Vision standards
Short answer
A prescription alone cannot establish legal blindness. In the US, it means best-corrected vision of 20/200 or worse in the better eye, or a visual field of 20 degrees or less.
In detail
There is no prescription that makes you legally blind. Legal blindness in the United States is defined as best-corrected visual acuity of 20/200 or worse in the better eye, or a visual field of 20 degrees or less — and the words that matter are best-corrected.
Someone at −10.00 D who reads 20/20 in glasses is not legally blind, while someone at −1.00 D whose vision cannot be corrected past 20/200 is. The familiar figure of around −2.00 to −2.50 D is what typically produces 20/200 vision without any correction, which is a different question and not the legal test.
Set your sphere power to see roughly what it means for unaided vision — then answer the one question that actually decides the legal side. The full chart is printed below, so nothing here depends on the converter loading.
Approximate unaided distance acuity by spherical prescription. These are rules of thumb — real acuity varies with pupil size, astigmatism, age and retinal health. None of these rows is a legal blindness threshold, because the legal test uses vision measured with correction.
Every threshold below is measured with best available correction. Each row links to the authority that sets it.
None of them. This is the rare case where the most-searched question has an answer that rejects the question, and the rejection is the useful part. Every legal definition of blindness in use — American, British, Canadian, Australian, and the WHO's — is written on visual acuity measured with the best available correction. A prescription describes how much correction you need, not what you can see once you have it.
That distinction has real consequences in both directions. A person at −12.00 D has vision most people would describe as almost useless without glasses, and is not legally blind at all if lenses bring them to 20/20. Someone with macular degeneration and a mild prescription may be legally blind, because no lens fixes damage to the retina. Prescription strength and legal blindness are measuring different things.
The number that circulates online — around −2.00 to −2.50 dioptres — is the point at which an average eye reads about 20/200 with no correction at all. It is a real rule of thumb about unaided vision, and it is simply not the legal test. Anywhere you see it quoted as the threshold for legal blindness, the source has confused the two.
In the United States, one of two criteria, either of which qualifies on its own. The first is best-corrected visual acuity of 20/200 or worse in the better eye — meaning that with your glasses or contacts on, the largest letter you can read at 20 feet is one a person with typical vision reads at 200 feet. The second is a visual field of 20 degrees or less, which covers tunnel vision from conditions like advanced glaucoma or retinitis pigmentosa even when central acuity is good.
Two details are widely missed. The test uses the better eye, so being blind in one eye does not on its own make a person legally blind. And it is a definition for administrative purposes — eligibility for particular benefits and services — rather than a medical diagnosis. A clinician measures the numbers; an agency applies the rule.
Other countries draw the line in different places and use different words. The UK registers people as severely sight impaired or sight impaired, on thresholds expressed in metric notation and weighing field loss alongside acuity. Australia sets its threshold at worse than 6/60. The table above gives the current position for each.
No, and the same answer applies to every number people search for. At −5.50 D, unaided distance vision is very poor — somewhere around 20/600 as a rough estimate — and daily life without correction would be genuinely difficult. With glasses or contacts that correct to 20/40 or better, none of the legal definitions apply.
What high myopia does carry is elevated risk of other eye conditions, including retinal detachment, myopic macular degeneration, glaucoma and early cataract. Those conditions can affect best-corrected vision, and that is the pathway by which a strongly myopic person could eventually meet a legal blindness threshold — through disease, not through dioptres. It is a good reason for regular dilated eye exams at high prescriptions, and a bad reason to worry about the prescription number itself.
If the underlying worry is whether your vision is getting worse rather than what a label means, that is a question for an eye exam. Prescription changes, floaters, flashes, or any loss of part of your field of view should be checked promptly.
20/200 is the level at which the big E at the top of a standard eye chart is the only letter readable at 20 feet. In practice it means large shapes and movement are available while detail is not: you can see that a person is standing in a doorway without recognising their face, and read a headline held close but not body text.
It is also far from no vision at all. Most people who are legally blind have usable sight, and only a small minority have no light perception. Magnification, high contrast, screen readers and orientation training all work with remaining vision rather than around its absence — which is why "legally blind" and "totally blind" describe very different daily experiences.
This is worth saying plainly because the phrase itself is misleading. Legal blindness is an administrative threshold that unlocks support, not a description of darkness.
By measurement, not by prescription. An eye-care professional records best-corrected acuity in each eye on a standard chart at a standard distance, and measures the visual field where field loss is suspected. Those measurements are what an agency, employer, or benefits programme relies on.
In the United States, a Social Security determination adds its own process on top of the clinical numbers, and eligibility for benefits depends on more than the vision criterion alone. Registration in the UK works differently again, through a certificate completed by a consultant ophthalmologist and then local authority registration. Neither is something an online tool or a prescription can decide.
If you think you may meet a threshold, the route is an eye exam and an explicit conversation about it. Bring your current correction, and say directly that you want your best-corrected acuity and visual field documented.
Visual field test guide
How peripheral vision is measured and why a prescription cannot describe it.
Astigmatism test
A fan-chart self-check for the other half of a typical prescription.
Vision & brain self-tests
Every screening on the site, in one place.
Is color blindness a disability?
The same kind of question for color vision, answered by jurisdiction.
Color blind test
Free color vision screening — a different axis of vision entirely.
Last reviewed July 26, 2026. General information about published vision standards, not medical, legal, or benefits advice. Legal blindness is determined from measurements taken by an eye-care professional and applied by the relevant agency.
Worried your vision has changed? Book an eye exam rather than working from a chart — see who to book and what the appointment involves.