Your eye doctor is required by federal law to give you a copy of your prescription at the end of every exam, free of charge. If you don’t have yours on hand, there are several ways to track it down or get a new one, depending on your situation.
Your Doctor Must Provide It for Free
The Federal Trade Commission’s Eyeglass Rule requires every ophthalmologist and optometrist to hand you a copy of your eyeglass prescription immediately after completing a refractive eye exam. They must do this automatically, whether or not you ask for it, and they cannot charge an extra fee for it. Simply asking if you want the prescription doesn’t satisfy the rule. The doctor has to physically provide it unless you specifically refuse.
If your last exam was recent and you never received a copy, call the office and request one. They cannot require you to buy glasses or contact lenses from them in exchange for releasing it. This is the simplest, most reliable way to get your current prescription.
Check Your Existing Glasses or Past Orders
If you bought glasses online, your prescription is almost certainly saved in your account on the retailer’s website. Log in and check your order history. If you purchased from a brick-and-mortar shop, they keep your prescription on file and can pull it up with a phone call.
Some people try to reverse-engineer their prescription from an old pair of glasses using smartphone apps or handheld lens-reading devices. These tools work by shining light through your existing lenses to estimate the prescription values. They can be reasonable for simple, moderate prescriptions, but if you have a complex correction (high power, prism, or significant astigmatism), they’re unreliable. For anything beyond a straightforward prescription, an in-person exam is the safer route.
Online Vision Tests for Renewals
If your prescription recently expired and your eyes haven’t changed much, online vision test services let you renew without an office visit. These typically work by having you stand about 10 feet from your computer screen and read letters, similar to an in-office chart. You’ll need your current prescription on hand, plus a brief eye health questionnaire. A licensed doctor reviews the results and, if everything checks out, emails you an updated prescription within about 24 hours.
These services are designed for people with stable, straightforward prescriptions who just need a renewal. They aren’t a substitute for a comprehensive eye exam, which checks for glaucoma, macular degeneration, and other conditions that don’t affect your ability to read a chart. If you have a high-power or irregular prescription, the American Academy of Ophthalmology recommends skipping the home tests and going in for a full exam.
How to Read Your Prescription
Once you have your prescription in hand, the abbreviations can look like a foreign language. Here’s what each column means:
- OD and OS: OD is your right eye (from the Latin “oculus dexter”), and OS is your left eye (“oculus sinister”). Some doctors use RE and LE instead.
- SPH (sphere): This is the main correction number, measured in diopters. A minus sign means you’re nearsighted; a plus sign means you’re farsighted. The higher the number, the stronger the lens.
- CYL (cylinder): This corrects for astigmatism, which happens when your cornea is slightly oblong rather than perfectly round. If this column is blank, you don’t have astigmatism.
- Axis: A number from 0 to 180 that tells the lab where on your cornea the astigmatism sits. It always appears alongside a CYL value.
- ADD: An additional magnifying power for the lower portion of bifocal or progressive lenses, used for reading. This value is added to your SPH number to create the near-vision correction.
- Prism: Not everyone has this. Prism correction shifts the image entering your eye to compensate for alignment problems that cause double vision. It includes a direction (base-in, base-out, base-up, or base-down) that tells the lab which way to orient the correction.
Your Glasses Prescription Won’t Work for Contacts
A detail that trips up many people: eyeglass and contact lens prescriptions are not interchangeable. Glasses sit a few millimeters in front of your eyes, while contacts rest directly on the surface. That small gap changes how much the lens needs to bend light, so the power values are adjusted accordingly. Contact lens prescriptions also include a base curve (the curvature of the lens) and a diameter measurement to ensure the lens fits your eye properly. You need a separate contact lens fitting to get these numbers.
Don’t Forget Your Pupillary Distance
If you plan to order glasses online, you’ll need one measurement that often isn’t included on your prescription: pupillary distance, or PD. This is the distance in millimeters between the centers of your pupils, and it tells the lab where to position the optical center of each lens.
You can ask your eye doctor’s office for this number, though some offices are reluctant to provide it since it signals you’re shopping elsewhere. If you can’t get it from them, you have a few options. Many online eyewear retailers offer camera-based PD tools that use your phone’s front-facing camera to measure the distance instantly. For the most accurate results, iPhones with depth-sensing cameras tend to perform best. You can also use a simple millimeter ruler held up to your face in front of a mirror: close your right eye, align the zero mark with the center of your left pupil, then open your right eye and close your left to read the measurement at your right pupil. The average adult PD falls between 54 and 74 mm.
Prescription Expiration Dates
Eyeglass prescriptions typically expire after one to two years, depending on the state and the doctor’s judgment. Contact lens prescriptions are required to carry an expiration date, and in many states a short extension (around two months) is available if you need time to schedule a new exam. Glasses prescriptions don’t always have a printed expiration date, but most doctors include one, and online retailers generally won’t fill a prescription older than two years.
If your prescription has expired and you’re not experiencing any vision changes, an online renewal service may bridge the gap. But if it’s been more than two years since your last comprehensive exam, or if you notice any shift in your vision, booking a full appointment catches both prescription changes and early signs of eye disease that a simple vision test won’t detect.