There’s a random myth that flies around in Accessibility that alt text is limited to 100, 150 or 200 characters long. It seems to be one of those things that’s been cited in multiple places but…this isn’t true. Screen readers, often used by people who are blind or have low vision, will read the alt text to describe the image, making web content accessible.
There is no hard limit or rule for alt text, no threshold and no advantage linked to those numbers. It’s a bit unclear where this has come from and been cited in many places, perpetrated as fact. The ideal length is determined by context, purpose, and content. It should be as long as it needs to be to clearly convey the image’s meaning.
TL;DR
There’s no limit, Alt text should be as long or short as it needs to be, just succinct and correct. Just not super long.
Why is this an issue?
The myth of a strict alt text character limit may force alt text writers to cut crucial details. For screen reader users, alt text is the image content. Arbitrarily restricting it may leave users with incomplete context and impacts the core principle of equal access.
It ends up in code, input fields are restricted
The big issue I see with this is the way alt text input fields are coded by developers, an input field might be restricted to 200 characters or less, not allowing someone to give enough information to the user. A CMS or social platform might be arbitrarily restricted. Content teams might again restrict the length when more information would be useful.
Canva is one example of a platform that has now embedded this myth as 250 alt text limit. Why?
Testing tools flag this myth as an error
I was also reading about an accessibility testing tool flagged over 100 characters as an an error, when it’s not. So inadvertently this myth has found itself embedded into systems already, making it harder to shake.
Large language models and misinformation
As more people new to accessibility rely on large language models (LLMs), this myth is amplified. If an LLM was trained on multiple sources repeating the false limit, it will present it as fact, further entrenching the misinformation without proper fact-checking.
Ok, but where’s your evidence?
This was written about before in a great post called by Eric Eggert “There is no character limit for “alt text” . Within this post was screen reader testing by Terrill Thompson demonstrating that screen readers can effectively read long alt text, debunking this myth from a technical point-of-view with testing.
Additionally, Christophe Strobbe provide very detailed insight with various accessibility books that also support no alt text limit.
- Constructing Accessible Web Sites by Jim Thatcher et al (Glasshaus, 2002): no character limit is mentioned; just advice to keep alt text short (page 77).
- Maximum Accessibility by John Slatin and Sharron Rush (Addison-Wesley, 2003): no character limit is mentioned (pages 249–250).
- Building Accessible Websites by Joe Clark (New Riders, 2003): page 63 (emphasis mine): Keep the alt text short. There is no set limit on the length of an alt text, but as we shall see shortly, a very long alt may not be fully displayed when image-loading is turned off or when the browser cannot locate the image file. By convention, limit alt texts to 1,024 characters (1 K) or less.
- Accessible XHTML and CSS Web Sites by Jon Duckett (Wrox / Wiley, 2005): nothing about keeping alt text concise, let alone a character limit (pages 259–260).
- Web Accessibility: Web Standards and Regulatory Compliance by Jim Thatcher et al (Friends of ED, 2006): no character limit is mentioned; just advice to keep alt text short (pages 136, 141).
- Access by Design by Sarah Horton (New Riders, 2006): no character limit is mentioned (pages 78–80). Design Accessible Websites by Jeremy J. Sydik (The Pragmatic Programmers, 2007): page 159: “Most of the time, the alt text should be no more than 40–80 characters or a couple of seconds when spoken.” This does not sound like a hard character limit.
- Web Accessibility: Practical Advice for the Library and Information Professional, edited by Jenny Craven (Facet Publishing, 2008): nothing on alt text length (page 33). Universal Design for Web Applications by Wendy Chisholm and Matt May (O’Reilly, 2009): no character limit is mentioned (pages 26–29).
- Barrierefreiheit verstehen und umsetzten by Jan Eric Hellbusch and Kerstin Probiesch (dpunkt.verlag, 2011): no character limit is mentioned (pages 157–158, 379–380, 389).
- Accessibilité Web. Normes et bonnes pratiques pour des sites plus accessibles by Armony Altinier (Eyrolles, 2012): no character limit is mentioned on pages 90–92, but page 96 mentions that the now defunct AccessiWeb guide “recommended” at most 80 characters as a guideline, i.e. without defining this as a hard limit. However, page 97 says that if more than 80 characters are needed, the image requires a long description.
NASA? What have they got to do with alt text?
In 2022 NASA released some images from the James Webb Space Telescope and the alt text of these images became the unexpected star of the show, the alt text on some of these images were around 500 characters, giving users information they need and receiving some really public positive feedback and press.
Takeaway
Alt text should be as long or short as it needs to be, just succinct and correct. Alt text is not about length. It’s about purpose and context, describing what an image shows and why it matters. Some images only need a few words, others deserve more detail.
Reading list
- For further discussions and community consensus, see the ongoing WCAG discussions.
- An alt Decision Tree
- Long alt text