What's the Evidence? Alt text should be half a tweet long, they say

May 2019

How would you know what an image was if you couldn’t see it? Like this one. You’d need someone to describe it to you.

Sculptures of Hairy Maclary and friends. He and four dogs are barking at cat Scarface Claw, crouched high up a post. Dog Hercules Morse is snoozing. Cat Slinky Malinky is sneaking along.

Figure 1: Hairy Maclary and friends on Tauranga’s strand. Take a selfie with them when you’re in town for the conference in October – they’re only a stone’s throw away! Photo by Earnsy Liu.

That’s what alternative text (alt text) is: a text description of an image. It’s not a caption. Alt text can be attached to the image behind the scenes (in the HTML alt attribute, which readers won’t see) or in the adjacent text (WebAIM, n.d.). It makes information accessible to everyone, including those who are blind or visually impaired, and anyone whose device doesn’t display images properly. Alt text helps with search engine optimisation (SEO) too, since key words can make pages more findable for search engines.

Let’s look at the useful length of alt text to people, not search engines. Because technology changes at breakneck speed, we’ll focus on literature from the last five years or so, looking at:

  • advice to keep alt text short
  • technical reasons for short alt text
  • lack of user experience findings
  • advice on describing complex images
  • user feedback on alt text for complex images
  • how much we should write.

Keep alt text short

A blue bird standing on a black iPhone, cupping its left wing to its mouth as if it were calling out.

Figure 2: Alt text should be half as long as a tweet. Image by Dsndrn-Videolar.

Writing alt text can be hard work! An unpublished master’s thesis found novices can take 15 minutes to describe an image (Maggard, 2008, cited in Tomashek, 2013).

There are plenty of tips on writing alt text, perhaps too many: ‘a lack of accurate and standardised guidelines … has resulted in many sets of recommendations published by different bodies’. The result? A ‘chaotic’ situation and ‘endless discussions’, including on the length of alt text (Vázquez, 2016, p. 34).

True enough, I found varied advice on length alone, such as:

  • 100 characters (University of Illinois, n.d.; WebAIM, 2011)
  • 125 characters (Pennsylvania State University, n.d.; University of Washington, n.d.)
  • 5–15 words (Kyrnin, 2018)
  • a few words. Web accessibility experts WebAIM tells us: ‘Typically no more than a few words are necessary, though rarely a short sentence or two may be appropriate’ (WebAIM, n.d.).

This sounds like alt text should be about half as long as a tweet, which can be up to 280 characters (‘Twitter’, n.d.). But is there evidence this is the right length for users?

Technical reasons for short alt text

Alternative text was developed for technical reasons (Tomashek, 2013), so it wasn’t surprising that some advice had technical grounds, such as SEO, a screen reader’s process, and page speed.

Four miniature figures doing construction work on an uncovered hard drive. They are wearing blue overalls and yellow hard hats.

Figure 3: Much of the advice on alt text length seem to have technical rather than user experience reasons. Image by Wilfried Pohnke.


Google seems to read only the first 16 words of alt text when it searches. We know because an SEO consultant conducted a simple experiment. He made up a word and added a running number to it: ukhobo1, ukhobo2, ukhobo3, and so on, then stuck the lot into alt text. When he googled the words, Google only found words up to ukhobo16 (Anderson, 2018).

A screen reader’s process

The screen reader Jaws reads alt text as separate 125-character chunks, ‘as if they were separate graphics’ (University of Washington, n.d.). This can be confusing, and suggests we should keep alt text to 125 characters.

It’s not clear if this limitation is still true, but I mention it to be safe. Jaws is a popular tool: a 2017 survey of screen reader users found nearly half used it as their primary screen reader, and two thirds used it often (WebAIM, 2017).

Download speed

It seems short alt text ‘keeps your pages smaller and smaller pages download faster’ (Kyrnin, 2018). Hmmm, perhaps, but does it make a significant difference? Only one article mentioned this, and without evidence. Tech expert Dave Gash didn’t mention alt text when discussing how to speed up web pages in his Page speed basicswebinar (Gash, 2018).

