What is a Snellen Chart? A Complete Overview

Published on September 22nd, 2022

Updated on April 1st, 2025

If you have ever had a comprehensive eye exam with an optometrist, it’s likely that you have been asked to read from an eye examing chart, with lines of letters that decline in size. This chart is called a Snellen Chart.

Keep reading for an overview of the Snellen Chart and how it can be used to help test your eyesight.

What is a Snellen Chart?

A Snellen Chart is a type of vision chart that tests the clarity and sharpness of your eyesight, also known as visual acuity. A Snellen Chart may be used during a routine eye exam to determine how well your eyes see the details of an object at a distance of 20 feet, or 6 metres, away. The results from the Snellen test can be helpful in deciding if you need any corrective glasses.

The history of the Snellen Chart

The Snellen Chart was first created in 1862 by Dutch ophthalmologist Herman Snellen. Snellen is supposed to have created the chart at the request of his associate Dr. Franciscus Donders, who used early versions of a wall-mounted eye chart to identify vision problems.

What do the lines and letters mean on a Snellen Chart?

You may have heard of 20/20 vision, which is commonly understood to refer to clear vision or normal visual acuity. But did you know that this ratio refers to the Snellen Chart? Each line of letters on a Snellen Chart is given a ratio like this. It is an indication of how clearly you can see.

The letters on a Snellen Chart are used to help test how well your eyes can identify details in objects. You may notice that the Snellen Chart does not use all letters of the alphabet. I, J, M, Q, W, X, and Y are not often used as they have been shown to be too difficult to read at a distance of 6 metres, regardless of how good your vision is.

Snellen Chart vs LogMAR Chart

At Oscar Wylee, we use certain elements of the Snellen Chart when testing your vision. However, we also use it in conjunction with another chart that tests visual acuity, called a LogMAR chart. LogMAR charts may look very similar to Snellen Charts, but they are not the same. They use a different measurement for scoring visual acuity and are formatted differently.

LogMAR charts will have the same number of letters per line regardless of size, and the spacing between letters is usually equivalent to the letter size. LogMAR scoring will usually involve a decimal. Snellen Charts by comparison will have more letters as the sizing decreases. They also have irregular spacing between letters and are scored as a ratio.

At Oscar Wylee our eye examing charts, which are digitised, are called ETDRS charts. ETDRS charts are a revised, modified version of a Snellen Chart. Our ETDRS charts have 5 letters per line, in keeping with LogMAR, and use the Bailey Lovie layout, which has a proportional spacing between the letters.

How is a Snellen Chart used during a visual acuity test?

A Snellen Chart usually consists of 11 lines of block letters. The first row will often be one or two large letters, with each line after this consisting of different letters declining in size. When using a Snellen Chart, your optometrist will position you 20 feet, or 6 metres back from the chart. They will then ask you to read out the letters from the largest at the top, going down until you either reach the bottom or cannot see the letters clearly.

The optometrist will then proceed to determine your prescription by having you look at the chart using a set of lenses. You will test each eye individually and then together. The test should take around 5 minutes and it should not cause any discomfort. This test will be conducted by an optometrist as part of your routine eye exam, alongside other testing such as a slit lamp test.

How do you interpret the results of a visual acuity test?

After testing your visual acuity with the Snellen Chart, your results will be recorded as a ratio. The first number of the ratio represents the distance at which the letters are being read, 20 feet or 6 metres, and the second number represents the line of letters you can read. If you can see objects clearly from 20 feet away, this will be expressed as 20/20 meaning you have normal visual acuity.

If you want to find out how sharp and clear your vision is, you can book an eye exam with an independent optometrist at Oscar Wylee on our online booking page.

Can you use a Snellen Chart at home?

It is possible to use a Snellen Chart at home. There are plenty of printable tests to be found online. However, home eye exams are not reliable and can provide inaccurate results. If you need an eye exam of any kind, it is best to book an appointment with an optometrist. They will perform a visual acuity test, which may include the use of a Snellen Chart, LogMAR chart, or ETDRS chart, and interpret the results correctly.

Read What is a Snellen Chart? A Complete Overview in other Oscar Wylee regions and their languages.