What Is the Snellen Eye Chart?

The 'Big E' eye chart tests vision acuity

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The Snellen eye chart is the classic chart of big and little letters you'll typically see used by eye care professionals, in schools, and in other settings where vision testing is done. Its name comes from Dr. Hermann Snellen, who invented the eye chart in 1862.

The eye chart is used to measure visual acuity, which refers to how well you can see without glasses or contacts. The results are used to determine if you need eyewear to correct your vision. It's part of a general assessment of your overall eye health.

This article explains how the eye chart works and what some benchmark results may mean. It adds information about modified eye charts that, for example, may be used with children who don't yet read letters.

Silver ballpoint pen pointing to letter in eyesight check table
megaflopp / Getty Images

The Standard Eye Chart

The Snellen eye chart is made of letters used as "optotypes," or symbols designed for vision testing.

This standard chart consists of 11 lines of block letters, beginning with a large single letter on the top row. The number of letters on each row increases moving from top to bottom.

The size of the letters progressively decreases, allowing for more letters on each subsequent line.

The traditional Snellen eye chart makes use of only the following letters: C, D, E, F, L, O, P, T, and Z.

Visual Acuity Testing

The Snellen eye chart is read by covering one eye and reading aloud the letters on the chart, beginning at the top and moving toward the bottom. The smallest row of letters that the patient reads accurately determines visual acuity in the uncovered eye.

The other eye also is tested, and both eyes are tested together.

The results are based on what you can see from 20 feet away compared to what someone with normal eyesight would see. It's why normal human eyesight is sometimes expressed as 20/20.

Your results are determined by the smallest line of letters you can accurately read on the chart.

What Is the 20/20 Line on an Eye Chart?

It seems like the answer should be straightforward, but the 20/20 line on an eye chart may vary. A standard Snellen eye chart example used by the American Academy of Ophthalmology puts 20/20 at Line 7, which is read by about 35% of all adults without correction. Be sure to ask your eye care professional which line on the chart you're using represents 20/20 vision.

How to Read Eye Chart Results

The results of your acuity exam (chart reading) will help to diagnose the quality of your eyesight. Your vision results (acuity) will be expressed as a fraction. Fractions are shown to the left of each row and identify how well you can see.

The expression of results, such as 20/40 vision or 20/25 vision, will depend on how far down you were able to read the lines on a specific chart.

Eye Chart Distance

The results of an eye chart exam are based on a reading distance of 20 feet. To interpret your reading, it helps to think of the numbers in feet. For instance, someone with 20/60 vision can read at 20 feet away what a person with normal vision could read at 60 feet away.

Modern optometric exam rooms are equipped with electronic eye reading charts, making it easier to produce accurate readings.

In the United States, a score of 20/40 or better is required to drive a car without the use of corrective lenses. A vision score of 20/200 (or worse) determines that an individual is legally blind.

Eye Chart Versions

There are several versions of the Snellen eye chart for people, such as young children, who cannot read the letters of the alphabet. 

Some of these special charts have pictures to identify in lieu of alphabet letters or numbers. The alternatives include three types of eye charts:

  • The tumbling E chart uses only the "E" letter in different positions. People can point to show they can see its direction.
  • The LEA test uses shapes (like an apple and square) instead of letters to test vision in very young children.
  • The Jaeger chart is used to evaluate near vision.

A Word From Verywell

You can access versions of the Snellen eye test chart online, but use caution with them. It can be hard to get accurate results with so many variables, such as monitor size and screen resolution. That's true of a printable eye chart, too. It's best to have regular examinations performed by an eye care professional. 

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Marsden J, Stevens S, Ebri A. How to measure distance visual acuity. Community Eye Health. 2014;27(85):16. PMID: 24966459

  2. American Academy of Ophthalmology. What Does 20/20 Vision Mean?

  3. Yeung WK, Dawes P, Pye A, et al. eHealth tools for the self-testing of visual acuity: a scoping reviewNature Partner Journals. Aug 2019;2(82):1-6. doi:10.1038/s41746-019-0154-5

By Troy Bedinghaus, OD
Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association.