Legal blindness is defined by two measurements: visual acuity of 20/200 or worse in your better eye with the best possible correction (glasses or contacts), or a visual field of 20 degrees or less. You only need to meet one of these criteria to be classified as legally blind. This definition comes from the Social Security Act and is used across federal agencies, from the SSA to the IRS.
The 20/200 Visual Acuity Threshold
The first criterion is about sharpness of vision. A Snellen eye chart, the familiar chart with progressively smaller letters, measures how clearly you see at 20 feet compared to someone with normal sight. If you have 20/200 vision, what a person with normal vision can read from 200 feet away, you need to be just 20 feet away to read. That’s a tenfold difference in clarity.
The critical detail is “best-corrected.” Your vision is measured after you’ve been fitted with the strongest glasses or contact lenses available. If corrective lenses bring your acuity above 20/200, you don’t meet this criterion, even if your uncorrected vision is far worse. The measurement also uses your better eye, not the worse one. So if one eye sees 20/200 but the other corrects to 20/100, you wouldn’t qualify under this threshold.
There is one exception worth knowing: if contact lenses could technically correct your vision beyond 20/200 but you can only wear them briefly due to pain, infection, or ulcers, the IRS still considers you legally blind for tax purposes.
The 20-Degree Visual Field Limit
The second criterion involves peripheral vision. A full visual field spans roughly 180 degrees horizontally. If your field of view narrows to 20 degrees or less in your better eye, you qualify as legally blind regardless of how sharp your central vision is. Twenty degrees is roughly the width of a credit card held at arm’s length. People with this kind of restriction sometimes describe it as looking through a narrow tunnel.
Visual field loss is measured using automated perimetry, typically a Humphrey Field Analyzer. During this test, you stare at a central point inside a dome-shaped device while small lights flash at various positions in your peripheral vision. You press a button each time you detect a flash. The machine maps which areas of your visual field are functioning and which are not, producing a detailed chart your eye doctor uses to measure the widest diameter of your remaining field.
Legal Blindness Is Not Total Blindness
Most people classified as legally blind retain some usable vision. They may be able to see colors, detect light and movement, or read large print up close. The term describes a threshold for government benefits and legal protections, not a complete absence of sight. Roughly 1.1 million Americans meet the criteria for legal blindness, according to CDC estimates from 2017, while another 6 million have milder vision loss that doesn’t reach the 20/200 cutoff.
The distinction matters because people often assume “blind” means seeing nothing at all. In reality, the spectrum runs from mild blur to no light perception, and legal blindness sits closer to the middle of that range than most people expect.
How the U.S. Definition Compares Globally
The World Health Organization uses a stricter threshold. Under the International Classification of Diseases, blindness starts at visual acuity worse than 3/60 (approximately 20/400), which is twice as severe as the U.S. cutoff. Many developed countries, however, use something closer to the American standard. Researchers have recommended the WHO adopt 6/60 (equivalent to 20/200) as a uniform global definition, aligning more closely with what the U.S. and other high-income countries already use.
Benefits and Protections
Meeting the legal definition of blindness unlocks specific federal benefits. Social Security recognizes “statutory blindness” as a qualifying disability. This has its own earnings rules that are more generous than standard disability programs, meaning you can earn more income while still receiving benefits.
The IRS provides a higher standard deduction for taxpayers who are legally blind. To claim it, you need a certified statement from an ophthalmologist or optometrist confirming that your best-corrected vision is 20/200 or worse, or that your visual field is 20 degrees or less. If your condition is unlikely to improve, the statement should say so, and you keep it in your records rather than filing it with your return.
Workplace protections come through the Americans with Disabilities Act. Employers are required to provide reasonable accommodations, which for someone who is legally blind might include screen reader software, materials in Braille or large print, modified work schedules, or accessible computer systems. The specific accommodations depend on the job and the individual’s remaining vision.
How Legal Blindness Is Certified
Your eye doctor performs two standard tests. Visual acuity is measured with a Snellen chart while you wear your best corrective lenses. If your acuity falls at or below 20/200, that test alone is sufficient. If your acuity is better than 20/200 but you suspect significant peripheral loss, a visual field test determines whether your field is 20 degrees or narrower.
For Social Security purposes, the visual field test must meet specific technical requirements: it needs to use a standard stimulus size, test points no more than 6 degrees apart, and cover the central 24 to 30 degrees around your point of fixation. The SSA then maps the results to determine the widest diameter of your remaining field. Your eye care provider handles the technical side, but it helps to know that a single office visit with these two tests is typically all that’s needed for certification.
Once certified, the documentation serves multiple purposes. The same statement from your eye doctor can support a Social Security claim, a tax deduction, state services for the blind, and eligibility for assistive technology programs.