Modular scale
How exactly do we decide on what a big heading should be? Is it 3em, or 3.5em? We usually decide on these arbitrarily based on whatever seems to work right. Doing it this way can cause a lot of inconsistencies. You may be using 3.2em at some place, but 3.3em at another.
Instead, set up your font sizes as relative percentages of other sizes. This is called a modular scale.
Why percentages?
Some designers rely on linear increments of font size. This isn't an ideal, and here's an example of why.
Modular scale
This system on relying on percentages is called a modular scale. Using the formula ms(n) = base × ration, you will be able to generate a list of font sizes you can use throughout your project.
In this example below, we use 12pt, as the base font size, and 1.2 as the ratio.
Also see
This article was adapted from Rico's design notes.
• Rico's design notes (slack.com - private)
• Modularscale.com (modularscale.com) — Modular scale calculator
• modularscale-sass (github.com) — Modular scale implementation in Sass