A “normal” eye prescription, meaning one that requires no corrective lenses at all, falls between -0.75 and +0.75 diopters. This range is considered effectively zero refractive error, a state called emmetropia. If your prescription sits within that window, your eyes focus light correctly on their own and you likely don’t need glasses. Most people who search this question, though, already have a prescription and want to know whether their numbers are typical or unusually strong. The short answer: most corrective prescriptions fall in the mild range, and anything under about 3 diopters in either direction is common and easy to correct.
How to Read Your Prescription
An eye prescription has a few standard abbreviations and columns. OD refers to your right eye (from the Latin “oculus dexter”), and OS refers to your left eye (“oculus sinister”). Some prescriptions also list OU, meaning both eyes together. Each eye gets its own row of numbers.
The main columns you’ll see are:
- Sphere (SPH): The primary correction for how your eye focuses. A negative number means you’re nearsighted (you see well up close but not far away). A positive number means you’re farsighted (distance vision is easier than close-up). The further this number is from zero, the stronger your prescription.
- Cylinder (CYL): This measures astigmatism, which happens when your cornea is shaped more like a football than a basketball. One curve is steeper than the other, so light doesn’t focus evenly. Not everyone has a cylinder value. If this box is empty, you don’t have significant astigmatism.
- Axis: A number between 1 and 180 that tells your optician exactly where on the cornea the astigmatism sits. It only appears when you have a cylinder value.
- Add: Extra magnifying power for reading, used in bifocals or progressive lenses. This shows up on prescriptions for people over 40 who have age-related difficulty focusing up close.
What Counts as Mild, Moderate, or Strong
Nearsightedness (Myopia)
If your sphere value is negative, your prescription corrects for nearsightedness. The American Academy of Ophthalmology classifies it this way:
- Mild: Less than -3.00 diopters
- Moderate: -3.00 to -6.00 diopters
- Severe (high myopia): More than -6.00 diopters
A prescription of -1.50 or -2.00 is extremely common and well within the mild range. You’d need glasses for driving and watching TV but could read a book or use your phone without them. Someone at -6.00 or beyond has significant blur without correction and faces higher long-term risk for retinal problems, so regular eye exams become especially important.
Farsightedness (Hyperopia)
A positive sphere value corrects for farsightedness:
- Low: +2.00 diopters or less
- Moderate: +2.25 to +5.00 diopters
- High: +5.25 diopters or more
Younger people with low hyperopia often don’t realize they’re farsighted because their eyes can compensate by squeezing the lens to bring things into focus. That ability fades with age, which is why some people don’t get their first glasses prescription until their 30s or 40s even though the underlying issue was always there.
Astigmatism
Cylinder values of -0.75 or less are so minor that many people skip correction entirely and never notice the difference. Values between -1.00 and -2.00 are moderate and typically worth correcting. Above -2.00, astigmatism has a noticeable effect on clarity at all distances, and you’ll likely want to wear your glasses most of the time. A large number of prescriptions include at least some astigmatism, so seeing a cylinder value on your script is perfectly normal.
The “Add” Power for Reading
Starting around age 40, the lens inside your eye gradually loses flexibility, making it harder to shift focus from far to near. This is called presbyopia, and it happens to virtually everyone. Your prescription will include an “Add” value representing the extra magnifying power needed for reading and close work.
The Add value climbs predictably with age. In your early 40s, you might need only +0.75 to +1.00. By your late 40s, that rises to around +1.25 to +1.50. Through your 50s it continues climbing, and by age 60 and beyond, most people need +2.50 to +2.75 for comfortable reading. This progression is universal and not a sign that something is wrong with your eyes. It’s simply the lens stiffening over time.
How Prescriptions Relate to 20/20 Vision
Your diopter measurement and your visual acuity (the 20/20 scale) are related but measure different things. Diopters describe how much correction your eye needs. The 20/20 scale describes how sharp your vision actually is, either with or without correction. Someone with a -4.00 prescription might see 20/400 without glasses but a crisp 20/20 with them. The prescription itself doesn’t tell you how well you see once you’re wearing the right lenses.
For reference, the threshold for legal blindness in the United States is 20/200 vision even with the best possible correction, or a visual field no wider than 20 degrees. That’s a very high bar. If your corrected vision reaches 20/20 or close to it, your prescription strength doesn’t affect your functional vision in any meaningful way.
Why Two Eyes Often Differ
It’s completely normal for your right and left eyes to have different prescriptions. A difference of a diopter or so between eyes is common. You might be -2.50 in one eye and -1.75 in the other, or farsighted in one and mildly nearsighted in the other. Your brain is remarkably good at blending the two images together. Larger differences between eyes (more than 2 to 3 diopters apart) can sometimes cause eye strain or headaches if not properly corrected, but your eye care provider will catch that during a routine exam.
What Changes Over Time
Children’s prescriptions often shift significantly as their eyes grow, with nearsightedness commonly increasing through the teenage years before stabilizing in the early 20s. Adults’ prescriptions tend to stay relatively stable from the mid-20s through the late 30s, with the main change being the arrival of presbyopia around 40. After 60, prescriptions can shift again as the lens inside the eye continues to change with age, and conditions like early cataracts can temporarily alter your numbers.
A small change between exams, something like a quarter-diopter shift, is normal fluctuation and may not even require new lenses. A sudden large change, especially in one eye, is worth investigating promptly, since it can occasionally signal something beyond a routine refractive shift.