OOH glossary · Operations
Legibility
Legibility is how easily drivers can read a billboard, set by letter size, contrast, word count and distance.
A board only works if people can read it in time. Legibility depends on letter height, font, color contrast and how many words you use.
Sign research gives a rule of thumb. Plan about 1 inch of letter height for every 30 to 40 feet (Bullough, 2017 review).
All caps read worse than mixed case. One study in that review found all-caps letters must be about 15% taller to read as well.
Formula
Minimum letter height (inches) = viewing distance (feet) ÷ 30 to 40
Example
Drivers can first read B-114 N from about 500 feet. Letters should be at least 500 ÷ 40 = 12.5 inches tall. About 17 inches (500 ÷ 30) is safer.
In Gabie
Studio’s artwork review flags images under 25 PPI at full size and text under 1 inch. It also flags more than 8 words, as advice only.