Font Size Calculator
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.
Gerben's thoughts
Brillant! Simple!! And just what I needed!!
John's thoughts
Remember that a lot of users are going to be wearing older or no glasses and as such be slightly below 20/20. Also, just because a person with 20/20 eyesight can read something doesn’t mean that they wouldn’t be more comfortable reading something twice the size.
Steve Collier's thoughts
This seems only to take into account visual acuity i.e. 1 minute of visual angle, times the feature separation in a typical font letter E i.e. x 5 giving 5 minutes minimum height. In fact, other guidelines recommend lager fonts. e.g. 15 minutes, or more. See for example NUREG 0700. Also we could use an indication of the conversion of point size to inches.
Patrick James's thoughts
Your trigonometry is strange, very strange, yet you did send a man to the moon! Minimum Height, x = d. tan a, thus for a = 1 minute of angle (1/60 degree) x = distance x 0.00029, For font size equal to five times Minimum Height formula becomes Font = distance x 0.00029 x 5 = 0.00145 x distance. Although the end result is about the same how you got there is important. Using the calculator the results are about twice what they should be. You state that The font calculator above takes some of these issues into account. To almost double the results seems to be excessive.
Of course I could have made a horrible mess of teh claculations!
Patrick
pchang's thoughts
Hi Patrick. Pretty sure the trig is right. So, not sure which part of the calculation you’re referring to. And regarding other issues, we use published data to take lighting and reliability into account. We’re not claiming this is the answer, only a guideline. So, hopefully you’ll find it useful as one data point in your design process. Thanks for the feedback.