page layout - How do I figure out the right size font / text for a given read distance?

To avoid printing single letters at varying sizes and measuring how far away I can see it from a distance is there a formula for determining what size font for a particular read distance? Is it better to take the midpoint between the ascender line and x-height or to go purely off of the x-height (since going purely off the ascender line could leave lowercase letters difficult to read)? How does vertical height play into this? Would I need to then figure the hypotenuse distance instead of a straight distance?

For example if I'm shooting for a 20' read distance but the sign is to be placed about 30' up should I use a distance of 35' instead of 20'?

I'm shooting for a 20' read distance for the headline and then possibly a 10' read distance for additional content depending on how it all fits on the active area.

Answer

Ignoring the how old the onlooker might be, how high up, low down, indoors, artificial light or not, dark train stations, weather, is it a print sign or a screen, reflective road sign etc etc.

There are a few tools that will help you calculate this, and there are some best practices. If you really want to get into this, your keyword will be signage. Designers have fought over this for hundreds of years, font designers have designed fonts meant for signage only... the result you can see in any or all of the cities of Europe...

Here is one piece of advice from the dutch masters and their wayfinding app:

According to the designers over at designworkplan points out the most important points when choosing a font family:

• A clear and straightforward type design, sans-serif


• Easy recognizable letterforms


• Positive letter spacing to enhance the visual appearance


• The Font Family includes a package of many different weights


• The typeface has a large X-height for good readability

However, there is a quick-and-dirty calculator that might get you started, and give you some surprising results. You can find it here, at The sign chief. Below, a screenshot to demonstrate that the calculator works in three ways:

• Visibility by viewing distance


• Visibility by letter size



• Visibility by travelling speed