I am not an expert with fonts, but I do have some experience ¹, and common sense. This post aims to debunk some misconceptions about font sizes!

11 px on your display is probably not 11 px on my display. Let's do some quick math. I have two displays, 1366x768 @ 21” and another with 1920x1080 @ 13”, call them A and B for now.

Display A has 1,049,088 pixels. A pixel is a square, of side say, s cm. The total area covered by my 21” display is about 1,066 cm² (41x26). Thus,

Display A
Dimensions: 1366x768 @ 21" (41x26 sq. cm)
1,049,088 s^2 = 1066
            s = 0.0318 cm (side of a pixel on Display A)

Bear with me, as I repeat the number crunching for Display B:

Display B
Dimensions: 1920x1080 @ 13" (29.5x16.5 sq. cm)
2,073,600 s^2 = 486.75
            s = 0.0153 cm (side of a pixel on Display B)

The width of a pixel on Display A is double the width of a pixel on Display B. The area occupied by a pixel on Display A is 4 times the area occupied by a pixel on Display B.

The size of a pixel varies from display to display!

A 5x11 bitmap font on Display A would be around 4 mm tall whereas the same bitmap font on Display B would be around 1.9 mm tall. A 11 px tall character on B is visually equivalent to a 5 px character on A. When you view a screenshot of Display A on Display B, the contents are shrunk down by a factor of 2!

So screen resolution is not enough, how else do we measure size? Pixel Density! Keen readers will realize that the 5^th grade math problem we solved up there showcases pixel density, or, pixels per cm (PPCM). Usually we deal with pixels per inch (PPI).

Note: PPI is not to be confused with DPI ² (dots per inch). DPI is defined for printers.

In our example, A is a 75 ppi display and B is around 165 ppi ³. A low ppi display appears to be ‘pixelated’, because the pixels are more prominent, much like Display A. A higher ppi usually means you can view larger images and render crispier fonts. The average desktop display can stuff 100-200 pixels per inch. Smart phones usually fall into the 400-600 ppi (XXXHDPI) category. The human eye fails to differentiate detail past 300 ppi.

So … streaming an 8K video on a 60” TV provides the same clarity as a HD video on a smart phone?

Absolutely. Well, clarity is subjective, but the amount of detail you can discern on mobile displays has always been limited. Salty consumers of the Xperia 1 ⁴ will say otherwise.

Maybe I will talk about font rendering in another post, but thats all for now. Don't judge a font size by its screenshot.