Did you know that you regularly read type set in size 8, or even smaller? In printed materials, captions and less important information (think: photograph credits, newsletter headline subtext, magazine staff listings) are usually reduced to something between 7.5 to 9 points. We generally read that size type without much issue, like glasses. The reason why we can comfortably read those small sizes is because the designers chose an effective font that keeps its clarity and legibility when shrunk.

Designers don’t make the font that tiny to give you a headache. They do it to establish a font hierarchy. Our brains interpret the biggest size as the most important and the littlest size as the least important. So we can create a hierarchy of font sizes to structure our work and communicate even more clearly.

Posters need to have large titles, often as large as 150 points, which allows someone to read it from about 25 feet away. In the poster below (by João Martinho and William Faulkner), the green title is set in TheSans Extra Bold at 90pt.

Headings on a poster, such as “who are you?”, should be set in about 40-point size or larger (this poster uses 45pt). Text at this size is legible from more than 5 feet. This means conference attendees can read your research poster title from down the aisle and come in closer to examine the details. It’s a good idea to pick a sans serif font here, even though these are on paper, because serif fonts tend to fall apart, with their thinner parts getting so thin that they begin to impact legibility.

This poster also has subheadings, like “Reasonably computer literate,” which is set at 30 points.

The narrative text is this poster is size 25 point. And serif or sans serif would work here because it is pretty small. 18-point size, give or take, is common for use on the narrative portion of poster text. At that size, it can be read comfortably from about 2.5 feet away.

The tiniest print on this poster is the names and email addresses of the authors. It’s snuck up right under the title, and should be something under 18 points.

Altogether, the sizes of the text sort all the content into a hierarchy of importance. This same method works in all of our reporting mechanisms, though they don’t all have as much content on one page.

Graphs within a page need to fit into the hierarchy as well (this page comes from Anne Roux & the team at Drexel University’s report on Autism Indicators). The most important part of the graph, usually its title, should be the largest in size to draw a viewer’s attention first. Notice that the title is written like a headline with a key takeaway point. Since the graph’s title fits within the hierarchy of this page, it’s got to be smaller than the orange headings. Graph titles here are set in Arial size 11, bolded.

If you had a subtitle to your graph, it would be a point or two smaller than the title. In some cases, graph designers like to exchange a subtitle for an annotation, and they might plunk a callout box right next to a key point in a graph. These annotations should be treated the same as subtitles, in terms of the font size hierarchy.

In the case of this graph, with no descriptive subtitle, the data labels at the end of each bar fill the second position in the importance hierarchy. They are still a larger size than the bar labels, which are larger than the axis label.

The smallest text of a report is likely to be in your graph, on your Source or Note information, and it can get as small as size 9. Figure 3.19 uses sans serif fonts within the graph but your favorite narrative text serif font might be too tiny to read at 9-point size, and here is why. For the tiniest reading look for a font that has what graphic designers call a taller x-height (named, cleverly, after the size of the lowercase x). For our purposes here, the point is simply the taller the letters, the more legible. Some, such as Verdana, are also wider, which is helpful for those of us who get headaches from squinting too much. But, what works at 9-point size does not always work at larger point sizes. Check out your nearest magazine. Chances are that the small-size captions are set in a different typeface than the larger text intended for narrative reading. Which means you might need three different fonts for a well-structured report. This graph has 3 fonts in 7 different sizes.

Audiences interpret larger size as higher importance. In a hierarchy of information, largest is at the top. Varying type size communicates the organizational structure of the report and provides the reader with clues to the author’s logic.

Nerd out with me on more topics like this in my book, Presenting Data Effectively, now out in its second edition. 

I’ve detailed out the other tiny but important formatting choices you should make so your graph tells your story in the Data Visualization Checklist, now living in an interactive website. Upload your image and rate it against the critical checkpoints.

We discuss how to choose the right fonts in an upcoming tutorial over at the Evergreen Data Visualization Academy