The number on your glasses or contact lens prescription tells you exactly how much correction your eyes need, measured in units called diopters. The higher the number, the stronger the lens required and the blurrier your uncorrected vision. A prescription of -1.00 is quite mild, while -6.00 or beyond enters “high myopia” territory with real health implications. Here’s how to make sense of your numbers.
What the Plus and Minus Signs Mean
A minus sign means you’re nearsighted: you see things up close clearly but struggle with distance. A plus sign means you’re farsighted: distance vision is easier but close-up work is harder. The number after the sign is what matters for severity. A -2.00 and a +2.00 are the same “strength” of lens, just correcting in opposite directions.
Nearsightedness: Mild to Severe
Eye care professionals generally classify nearsightedness (myopia) into tiers based on how many diopters of correction you need:
- Mild: -0.50 to -3.00
- Moderate: -3.25 to -6.00
- High: -6.25 to -9.00
- Severe: beyond -9.00
If your prescription is -2.00, you’re solidly in the mild range. You need glasses for driving and movie theaters, but your eyes aren’t in any unusual medical danger. At -4.50, you’re in moderate territory. Without correction, most text beyond arm’s length is a blur. At -7.00 or stronger, you’re dealing with high myopia, and the health picture starts to change (more on that below).
How Blurry Is Your Uncorrected Vision?
Diopters don’t translate perfectly to the 20/20 scale your eye doctor uses on the wall chart, but rough estimates give a useful sense of what you’re actually seeing without glasses. At -0.50, your uncorrected vision is roughly 20/30 to 20/40. At -1.00, it drops to about 20/60. By -1.50, you’re around 20/100, and at -2.50, you’ve reached approximately 20/200, the threshold used to define statutory blindness (though that definition applies to corrected vision, not uncorrected).
This is why even a “small” prescription makes a noticeable difference. A person at -1.00 already can’t read road signs as quickly as someone with perfect vision. And someone at -3.00 without glasses sees a world that’s genuinely blurry at any useful distance.
Farsightedness: Mild to Severe
Farsightedness (hyperopia) uses a different scale because younger eyes can partially compensate for it by flexing the lens inside the eye. The American Optometric Association classifies it as:
- Low: +2.00 or less
- Moderate: +2.25 to +5.00
- High: above +5.00
A child with +3.00 of farsightedness may still see 20/20 because their flexible lens compensates. The same prescription in a 50-year-old causes much more noticeable blur because the lens has stiffened with age. This is why farsightedness often “appears” in middle age even though the eye shape hasn’t changed.
The “ADD” Number on Your Prescription
If you’re over 40, your prescription likely includes an ADD (addition) value. This isn’t a separate eye problem. It reflects presbyopia, the normal, universal loss of close-up focusing that comes with age. Typical ADD values follow a predictable pattern:
- Ages 35 to 45: +1.00
- Ages 46 to 50: +1.50
- Ages 51 to 55: +2.00
- Over 55: +2.50 or higher
A higher ADD number doesn’t mean your eyes are “worse” in any concerning way. It just means your lens has become less flexible, which happens to everyone. If you’re 53 with an ADD of +2.00, that’s completely typical.
When a High Prescription Becomes a Health Risk
For most people, a glasses prescription is an inconvenience, not a medical problem. The correction fully restores your vision, and your eye health stays normal. But high myopia, generally -5.00 or stronger (the threshold the World Health Organization uses), carries real risks that go beyond blurry vision.
Highly myopic eyes are physically longer than average. That extra length stretches the retina thinner, like pulling a balloon before it’s inflated. This stretching creates vulnerability to several conditions:
- Retinal detachment: The risk is five to six times greater in people with high myopia compared to those with mild prescriptions. The stretched retina is more prone to tears, and the gel inside the eye is more likely to pull away and create problems.
- Glaucoma: People with moderate to high myopia face roughly 2.5 times the risk of developing open-angle glaucoma compared to those with low prescriptions.
- Cataracts: High myopia increases the likelihood of needing cataract surgery, with odds about 17% higher than for moderate myopia.
- Myopic macular degeneration: The risk rises sharply with both age and increasing prescription strength. This condition affects central vision and can cause permanent changes.
None of this means vision loss is inevitable. It means that if your prescription is -5.00 or stronger, regular dilated eye exams become more important. Your eye doctor will check for early signs of retinal thinning or other changes that are treatable when caught early.
What Counts as “Legally Blind”
Legal blindness is defined by the Social Security Administration as corrected visual acuity of 20/200 or worse in your better eye, or a visual field narrowed to 20 degrees or less. The key word is “corrected.” If your prescription is -8.00 and your vision is terrible without glasses, but you see 20/20 with them, you are not legally blind. Legal blindness refers to what your eyes can do with the best possible correction in place.
So even a very strong prescription doesn’t make you legally blind as long as glasses or contacts bring your vision back to a functional level.
Practical Benchmarks That Matter
Beyond the clinical categories, a few real-world thresholds help put your prescription in context. Nearly every U.S. state requires corrected visual acuity of at least 20/40 to hold a standard driver’s license. If your glasses or contacts get you to 20/40 or better, you meet that bar regardless of how strong your prescription is.
For daily life, people with mild prescriptions (up to about -2.00 or +2.00) can often function without glasses in many situations, even if things aren’t perfectly sharp. Moderate prescriptions make glasses essentially necessary for driving, work, and most activities. High prescriptions mean you’re reaching for your glasses the moment you wake up.
The number itself isn’t a judgment. A prescription of -4.00 is extremely common and fully correctable. What matters is whether your corrected vision is good, whether your prescription is stable, and whether you’re getting appropriate monitoring if you’re in the high range.