Vision standards
Short answer
Legally blind means best-corrected vision of 20/200 or less in the better eye, or a visual field of 20 degrees or less. It is an eligibility threshold; most people who meet it still have some sight.
In detail
Legally blind is an administrative label, not a description of seeing nothing. US law sets two tests and meeting either one is enough: central visual acuity of 20/200 or less in the better eye while wearing the best correcting lens, or a visual field narrowed to 20 degrees or less.
Both are measured by an eye-care professional, and both describe the vision that remains after glasses or contacts have done everything they can. Most people who meet the definition still have usable sight. Agencies use the label to decide eligibility for benefits, tax treatment and services, which is why the wording comes from the Social Security Act rather than from a medical textbook.
This page explains published definitions. It cannot tell you whether you are legally blind: that is decided from measurements taken by an eye-care professional and applied by the relevant agency. Looking for what a specific prescription means? See what prescription is legally blind.
Five terms that get used interchangeably and should not be. Acuity figures are best-corrected, meaning measured with glasses or contacts on.
The same acuity written three ways. US clinics use feet (20/x), most other countries use meters (6/x), and research papers often use the decimal.
It means a person's vision falls at or below a line that a law has drawn, measured in a specific way. In the United States the line comes from the Social Security Act, and other federal and state programs borrow it. The acuity test is central visual acuity of 20/200 or less in the better eye with the use of a correcting lens. The field test is a visual field whose widest diameter is 20 degrees or less. A person who meets either one is legally blind for the purposes of those programs.
Three words in that definition do most of the work. Better eye means the test looks at the stronger of the two, so losing the sight of one eye does not meet it. Correcting lens means the measurement is taken with the best glasses or contacts in place, so a strong prescription is irrelevant if it corrects well; our guide to what prescription is legally blind covers that misunderstanding in detail. And legally signals that this is an eligibility rule. A clinician measures the numbers and an agency applies the rule to them.
The fraction compares you with a reference observer. The first number is the testing distance, 20 feet. The second is the distance at which a person with typical acuity could read the same line. At 20/200, the smallest letters you can read from 20 feet are ones that reference observer reads from 200 feet. On a classic Snellen chart that is the single large letter at the top.
How the chart is built matters at this level. Traditional Snellen charts jump straight from 20/100 to 20/200 with nothing in between, while newer designs such as ETDRS and Bailey-Lovie charts have extra lines. The Social Security Administration's rule for those newer charts is that a person who cannot read any letters on the 20/100 line is treated as having acuity of 20/200 or less. It is a small technical detail that decides real cases.
Outside the US the same acuity is written 6/60, because the testing distance is 6 meters, or as the decimal 0.1. The table above lines the notations up.
Yes, through the visual field test. Acuity describes how fine a detail you can resolve in the very center of your vision. It says nothing about how wide your view is. Someone with advanced glaucoma or retinitis pigmentosa can read the 20/20 line while seeing the world through a narrow tunnel, missing steps, curbs and anything approaching from the side.
The law treats that as equivalent. A field whose widest diameter is 20 degrees or less meets the definition regardless of central acuity. For scale, a typical field of view spans close to 180 degrees horizontally with both eyes open, while 20 degrees is roughly the width of a dinner plate held at arm's length. Field loss is measured with perimetry; our visual field test guide explains how that appointment works.
No. Most people who are legally blind have sight they use every day. They may read large print or a magnified screen, recognize people at close range, and get around familiar places by sight. Total blindness, meaning no perception of light, describes only a small share of people who meet the legal definition.
Low vision is a broader, clinical idea. The National Eye Institute describes it as vision loss that cannot be fixed with glasses, contacts, medicine or surgery and that makes everyday tasks hard. In the US it is commonly counted from a best-corrected acuity of about 20/70. Everyone who is legally blind from an eye condition has low vision, but many people with low vision are well above the legal line.
Not under the US definition, as long as the other eye sees better than 20/200 with correction and has a field wider than 20 degrees. The test is written on the better eye for a reason: a person with one well-functioning eye keeps most of their functional vision, though they lose some depth perception and part of the field on the affected side.
That does not make monocular vision trivial. Some jobs and license classes set their own rules for it, and adapting takes time. It is simply a different category from legal blindness, handled by different rules.
Almost always an eye disease or injury, because the definition only counts vision that lenses cannot improve. In adults the leading causes are age-related macular degeneration, which erodes central vision, diabetic retinopathy, glaucoma, which narrows the field, and cataract that has not been treated. In children and younger adults, inherited retinal conditions, optic nerve disorders and complications of prematurity account for more cases.
Refractive error alone, meaning nearsightedness, farsightedness or astigmatism, does not cause legal blindness when correction is available, since glasses restore the acuity. Very high myopia is a partial exception only because it raises the risk of retinal disease later in life. The disease, not the diopters, is what changes best-corrected vision.
In the US the label connects to three main things. The Social Security Administration uses it in disability determinations, with rules for blind applicants that differ from those for other disabilities, including a higher monthly earnings limit. The IRS gives taxpayers who meet the definition a larger standard deduction, and asks for a statement from an eye doctor to support it. State agencies use it for vocational rehabilitation, orientation and mobility training, and related services, and the Library of Congress runs a free braille and talking-book service for people who cannot read standard print.
None of these is automatic. Each program has its own application and its own non-vision criteria, and meeting the definition is one requirement among several. The starting point is always the same: an eye exam in which best-corrected acuity and, where relevant, the visual field are documented in writing.
The phrase gets used for two different things. The first is a prescription strength people assume must count as legal blindness. No such prescription exists, because the definition is measured with correction on; the prescription chart and per-diopter answers show what each strength means for unaided vision instead.
The second is eyewear made for people whose vision stays at or below 20/200 even with ordinary lenses. These are low vision aids, and an optometrist who specializes in low vision prescribes them. High-plus reading spectacles and prismatic half-eye glasses magnify near work. Bioptic telescopes mount a small telescope in the upper part of a lens for spotting distant detail, and some US states allow them for restricted driving. Head-worn electronic magnifiers use a camera and screens to enlarge and boost contrast. Tinted and filter lenses help with glare and contrast in conditions such as macular degeneration.
All of these help a person use the vision they have. None changes the underlying measurement, so someone who is legally blind remains so while wearing them. A low vision evaluation is the way to find out which, if any, would be worth it for a particular task.
No. Canada uses the same 20/200 and 20-degree structure as the US. The UK does not use the phrase legally blind at all: a consultant ophthalmologist certifies a person as sight impaired or severely sight impaired, on thresholds written in metric notation with field loss weighed alongside acuity. Australia sets its line at worse than 6/60 or a field under 10 degrees. The World Health Organization's blindness category, used for statistics, starts below 3/60.
The thresholds by country table on the prescription page lists each one with a link to the authority that sets it.
What prescription is legally blind?
Prescription chart, converter, per-diopter answers and thresholds by country.
Visual field test guide
How the 20-degree field criterion is actually measured.
Contrast sensitivity test
The part of functional vision an acuity chart does not capture.
Vision & brain self-tests
Every screening and vision guide on the site, in one place.
Is color blindness a disability?
The same kind of legal question, answered for color vision.
Last reviewed September 18, 2026. General information about published vision standards, not medical, legal, tax, or benefits advice. Legal blindness is determined from measurements taken by an eye-care professional and applied by the relevant agency.