There are several tactics for deciding where to put breakpoints in a responsive design. There is the rusty idea that they should be based on common screen sizes, but this doesn’t scale well. There are no “common” screen sizes. Another popular tactic is to create a breakpoint wherever the layout breaks.
This sounds much better. But it still leaves us with the question, How do you determine whether the layout is broken? One logical answer is to look at classic readability theory and to define our breakpoints based on that.
According to Robert Bringhurst, “Anything from 45 to 75 characters is widely regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size.” And Josef Müller-Brockmann writes that, “A column is easy to read if it’s wide enough to accommodate an average of 10 words per line.” A few variables determine the exact number of characters or words, but this is the basic theory: If you start with a small screen and you grow it, every time the width of the main content grows wider than either 75 characters or 10 words, something should happen. Simply said, these are your breakpoints.