There is no character limit for “alt text”
When you search for alternative text length on the internet, there seems to be a pretty common understanding that “some screen readers” truncate alternative text1 after 125 characters:
The character limit for alt text is 125 characters, which is short, but just do the best you can.
Keep your alt text fewer than 125 characters. Screen-reading tools typically stop reading alt text at this point, cutting off long-winded alt text at awkward moments when verbalizing this description for the visually impaired.
Terrill Thompson did create a test page for it and documented his results: No screen reader cut off text altogether. JAWS – and I have tested this with a more recent version of it as well – will split up the alternative text into multiple graphics with shorter alternative text.
Screen readers do not just hide the rest of the text.
Additionally, I am pretty sure that the cut-off mentioned in Firefox does not exist anymore. At least I did not come across any truncated alternative texts.
My best guess is that some people have observed JAWS’ behavior and assumed that the text was truncated where it was only split up.
Alternative text best practices
I helped write a whole tutorial on images, so I don’t want to go into the details here, but the general rules are:
- If an image meaning or shows something, it needs alternative text.
- Alternative text depends on the context and usage of the image.
- Describe the image as succinct as possible, but also as detailed as needed.
Writing alternative text is an art more than a science, and sometimes you’ll leave out essential information. My litmus test is “would I picture an approximation of the image when it was described to me over the phone using the alternative text.”
Note that many people use the phrase “alt text”, because the “traditional” way to use alternative text is through the
altattribute in HTML. These days, alternative text for images can come in different methods, including ARIA labels and descriptions or in some cases SVG titles. ↩
Comments & Webmentions
Sure, currently there is no limit. However there was a time when that was true. But you forget one thing... There is no way to pause alt-text. Your only choice is to stop and refocus to listen to it all over again. VERY frustrating user experience. 2-3 sentences describing the main takeaway still remains best practice.
I did not forget about it, the post explicitly says to be “as succinct as possible”.
People have indicated length to help guide in writing succinct alternative text. Alt text can't be structured and if you really need to write more than a sentence or two it needs to be text that can be read visually for people with low vision. who may miss small details.
See also the link to the images tutorial I helped to write. This article does not aim to cover everything alternative text.
My understanding was not that 125 characters was a hard limit imposed by screen readers. Rather, that keeping alt text to about 125 characters is a suggested best practice based on screen readers' inability to navigate forward/backward or pause and resume midway through alt text.
I came across several people who indicated there would be a hard limit over the last few weeks, and so I thought I’d write it up. Details on how to write alternative text is linked in the article above.