Hardly any user experience findings

For Pete’s sake, we’re technical communicators — user advocates. I want to know what people like. Show me the evidence! I googled and dug and hunted and found … precious little.

Our very own accessibility expert, Kevin Prince, said, ‘It’s something I’ve looked for and failed to find myself. There used to be a 255 character [limit] but that’s no longer an issue as far as I can see by inspection’ (personal communication, February 10, 2019).

An expert from West Virginia University’s Center for Excellence in Disabilities didn’t know of such research either (Tatiana Solovieva, personal communication, February 11, 2019).

I kept digging and eventually found a mention of focus groups and a guide developed by advocates for the blind.

Blurry photo of neat cursive writing in German.

Figure 4: There seems to be a dearth of user experience research on good alt text length. Image by Cocoparisienne.

Focus groups

A student paper briefly mentioned focus group research on alt text. It didn’t mention text length, except that participants ‘commented that too much information was not good, but would rather have too much than none’ (Tomashek, 2013, p. 2). Although this doesn’t give us clear guidance on length, it indicates that overly long alt text is a ‘thing’, so let’s not go to town with it.

A guide on reducing waste

Feeling rubbish is a guide designed for Aucklanders with visual impairments. Since it was published by Blind Citizens NZ, which calls itself ‘New Zealand’s largest, generic, blindness consumer organisation’, the alt text length would presumably be useful. The varied examples below (Blind Citizens NZ, 2017) suggest we don’t have to stick to recommended lengths religiously:

Alt text Length
Two men stand in the yard of a community recycling centre inspecting a couple of wooden chairs (cover page). 1 sentence

17 words

94 characters

A photograph of Maungakiekie (One Tree Hill) with sheep grazing on its lower slopes. The obelisk can be seen clearly on the top of the volcanic mountain (p. 4). 2 sentences

27 words

152 characters

An empty blue plastic bottle that has been crushed (p. 7). 1 sentence

9 words

50 characters

A photograph of a bokashi composting system. The image features two buckets, with one inside the other. Food scraps are visible in the top bucket. A black spoon sprinkles Compost Zing onto the food scraps below (p. 43). 4 sentences

36 words

210 characters

Want to check out the alt text yourself? Download the guide and hover over the images with your mouse.

Advice on describing complex images

It’s fine if you’re describing straightforward images like photos. What about more complex images like graphs or tables — how do you cram a decent description into half a tweet? Or do you get a waiver on the limit?

There are two main solutions (Oregon State University, 2017; WebAIM, 2011, WebAIM, n.d.):

  • Describe the image on the page itself. This was also the Blind Foundation’s advice (personal communication, Feb 18, 2019). That would benefit all visitors, regardless of whether they use screen readers or need alt text because images don’t display.
  • Create a web page with the description and place a link near the image. See an example of a text descriptionof a graph.

Another possibility the HTML long description attribute (longdesc) but it doesn’t work as well. It’s not widely supported by browsers, it’s not supported in HTML5, it’s rarely implemented correctly, and it’s only accessible via a screen reader (Oregon State University, 2017, Pennsylvania State University, n.d.; Tomashek, 2013; WebAIM, n.d.).

Whichever solution you choose, you’d still need to write alt text for the image. Every image must have alt text — even social media images. If it’s purely decorative, insert two double quotation marks (Oregon State University, 2017; WebAIM, n.d.).

User feedback on alt text for complex images

While those solutions make sense, I don’t know any evidence for them, sigh.

I previously wrote alt text for graphs and tables, some of which would have exceeded the length recommendations. What would users think? I sent two examples to Harriet Kay, who uses a screen reader and co-presentedan accessibility webinar. I asked, ‘Are they about the right length, slightly too long, or way too long?’ (personal communication, Feb 27, 2019).

