In order to be legally blind, you must have a visual acuity of 20/200. This means that even with glasses or contacts, you can only read the first letter at the top of the Snellen Chart, if that. You can also be legally blind if you can see, but only in a very small window in your eye.

In the United States and many other regions, "legally blind" is defined by a specific visual acuity of 20/200 or worse in the better-seeing eye while wearing corrective lenses (glasses or contacts). This means that a person must be as close as 20 feet to see what someone with "normal" 20/20 vision can see from 200 feet away. Additionally, legal blindness can be determined by a visual field (peripheral vision) that is restricted to 20 degrees or less (often called "tunnel vision"), even if the central vision is clearer. These specific numerical thresholds are used by government agencies and insurance companies to determine eligibility for disability benefits, tax exemptions, and specialized services. It is important to note that many people who are legally blind still have some level of usable vision and are not in total darkness.