A 20/20 result is the benchmark most people know. But the vision scale runs both sharper and softer than that, from 20/15 to 20/25 and beyond. This guide covers what those numbers mean, how eye doctors measure them and why your prescription is a separate reading.

Visual acuity and the Snellen chart

Visual acuity (VA) is a term for how sharp or clear your vision is. It reflects how well you can differentiate fine detail at a set distance. There are a few ways to measure it, but the Snellen chart is the most common tool eye doctors use.

The Snellen chart is an eye chart with several rows of letters, and each row represents a different level of visual acuity. The biggest letters sit at the top, starting with a single E. As you move down each row of the chart, the letters gradually get smaller.

Visual acuity is represented as a fraction. The first number is the distance you view the chart from. Eye doctors usually set the chart about 20 feet away (about 6 meters, which is why the same result is written 6/6 outside the US). So in the U.S., that first number is almost always 20. [2]

The second number is the distance at which a person with normal vision could read the same line. If you have 20/20 vision, you see at 20 feet what a normal eye sees at 20 feet. That makes 20/20 the standard for good, or normal, vision. [1]

If you have 20/40 vision, you see at 20 feet what an average eye sees at 40 feet. So, your vision is considered slightly poorer than average. But if you have 20/15 vision, you see at 20 feet what a normal eye would only see at 15 feet. That makes 20/15 vision better than average.

The values in between follow the same logic. A 20/25 vision score means you see at 20 feet what a normal eye sees at 25 feet, one line below the 20/20 benchmark. A 20/15 vision score is the opposite, one step sharper than 20/20. Here is how the most common points on the scale compare:

Most people have an "uncorrected" and a "best corrected" visual acuity. Uncorrected VA is how well you see with no lenses on. Best corrected VA is how well you see with the right lenses in place. In other words, it is your acuity while wearing your glasses or contact lenses.

Acuity is only part of the picture, though. Color vision, depth perception, peripheral vision and binocular vision (how your eyes work together) all shape how well you see.

Visual acuity vs vision prescriptions

Visual acuity tells you how sharp your vision is. A vision prescription tells you the lens power you need to reach your best corrected VA. The two are linked, but they are not the same thing.

Prescriptions are measured in diopters, in quarter steps (0.25 D, 0.50 D, 0.75 D, 1.00 D). If your acuity is 20/20 or better, your prescription is often plano (0.00). "Plano lenses" don't contain any corrective power.

The farther a prescription sits from zero, the stronger it is. A plus sign (+) means the lens corrects hyperopia (farsightedness), while a minus sign (-) means it corrects myopia (nearsightedness).

A prescription holds every detail a lab needs to correct your vision. Your optician uses it to make the right lenses for you.

Convert eye prescription to 20/20 scale

is the key point: there is no direct correspondence between your visual acuity and your vision prescription. A vision prescription provides details that you don't get from a visual acuity measurement.

That is also why there is no calculator or single formula that turns a prescription into a 20/20-scale number. Two people can share the same -2.00 D prescription and still read different lines on the chart. Acuity depends on factors a diopter value can't capture. The only reliable way to learn your acuity is to have it measured.

To find your visual acuity and get a valid contact lens prescription, book a comprehensive eye exam and contact lens fitting. If cost is a concern, your vision insurance or FSA/HSA dollars can often go toward the exam. Your eye doctor can then point you to the clearest vision for your eyes.

Once you have a current prescription, you can order the exact contact lenses it calls for online, with free shipping on every order.

Sources

[1] American Academy of Ophthalmology (aao.org) — visual acuity describes the sharpness of vision, and 20/20 is the standard for normal distance vision.

[2] MedlinePlus (medlineplus.gov) — a visual acuity test uses a Snellen eye chart, typically read from about 20 feet, to measure how clearly you see.