text - What is the best number of paragraph width for readability?

Since my current project has a lot of text/paragraph content, I consider about the readability of the website.

I've seen and try to inspect any of sites that I feel have a good readability such as medium.com, Quora.com, and so on.

Their width of the paragraph always between 500-700px.

My question is, what is the best number of paragraph width for readability?especially on desktop view.

Answer

According to studies, the line length should not exceed 70 characters. So keep your paragraph width between 50 and 70 characters.

So actually you should not care about the width in pixels, but rather the width in ems (The width relative to the font-size). So go for 30-50em.

Also wikipedia says:

Some studies have shown that 100 cpl can be read faster than lines with 25 characters, but the level of comprehension remains the same. In order for on-screen text to have both the best speed and comprehension possible about 55 cpl should be used. Like with printed text if lines are too long or too short it will result in slower reading. If lines are too long it is difficult for the reader to quickly return to the start of the next line (saccade) whereas if lines are too short more scrolling or paging will be required. Research suggests that longer lines are better for quick scanning, but shorter lines are better for accuracy. Longer lines should be used when the information will likely be scanned, while shorter lines should be used when the information is meant to be read thoroughly. Web design is often intended to be read in full rather than skimmed, so shorter lines should be used when possible.

See: https://www.researchgate.net/publication/234578707_Optimal_Line_Length_in_Reading--A_Literature_Review