What Is the Snellen Chart?

The Snellen eye chart is used to test your visual acuity, which is how clearly and sharply you can see without glasses or contact lenses. It’s the most common eye chart used by eye doctors, in schools and in other places where eye exams and vision screenings are done.

The Snellen eye chart has 11 lines of block capital letters, also called optotypes. The largest letter is an “E” on the first line. As your eyes move down the chart, the number of letters on each line increases as the size of the letters gets smaller.

The letters C, D, E, F, L, O, P, T and Z are the only ones used on the common Snellen chart. Some letters aren’t used because your brain can easily identify the letter even if it looks blurry.

An example is the letter Q. No other capital letter has a tail like a Q. Even if your eyes can’t clearly see it’s a Q, your brain will know which letter it is if it sees the shadow of the tail. Recognizing letters on an eye chart that you can’t see clearly could make it appear that your vision is better than it actually is.

History of the Snellen Eye Chart

Dutch eye doctor Herman Snellen created the chart in 1862 for his colleague Dr. Franciscus Donders. Dr. Donders conducted eye exams by having patients look at the chart on a wall and tell him what they could see.

Before Snellen created his chart, eye doctors used their own charts to test their patients’ vision. Some charts included pictures of common objects, such as a house or flower, and others used words.

Being able to see clearly was critical when Snellen created his standardized eye chart. The industrial revolution was changing the world and workers needed good vision to operate machinery.

The Snellen eye chart is still used today to measure visual acuity.

How the Snellen Chart is Used for Visual Acuity Testing

During your eye exam, your eye doctor will have you read the Snellen chart to help determine how clearly and sharply you can see. You will:

• Sit or stand 20 feet away from the chart.
• Cover one eye.
• Read aloud the smallest line of letters you can see clearly.
• Cover the other eye and follow the same steps.

You can test your vision at home by printing your own Snellen eye chart. A home test is convenient, but it is not a substitute for a comprehensive eye exam. Seeing your eye doctor is the best way to make sure your eyes are healthy and determine if you need glasses or other vision correction to improve your eyesight.

Understanding the Results

After testing your vision with the Snellen chart, your visual acuity will be given as a fraction such as 20/20 or 20/40. The smallest row of letters that you can read clearly and correctly determines your visual acuity. These fractions are listed on the left side of the chart.

If you have 20/20 vision, that means you can comfortably read the row of letters on the eye chart from 20 feet away that the average person should be able to read at that distance. In the United States, 20/20 is widely considered to be “normal” eyesight.

About 30% of American adults have 20/20 vision without any kind of vision correction such as glasses or contacts. Approximately 75% have 20/20 vision with vision correction.

If your results are 20/30, 20/40, 20/50 or higher, that means you have to be no more than 20 feet away to see something a person with normal vision can see at 30, 40 or 50 feet away. The higher bottom number shows that your vision is weaker than normal and that you may need glasses, contacts or other vision correction to help you see more clearly.

Although rare, it is possible to have vision better than 20/20. For example, 20/10 vision means that you can see clearly from 20 feet what a person with “normal” vision sees well from 10 feet away.

Here are some interesting things to know about visual acuity measurements:

• Most young people with healthy eyes have 20/15 vision and sometimes even a bit better.
• 20/40 vision or better — natural or with vision correction — is what’s required by most states to get an unrestricted driver’s license.
• People with 20/200 vision or lower can only read the single large letter at the top of a Snellen chart without vision correction. They are considered legally blind if their vision can’t be improved beyond 20/200 in their better eye, even with vision correction.

A Snellen chart is just one tool your eye doctor uses to determine your visual clarity and what type of vision correction you may need, if any. If you’re having any vision problems, make an appointment with your eye doctor. They can give you a comprehensive eye exam to ensure your eyes are healthy and that you’re seeing as clearly as possible.