Your eye prescription is a set of numbers that describe exactly how your lenses need to be shaped to correct your vision. You can find it on the paperwork from your last eye exam, printed on your contact lens boxes, or by requesting a copy from your eye doctor, who is legally required to provide one. Understanding what those numbers mean, and knowing when they might need updating, helps you shop for glasses or contacts with confidence.
Where to Find Your Prescription
The most straightforward place is the slip of paper your eye care provider handed you after your last exam. If you’ve lost it or never received one, federal law in the United States is on your side. The FTC requires eye care providers to give you a copy of your prescription every time they perform a refraction exam, at no extra charge, even if you don’t ask for it, and even if the exam itself was free. You don’t need to buy glasses or contacts from that office to get your numbers.
If you wear contact lenses and have a recent box on hand, your prescription details are printed on the side or end of the box and on each individual blister pack. You’ll see your lens power (labeled PWR or D), base curve (BC), and diameter (DIA). Keep in mind that a contact lens prescription is not the same as a glasses prescription, so you can’t use one interchangeably for the other without conversion.
Reading a Glasses Prescription
A glasses prescription looks like a small table with abbreviations that can seem cryptic at first. Here’s what each one means:
- OD and OS: OD stands for “oculus dexter” (right eye) and OS stands for “oculus sinister” (left eye). Some newer prescriptions just write “right” and “left.”
- SPH (Sphere): This is the main lens power needed to correct your vision, measured in diopters. A minus sign (−) means you’re nearsighted; a plus sign (+) means you’re farsighted.
- CYL (Cylinder): This number indicates how much astigmatism you have, if any. Astigmatism means your cornea is shaped more like a football than a basketball, causing blurred vision at certain angles. If this column is blank, you don’t have significant astigmatism.
- Axis: Written as a number between 1 and 180, this tells the lens maker the angle at which your astigmatism correction needs to be oriented. It only appears when there’s a CYL value.
- Add: This is extra magnifying power added to the bottom portion of bifocal or progressive lenses, typically prescribed for people over 40 who need help with close-up reading.
What the Numbers Actually Tell You
The SPH value is the number most people focus on because it reflects how strong your correction is. A prescription of −1.00 is mild nearsightedness, while −6.00 or beyond is classified as high myopia. Most nearsighted people fall somewhere between −0.50 and −6.00. The higher the number (ignoring the sign), the stronger the lens and the blurrier your uncorrected vision.
For farsightedness, the same logic applies with a plus sign. A prescription of +1.50 is a mild correction, while higher numbers mean more difficulty focusing on nearby objects. If your CYL value is something like −0.75 or −1.25, that’s a modest amount of astigmatism that’s easily corrected with standard lenses. Values above −2.00 represent more significant astigmatism.
Why Glasses and Contact Prescriptions Differ
If you’ve compared the numbers on your glasses prescription to the ones on your contact lens box, you may have noticed they don’t match. This isn’t an error. 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, contacts typically need a slightly weaker (less minus) power than glasses. For farsighted prescriptions, contacts often need a slightly stronger (more plus) power. This difference becomes clinically meaningful once your prescription exceeds about ±4.00 diopters. Below that threshold, the numbers are often similar or identical. Contact lens prescriptions also include a base curve and diameter measurement that glasses prescriptions don’t, because contacts need to physically fit the shape of your cornea.
Pupillary Distance: The Number Often Missing
One measurement you need for ordering glasses online that isn’t always included on your prescription is your pupillary distance, or PD. This is the distance in millimeters between the centers of your two pupils, and it tells the lab where to position the optical center of each lens. For most adults, PD falls between 54 and 66 mm. For children, it’s typically between 41 and 53 mm.
You can measure your PD at home with a ruler and a mirror, or have someone hold a millimeter ruler across the bridge of your nose while you look straight ahead. Smartphone apps that calculate PD from a selfie are another option and tend to be the easiest method. Some eye care offices include PD on the prescription, but many don’t because it’s considered a fitting measurement rather than a medical value. If yours isn’t listed, just ask for it at your next visit or measure it yourself before placing an online order.
Signs Your Prescription Has Changed
Prescriptions don’t stay accurate forever. Your eyes change gradually, and several everyday symptoms can signal that your current lenses are no longer doing their job.
Frequent headaches, especially tension-type pain around your temples or forehead, are one of the most common signs. When your eyes strain to compensate for an outdated prescription, the muscles around them work harder than they should. You might also notice yourself squinting more often, holding your phone or books closer to your face, or struggling to read street signs while driving. These are all signs your lenses aren’t providing the correction you need.
Low-light and nighttime vision tend to suffer first. If driving after dark feels harder than it used to, or glare from oncoming headlights and streetlights has become more bothersome, your prescription may have shifted. Persistent eye discomfort like scratchiness, excessive watering, or a general feeling of tired eyes can also point to lenses that are no longer quite right.
Most eye care providers recommend a comprehensive eye exam every one to two years for adults. Children’s prescriptions can change more rapidly, so annual exams are standard through age 18. Even if your vision feels fine, regular exams catch changes before symptoms become obvious and screen for conditions unrelated to prescription strength.