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

Question

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 1

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

Answer 2

Please look at these resources

Distance Legibility Chart

additional explanation

crazy resource

also, you probably want to learn what the visual acuity is...

If you want to calculate the proper 20/20 letter size based on distance you can use this formula:

tan(5 minutes) = distance in feet/20

but it is not the BEST letter size, only the size that a person with 20/20 vision can clearly see, thus you should not use a letter size smaller than the result of this formula.

I paste here some reference for the case the main link is broken -

(distance -> font size)

1.2 feet (0.35m) -> 8 points
This is the typical reading distance for a book. Most people prefer text to be 10, 11, or 12 points at this distance.

2.4 feet (0.7m) -> 16 points
This is the closest comfortable distance for reading a large poster.

5.0 feet (1.5m) -> 32 points
In many settings this is as close as one can get to a poster. Sometimes this is because the poster is roped off, or in other cases, large crowds simply make close approach difficult.

25.0 feet (7.5m) -> 160 points For almost any setting, you want a title that can be read from at least this far away.

Answer 3

There are too many variables for one answer.

The first thing to evaluate is the typeface. If you are using a face designed for signage, the general references Ilan provided are probably roughly accurate. On the other hand, if you're working on a branded piece where the typography is part of a larger brand standard, you'll have to do your own research.

The x-height is the best indicator of legibility. Following that, ascenders contribute more to legibility than descenders due to their greater usefulness in character differentiation.

I've posted some information on legibility/readability in another answer that might be helpful in this case.

Next you have to define your viewing conditions. You're on the right track with distance. Don't forget viewing angle, movement, and environmental conditions.

Legibility of narrow width fonts will drop off much more quickly when viewed at an angle. Stroke contrast and weight effect the importance of lighting conditions. The relative complexity of the characters will determine how well read they are when moving.

Define your own formula.

Make a scale model and try it out. Take into account as many of the variables as possible. Figure out the right size at a given distance then apply a ratio to find the right size.

If your font and viewing conditions work at 16pt at 4 ft, you have a ratio of 4:1 which means you multiply the distance by 4 to get the right size. It's a relatively simplistic experiment but you'll get the desired result with more confidence than relying on generalized calculations.