for the prescription itself. The prescription includes written specifications for your lenses derived from a refractive eye examination.
Most states require the prescription to include your name, the exam date, an expiration date, and the prescribing doctor’s name, contact information, and signature. If you need a copy months or years later, call the office. They’ll typically have it in your patient records and can share it over the phone, by email, or through a patient portal. Just keep in mind that prescriptions do expire, usually after one to two years depending on your state.
How a Prescription Is Determined
Your prescription comes from a refraction test during an eye exam. You sit behind a device called a phoropter, which holds dozens of lenses that can be swapped into your view. While you focus on an eye chart about 20 feet away, the doctor flips between lenses and asks which option looks clearer. This is done one eye at a time. Many offices also use an autorefractor, a machine that shines a specialized light into your eyes and estimates your prescription automatically, without needing any responses from you. The doctor then fine-tunes that estimate with the phoropter to land on your final numbers.
How to Read Your Prescription
A glasses prescription can look intimidating, but each abbreviation has a straightforward meaning. The two rows are labeled OD (right eye) and OS (left eye).
- SPH (sphere): This is the main power of your lens. A negative number means you’re nearsighted, and a positive number means you’re farsighted. The higher the number in either direction, the stronger the correction.
- CYL (cylinder): This corrects astigmatism, which happens when your eye’s surface is more oval-shaped than round. SPH handles the round part of your eye, while CYL accounts for the oblong curvature. If this column is blank, you don’t have significant astigmatism.
- Axis: This works alongside CYL and describes where on your cornea the astigmatism sits, measured on a scale of 0 to 180 degrees. If you have a CYL value, you’ll always have an axis value too.
- Add: This is the additional magnifying power for the bottom portion of bifocal or progressive lenses, used for reading and close-up work. Adding this number to your SPH gives the prescription for the lower part of the lens.
- Prism: Some prescriptions include a prism value, which is a special correction for people who see double. The prism built into the lens fuses two separate images into one. Most people won’t have this on their prescription.
Pupillary Distance Isn’t Always Included
When ordering glasses online, you’ll need your pupillary distance (PD), which is the distance in millimeters between the centers of your pupils. This measurement ensures each lens is centered correctly in front of your eyes. Some doctors include it on the prescription, but many don’t, since it’s technically a fitting measurement rather than a prescription value.
You can measure it yourself at home. Stand about 8 inches from a mirror and hold a millimeter ruler against your brow. Close your right eye and line up the ruler’s zero mark with the center of your left pupil. Then, looking straight ahead, close your left eye and open your right. The millimeter mark that lines up with the center of your right pupil is your binocular PD. Most adults fall between 54 and 74 mm. It helps to repeat this a few times to confirm your number.
The Numbers on Your Frames Are Not Your Prescription
A common point of confusion: the numbers printed on the inside of your glasses’ arm are frame measurements, not your lens prescription. These typically show three values separated by dashes or spaces. The first is the lens width (how wide each lens opening is), the second is the bridge width (the gap between the two lenses that sits on your nose), and the third is the temple length (how long the arm is from hinge to tip). These numbers help you find frames that fit your face, but they tell you nothing about the corrective power of your lenses.
A Glasses Prescription Is Not a Contact Lens Prescription
If you’re hoping to use your glasses prescription to order contact lenses, you’ll need a separate prescription. The two aren’t interchangeable because glasses sit about 12 to 14 millimeters away from your eye, while contacts rest directly on the cornea. That difference in distance changes the effective power of the lens. For nearsighted prescriptions, a contact lens typically needs slightly less power than the glasses version. For farsighted prescriptions, it needs slightly more. This adjustment becomes especially important once your prescription exceeds about 4.00 diopters in either direction.
Contact lens prescriptions also include additional specifications like the lens brand, base curve, and diameter, which are determined by the shape of your eye and require a separate fitting.
Other Ways to Find Your Prescription
If you can’t reach your eye doctor and don’t have a paper copy, there are a few other options. The retailer where you bought your last pair of glasses may have your prescription on file, whether that’s an optical shop, a big-box store, or an online retailer. Check your account or call their customer service line.
Some optical shops and online services offer to read your existing lenses using a device called a lensometer, which measures the power built into the glasses you’re already wearing. This can give you approximate values, but it won’t replace a full eye exam since your vision may have changed since those lenses were made. A handful of smartphone apps and online tools claim to test your vision remotely, but these vary widely in accuracy and aren’t accepted everywhere as a valid prescription.
For the most reliable and up-to-date numbers, a comprehensive eye exam remains the standard. Beyond just producing a prescription, it checks for eye health conditions that don’t affect your vision in the early stages, like glaucoma and macular degeneration.