A graph showing the proportion of transgender people. Three dots are around 0.5 per cent for adults. Another dot shows 1.2 per cent for teenagers.

Figure 5: How would you write for graphs and tables, when there’s lots of information to describe?

Example 1: A graph showing the proportion of transgender people

Alt text Length
Four dots on a graph representing estimates of the percentage of transgender people in a population. Three blue dots close together represent estimates in adult populations, at 0.3, 0.5, and 0.6 per cent. A fourth green dot further right is the estimate for teenagers at 1.2 per cent. 3 sentences

48 words

284 characters

Example 2: A table showing preferences for information in different formats.

Alt text Length
Table listing five formats. The first preference was a table, winning 11 out of 35 votes and rating 17 out of 20 for clarity. The next preference was a question and answer format, winning 8 votes and rating 18 for clarity. 3 sentences

41 words

221 characters

Her verdict: ‘it’s perfect!’ (personal communication, Feb 27, 2019). Phew! But knowing what I know now, I’d probably describe the images in the text or link to a more detailed description.

So how much to write?

It appears alt text longer than the guidelines above may be acceptable to users. However, as with all communication, it’s important to be succinct. I’d aim for about 125 characters, just under half a tweet.

And you’ll include alt text in your social media posts, won’t you?


Thank you, Harriet Kay, for answering my questions and for reviewing what I’ve written. Thank you too, Tatiana Solovieva, for directing me to articles. And thanks, Kevin Prince, for answering yet more of my accessibility questions.


Anderson, S. (2018). How many words in ALT text for Google? Retrieved from https://www.hobo-web.co.uk/how-many-words-in-alt-text-for-google-yahoo-bing/

Blind Citizens NZ. (2017). Feeling rubbish: A guide to reducing waste for blind and vision impaired Aucklanders. Auckland, New Zealand: Author.

Gash, D. (2018). Page speed basics: Easy performance wins in the battle for broadband [PowerPoint slides]. Retrieved from https://techcomm.nz/Category?Action=View&Category_id=685

Kyrnin, J. (2018). Writing great alt text for website images. Retrieved from https://www.lifewire.com/writing-great-alt-text-3466185

Oregon State University. (2017). Alternative text for images. Retrieved from https://accessibility.oregonstate.edu/alttext

Pennsylvania State University. (n.d.). Image ALT tag tips for HTML. Retrieved from http://accessibility.psu.edu/images/imageshtml/

Rodríguez Vázquez, S. (2016, April). Measuring the impact of automated evaluation tools on alternative text quality: a web translation study. In Proceedings of the 13th Web for All Conference (p. 32). ACM. Retrieved from https://archive-ouverte.unige.ch/unige:83932/ATTACHMENT01

Tomashek, D. B., Edyburn, K. D., Baumann, R., & Smith, R. O. (2013). The case for next generation text description solutions for visual information accessibility. In RESNA Annual Conference . Retrieved from https://www.resna.org/sites/default/files/legacy/conference/proceedings/2013/PDF%20Versions/Computers%20and%20Communication/Tomashek.pdf

Twitter. (n.d.). In Wikipedia. Retrieved Mar 18, 2019, from https://en.wikipedia.org/wiki/Twitter#Format

University of Illinois (n.d.). Alt text no more than 100 characters. Retrieved from https://fae.disability.illinois.edu/rulesets/IMAGE_4_EN/

University of Washington. (n.d.). How long can an "alt" attribute be? [Factsheet]. Retrieved from https://www.washington.edu/accessit/print.html?ID=1257

WebAIM. (2011, August). Quick tip: succinct alternative text. WebAIM newsletter. Retrieved from https://webaim.org/newsletter/2011/august

WebAIM. (2017). Screen reader user survey #7 results. Retrieved from https://webaim.org/projects/screenreadersurvey7/

WebAIM. (n.d.). Alternative text. Retrieved from https://webaim.org/techniques/alttext/