You can get a copy of your glasses prescription by calling the eye doctor who performed your last exam. By federal law, they’re required to give it to you at no extra charge. If it’s been a while since your last exam or you’ve lost the paperwork, there are a few other ways to track it down, including reading it off your current glasses or using an online refraction tool.
Your Eye Doctor Must Provide It Free
The FTC’s Eyeglass Rule requires optometrists and ophthalmologists to hand over a copy of your prescription after any eye exam, without charging you for it. They also can’t require you to buy glasses from them as a condition of the exam. If you weren’t given a copy at the time, you can request one later by calling the office. Most practices can look it up in their records and email, fax, or mail it to you.
Keep in mind that glasses prescriptions expire, typically after one to two years depending on your state. If yours has expired, you’ll need a new eye exam to get an updated one.
How to Read Your Prescription
A glasses prescription can look like a string of confusing abbreviations, but each part has a straightforward meaning. OD refers to your right eye, and OS refers to your left eye. Sometimes you’ll see OU, which means both eyes.
The main values you’ll find on a prescription include:
- SPH (Sphere): The basic lens power needed to correct your vision. A minus sign means you’re nearsighted (you see well up close but need help with distance). A plus sign means you’re farsighted (distance is clearer than things nearby).
- CYL (Cylinder): The amount of astigmatism correction you need, if any. Astigmatism happens when part of the cornea curves differently than the rest.
- Axis: A number between 1 and 180 that indicates the angle of your astigmatism on the cornea. This only appears if you have a CYL value.
- Add: Extra magnifying power for the lower portion of bifocal or progressive lenses. This is common for people over 40 who need help with reading.
A typical prescription might read: OD −2.50, CYL −0.75, Axis 180. That means the right eye is moderately nearsighted with a small amount of astigmatism.
You Also Need Your Pupillary Distance
If you’re ordering glasses online, you’ll need one more number that often isn’t included on your prescription: your 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. Most adults fall between 54 and 74 mm.
You can ask your eye doctor for this measurement, though some offices don’t include it on prescriptions automatically. You can also measure it yourself at home using a millimeter ruler and a mirror. Stand about 8 inches from a mirror, hold the ruler flat against your brow, close your right eye, and align the zero mark with the center of your left pupil. Then open your right eye, close your left, and read the number that lines up with the center of your right pupil. That number is your single PD.
Some online retailers ask for a “dual PD,” which is two separate measurements from the center of each pupil to the bridge of your nose. A dual PD is written as two numbers like 32/30, with the right eye listed first.
Can You Figure It Out From Your Current Glasses?
If you have a pair of glasses that work well for you but no written prescription, an optical shop can use a device called a lensometer to read the prescription directly from your lenses. This takes about a minute and many shops will do it for free, even if you didn’t buy the glasses there. The measurement is usually accurate enough to order a new pair with the same correction.
While you’re looking at your glasses, you might notice a set of numbers printed on the inside of one of the temples (the arms that hook over your ears). These aren’t your prescription. They describe the frame dimensions: the first number is the lens width, the second is the bridge width (the gap between the lenses that sits on your nose), and the third is the temple length. All three are in millimeters. These are useful when you’re shopping for new frames and want a similar fit.
Online Vision Tests Have Limits
Several companies now offer online refraction tests that let you check or update your prescription from home using a computer screen and your smartphone. These tools can measure the power correction your eyes need, but they can’t detect eye diseases, many of which have no obvious symptoms in their early stages.
The American Academy of Ophthalmology considers online vision tests reasonable only for healthy adults between 18 and 39 who already have a mild or moderate prescription, have had a comprehensive eye exam in the past, and have no symptoms of eye disease. If you have diabetes, a family history of glaucoma, a high or irregular prescription, or haven’t had a full eye exam recently, these tests aren’t a reliable substitute. Any prescription generated through an online tool still needs to be reviewed and approved by an optometrist or ophthalmologist before it’s finalized.
These services work best as a way to update an existing prescription when getting to an office is inconvenient. They’re not a replacement for the full exam that checks eye health, screens for conditions like glaucoma and macular degeneration, and sometimes catches systemic health issues like high blood pressure.
A Glasses Prescription Isn’t a Contact Lens Prescription
If you’re hoping to use your glasses prescription to order contact lenses, you’ll need a separate fitting. The two prescriptions aren’t interchangeable. Glasses sit about 12 millimeters from the surface of your eye, while contacts rest directly on it. That small gap changes how much the lens needs to bend light, so the power values are often slightly different.
A contact lens prescription also includes measurements that a glasses prescription doesn’t: the base curve (the curvature of the lens to match your eye’s shape) and the diameter. Astigmatism correction in contacts can be trickier to match as well, since the lens has to stay oriented correctly while sitting on an eye that moves and blinks. You’ll need a contact lens exam and fitting to get those additional measurements.