Georgia Tech Research Institute
 

Font Size Calculations

For 20/20 vision, the Human Factors Design Standard (HFDS) recommends that the height of characters occupy a visual angle of 16 to 24 minutes of arc. To compute the character height, use the following formula:

h = 2dTan(x/2)

where h is the character height, d is the viewing distance, and x is the desired visual angle in radians. (One radian equals 3437.747 arc minutes; divide the visual angle in arc minutes by 3437.747 to obtain the visual angle in radians.)

1194.31(b) of Section 508 states that products should be accessible to users with 20/70 visual acuity. Multiplying the character height (h) calculated for 20/20 vision by 3.5 (70/20) yields the recommended character height for users with 20/70 vision for the specified viewing distance.