Calculating Optimum Sign Legibility

Use a formula to make sure your sign will be as legible as you want it.

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Stand back … way back, way-way back and look at your design on the monitor. What can you see? Can your eyes pick out the letters and recognize them? Can you read the message on the sign? Will it stand out in a sea of visual clutter in the real world? In other words, is it legible?

Designing monument signs is a balancing act of blending parameters, colors, objectives and graphics into a monolithic mass of aluminum and electricity whose only function is to be noticed, to inform and to bring in revenues for the one who paid for it. In discussing the effectiveness of a sign, we must address three factors that dominate the design process: visibility, legibility and readability. They are defined as follows:

–          Visibility-The purpose of having a sign is so the intended audience “notices” the sign and reads the message. Why do you think truck stops along the interstate use 100′ tall pylon signs with 8′ tall gas price numbers? Visibility is about being seen: Can you see the sign from the highway, yes or no? There are at least four factors that affect visibility: shape, size, color and location. An effective sign is one that makes the best use of all four of these factors based on the environmental conditions at hand.

–          Legibility-In the sign industry, this means, “Can you tell if that is a capital D or a lowercase P from 300 feet away?” Does the font have a unique style that makes it hard to recognize? If so, you may want to reconsider your options.

–          Readability-Have you ever looked at a sign with a lot of copy and felt your eyes strain while reading it? If your eyes strain, the designer either used the wrong font, the wrong size of letters or incorrect spacing and kerning, conflicting colors … the list goes on and on.

Calculating Optimum Legibility

If your sign is 35′ tall and you need it to be legible and readable at 300 feet, what formula do you use to calculate letter height? Calculating this is fairly complex and involves the use of a left-handed abacus, however an easy shortcut is to simply add a zero to the letter height (in inches) and convert the number to feet. Follow this example:

–          Start with your letter height of 6″, then add a zero to it and change the measurement from inches to feet. That means a 6″ tall letter would have an optimum legible distance of 60′. Letter 4″ tall are best read at 40′ and 12″ tall letters are best viewed at 120′, and so on.

–          To have optimum legibility at 300 feet, the letter height needs to be at least 30″ tall. Many of the digital display and plastic letter companies offer useful charts that show letter height readability distances for letters from 1″ all the way to 60″ tall. Posting one of these charts near your desk is always a good measure

Matt Charboneau owns and operates Charboneau Signs of Drake, Colo., an over-the-Web sign design service. He can be reached through his Web site at or

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Tony Kindelspire

Tony Kindelspire is digital content editor of Sign & Digital Graphics & WRAPS magazines.

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