People choose font sizes every day, yet seldom have a good reason for their choices. Appropriate font size will vary with color, font face, and other paragraph formatting considerations. These factors make proper font-size selection difficult. However, a good rule-of-thumb relies on the acuity of the eye as the basis for font-size selection.

A person with 20/20 vision (or corrected vision) can distinguish approximately 1′ (1/60 degree) of angular spread from the eye. Based on this value, a standard eye chart is designed such that the arms of an E (see measure x in the figure below) correspond to 1′ of angular spread from the subject’s eye. From this angular measurement, we can calculate the proper font size for a given distance.

Let’s use these facts to derive a rule-of-thumb for choosing proper font size. The value x can be related to distance (d) and angular spread (a) by trigonometry:

x = 2dtan(a/2)

Substituting 1′ (1/60 degree) for a gives us the minimum value for x:

x(min) = 2dtan(1/120) = 0.0003d

The minimum height of the “E” should then be 5 times x(min):

E(min height) = 5x(min) = 0.0015d

The following graphs show values for the minimum height of E. Note that this is a minimum value and that the designer would want to consider vision impairment, lighting conditions, and other environmental considerations. The font calculator above takes some of these issues into account.