So, how long should alt text be? The general consensus here is that there is no hard limit, but more of a contextual awareness of what purpose the image serves and adapting to it accordingly.

— Just How Long Should Alt Text Be? (CSS-Tricks)

As I was revamping my website, I took the chance to make sure all images on my site now have an alt tag. This was not an easy exercise and I find myself getting stuck naming a lot of the images. As much as possible I just try to type one single sentence that pops up in my mind when I see the image.

The HTML specification doesn’t outright define a maximum length for “alt” attributes. Current versions of the leading screen reader programs have no limits on the amount of alternate text they will read. However, there are at least two good reasons to keep alt text “short and sweet”.

First, if alt text is too lengthy, it can interrupt the content reading flow for screen reader users unnecessarily. Alt text should provide equivalent access to the content of an image, but must do so efficiently in order to avoid burdening users with extraneous information.

Second, most browsers display alt text visually if the image fails to display. It will also happen on mobile devices if the user has disabled images in order to conserve bandwidth and keep costs down. Although nowadays data is abundant enough that most people have forgotten this feature even existed. Alt text should therefore be short enough to reasonably fit within the space allocated for the image.

Essentially, alt text should be as long as it needs to be in order to effectively describe the content, but should be succinct.