Image alt text purposeWhen looking at making sure your image alt text works for your SEO purposes, don’t forget that one of the main purposes of alt text is to provide visually impaired people a description of the content they’re looking at. It can be easy to get carried away filling it up with SEO-oriented keywords, leaving it useless for screen readers and affecting the readability of your website for those who rely on screen readers. So, that said… When looking at your image alt text for SEO purposes, is longer better? Not necessarily!
Current thinking regarding alt textAccording to here, here and here below 125 characters is good – probably 10-15 words is a sweet spot to aim for. Beyond that doesn’t seem to provide much usefulness for SEO.
For visually impaired readers, however, there may be a benefit from having longer alt text component to the image – something to keep in mind when looking at your content. From the Penn State University link it seems that some screen readers break the text up into 125-character chunks, though, so longer than that limit may not work well anyway.
Images that aren’t part of contentIf you don’t want to put alt text in for every image or have on-page images that aren’t part of the content, at least put in an empty entry – e.g.
img src=”blah.jpg” alt=””That way screen readers will skip it and not say “image”.