Designing signs for legibility: Letter size

By Ken Millar

Posted on Wednesday, February 10th, 2021

To sell signs effectively, we must know what it takes to create a sign that works—a sign that is both attractive and readable in the setting that it is used. The first part, coming up with an interesting design, is usually easier to do than the second part, which is “How do you make sure it will be readable on site?”

Today, when everyone can choose fonts and clipart and do fades and shadows and distort letters with a click here and a click there, clients may not place value in the design alone. But if you can show them that you know how to make sure their sign can be read and deliver their message, then you’ve got something to sell. It will separate you from the competition. When you can explain and discuss these issues with clients, they’ll often realize they’re dealing with a professional who has their best interest in mind.

Why signage fails

We can learn a lot about creating successful sign layouts by considering what causes a sign to fail to deliver its message to potential readers. By learning the common pitfalls we may be able to avoid them in our layouts.

I find that when signage is not readable, it’s usually because of one or more of the following reasons:

  1. Too much copy or wording
  2. Poor design or layout
  3. Bad choice of colors
  4. Letter style is not legible from the reading distance

Another possible reason is when there are too many signs concentrated in the area. There is nothing the sign designer can do about this—except to make the sign he or she is designing the most legible and appealing sign in the crowd.

Traditionally, advertising design houses stuck to the golden rule of “a trademark plus seven words or less” for design for painted bulletins and billboards. This rule is hard to apply in a sign shop. We may not have the luxury of eliminating unnecessary wording. But nonetheless, the copy must be managed in the layout or readability will suffer.

Guidelines for letter readability

Most exterior signs are read from a considerable distance. On top of this distance issue, the reader is often in a moving vehicle. There are physical limitations to what the viewer is capable of reading at that distance and in the time they have to read it. A wise designer plans for this.

Over the years, charts have been published showing the readable distance for various letter heights for maximum impact. These are tools to help designers determine what size letters should be used on a sign given its viewing distance.

When applying for a driver’s license, your eyes are tested for the ability to distinguish letters and numerals at various distances, based on a minimum acceptable vision of 20/40. With 20/40 vision, you are able to read the following size letters at these distances, assuming they’re black block letters on a white background:

There are some important things to remember about letter legibility when considering charts like these:

  • Remember that the readable distance will vary the style of lettering and stylizing of individual letters. Some letter styles, like Helvetica, are highly legible while others, like script styles, are less legible.
  • Color contrast has a huge impact on letter legibility. Charts like these assume strong contrast between the letter and background, usually black on white. As color contrast decreases (say tan on pale yellow) so does legibility.
  • Letter weight matters. Compared to a lighter weight letter, a bolder letter is often readable at a lesser height. So if 6-in. Futura Medium lettering is readable from a certain distance, you may find that 4-in. Futura Extra Bold is also readable from that same distance.
  • Readability may be improved by opening the interior portion of lettering by extending the letters horizontally or vertically. Designers of roadside bulletins and billboards often do this:

Get real-life knowledge

Have you ever measured off the distance between a sign and the place where it is typically read from to see how much of the copy was actually legible from that distance? It’s a great exercise—and essential if you want to design effective signs and be able to explain your designs to your clients.

Choose a sign—say a storefront or freestanding sign that must be read from the street—then measure the viewing distance and the size of the lettering. Is all the copy legible from the typical reading distance? This will give you firsthand knowledge of how letter size and reading distance factor into legibility.

Viewing time is a key factor

Along with letter size, we also must consider how long the viewer has to read the lettering. That is actually a topic of its own, but here is a chart that shows the viewing time at various speeds and the letter size that can be read at those distances:

It’s easy to see how things get more complicated when the viewer is in motion. When the person who is passing a storefront sign in a car going 45 mph is 330 feet away, he or she can read a 12-in. letter. They will be five seconds from the sign.

Two seconds later they will be 198 feet from the sign and will be able to read an 8-in. letter. In three more seconds they will be past the sign with no chance of reading it.

Will your design be able to deliver its essential message in those few seconds?

The experience gained from studying successful, effective signs is priceless to a designer. Coupled with distance legibility charts it gives you the basis for making sure your sign designs work in their environment. And that’s really our job.

Ken Millar spent many years as a sign designer in the Chicagoland area and was also the instructor at the International Brotherhood of Painters and Allied Trades sign school in Chicago.

SignCraft, January/February 2011