Alt text should be long enough to convey, short enough not to stall
Aliases: verbose alt · concise alternative text · alt length
What it is
Alternative text has a job and a time cost. Too short — “chart,” “image” — and the fact never ships. Too long — a marketing paragraph — and the reading stream stalls so people skip. The right length is the shortest sentence that carries the extra information that image adds, usually a phrase or one sentence, not a paragraph. A large hero need not be longer; a small thumbnail need not be shorter. Length follows unique information, not pixel area.
Why it happens
Screen-reader users often move by heading and link; every image still inserts a name along the way. A grid of forty-word thumbnails taxes browsing. A results figure named “chart” might as well be absent to AT. A second layer: when a name will not go short, appearance, mood, photographer, and copyright have usually been piled into it — the wrong layer, which also makes it long. If equivalence truly needs a paragraph, the short attribute is the wrong container; that is a long description, not a stuffed alt.
No universal character cap sits in every standard. Sentences that fit in one breath are rarely complained about; paragraphs that fill several virtual-buffer screens are almost always skipped. A leading “image of” is both the wrong layer and wasted length. Boilerplate on a product grid (“high-resolution photo of the product on a white background”) is a length problem and empty information.
Studying it
Listen to every image name on a page. Log time, skips, and whether the fact the image was meant to add can be restated. Write the same image three ways — three words, one sentence, one paragraph — and compare comprehension against skip rate.
Independent variables: name length, image count on the page, information density. Dependent variables: traversal time, skip rate, accuracy of restating the fact.
Sample WCAG-EM pages, not only the home hero. Automation can flag unusually long alts or filename alts; “long enough” still has to be judged against the task.
Where it stops holding
News photography and art notes sometimes must run longer than a button name; they should still aim at a sentence, not an essay. On social media the image is the content, so length rises — still the unique fact, not a tag pile. Mail clients truncate alt more aggressively, which pushes shorter. The same image may take a short name in a list and fuller adjacent copy on a detail page — layering, not two empty slogans.
Applying it
- Say the unique information in one sentence first. If it will not fit, do not add adjectives; ask whether the image has already outgrown a short alt.
- Drop “image / graphic / photo of” and filenames.
- In a thumbnail set, name what differs (which variant, which step). Do not paste the same mood sentence.
- Verify: listen through a page of images at 1.5× speech. Wanting to fast-forward means too long; finishing unable to state the fact means too short. With images off, the name should be just enough to do the job.