How to write alt text people can actually use
A screen reader says graphic before it reads your text, which is why image of openers stutter. WCAG, Google and the platforms each have a rule here, and they do not all say the same thing.
8 minute read
Alt text replaces the image, it does not caption it
WCAG success criterion 1.1.1, at level A, asks that all non-text content carry a text alternative serving an equivalent purpose. Equivalent purpose is the load-bearing phrase, and it is not the same thing as a description. A caption sits under an image and adds something for everyone; alt text stands in for the image entirely, for someone not receiving it. It is read aloud by a screen reader, printed in place when an image fails to load, and used by search engines: Google's image SEO documentation says it combines alt text with computer vision and page context to work out what an image shows. Most images still go without. WebAIM's 2026 Million report, a scan of the top million home pages, found 16.2% of images had no alternative text, and a further 10.8% of those that did carried a file name or a placeholder like image or graphic.
How a screen reader actually announces it
This is the part that changes how you write. Screen readers announce the role before the content, so JAWS and NVDA say graphic and then read your text. Write photo of a woman speaking on stage and a listener hears graphic, photo of a woman speaking on stage. That is why every accessibility guide tells you to drop image of and picture of openers: the software has already said it. The second thing to know is that alt text is delivered as one uninterrupted run. There is no skimming it, no glancing back at the middle, and no scanning ahead to see whether it is worth the wait. So put the point in the first few words, keep the sentence structure simple, and use full stops, because punctuation is what gives the voice somewhere to breathe.
A good description says what the image is doing
The W3C's alt decision tree is not a template, it is a series of questions about context, and that is the real lesson in it. The same photograph needs different alt text depending on the post it sits in. A picture of your desk illustrating a story about a microphone should lead with the microphone; the same picture illustrating a story about working from a spare room should lead with the room. Describe the point, not the inventory. For a chart, that means the finding rather than the axis labels, so revenue tripled after the pricing change beats bar chart showing quarterly revenue. If the image contains words, the words go in the alt text, because they are the content. Quote cards and screenshots fail hardest here, and they are exactly what creators post most.
Keyword stuffing fails twice over
Google's image SEO documentation is unusually direct about this. It says filling alt attributes with keywords creates a bad user experience and can get a site treated as spam, and it walks a single puppy photograph through four versions: no alt text, a stuffed run of synonyms, the bare word puppy, and finally the descriptive version it prefers, Dalmatian puppy playing fetch. The best answer is also the most human one, which is the whole point. The second failure is the one nobody sees. A stuffed field is not skipped by a screen reader, it is read aloud, word by word, to the person who needed the description. Whatever the field is called, treat it as a sentence written for a listener, not as somewhere to repeat the hashtags you already used.
Where each platform puts the field
The field moves around and the apps do not agree. On X you tap the +ALT button on the image on web, or Add description underneath on mobile, and an ALT badge then appears on the published post that anyone can tap to read. LinkedIn's help pages tell you to click Alt text below the image on desktop or tap ALT on mobile. Instagram keeps it with the pre-publish options and lets you edit it afterwards from the post's own menu. TikTok added alt text for photo posts, addable in the composer or after publishing. Bluesky puts it under the image and offers something no one else does: a setting that stops you posting without it. One trap worth knowing, flagged by Accessible Social, is that since April 2026 you cannot add alt text through Meta Business Suite at all, so scheduled posts are where descriptions quietly disappear.
What the platforms write for you is a floor, not a finish
Several apps fill the field for you if you leave it empty. Meta's automatic alt text launched in 2016 recognising about a hundred common concepts, and the rebuilt version the company announced in 2021 covers more than 1,200 concepts across 45 languages. Meta says accuracy matters more than coverage, which is why every generated description opens with may be, and why a listener gets hedged object counts along the lines of may be an image of five people. LinkedIn's help notes it may assign alt text itself where none was supplied, and TikTok has been testing AI-generated descriptions for photo posts without one. All of these name objects. None of them knows why you posted the image, what the text inside it says, or which of the five people is the reason it exists.
Video needs a different mechanism entirely
Alt text does not apply to a video file, and this is a genuine limit rather than an oversight. Success criterion 1.1.1 has a carve-out for time-based media: the text alternative only has to identify the media descriptively, because the real work moves to the 1.2 criteria, which is where captions, transcripts and audio description live. The accessibility guide elsewhere on this blog covers those with the thresholds attached. What does take alt text is everything still that travels with a video: the thumbnail, the cover frame, carousel images, quote cards, the screenshot you drop into a post. Write it in the same minute you write the caption, one sentence, point first, and check whatever the platform generated before you publish rather than after.
FAQ
How long should alt text be?
There is no limit in HTML and WCAG names no number, so the widely repeated 125-character rule is convention rather than standard. The real ceilings are the platforms': Accessible Social lists 300 characters on TikTok and 2,000 on Bluesky. One clear sentence covers most photographs. Go longer only when the image itself carries the content, such as a quote card or a chart.
Do I need alt text on every image?
No. The W3C's alt decision tree routes purely decorative images, and images whose information already appears in adjacent text, to an empty alt attribute so screen readers skip them. That option only exists on sites you control. On social platforms there is no skip, so describe anything that carries meaning and keep decorative images to one short line.
Should I use the alt text platforms generate automatically?
Treat it as a starting point. Meta's automatic alt text recognises over 1,200 concepts in 45 languages, but it opens every description with may be because the company prioritises accuracy over completeness, and it can only name objects. It cannot know your intent, read the meaning of text inside the image, or say which person in frame matters.
Does alt text help SEO?
On your own site, yes, modestly. Google's image SEO documentation says it uses alt text alongside computer vision and page context to understand an image, and treats it as anchor text when the image is a link. The same page warns that stuffing the attribute with keywords creates a poor experience and can make a site look like spam.
Sources
- Understanding Success Criterion 1.1.1: Non-text Content · W3C Web Accessibility Initiative
- An alt Decision Tree · W3C Web Accessibility Initiative
- The WebAIM Million: the 2026 report on the accessibility of the top 1,000,000 home pages · WebAIM
- Image SEO best practices · Google Search Central
- Platform Image Accessibility · Accessible Social
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