How to write alternative text that is actually useful

Alt text exists to convey what an image communicates to someone who cannot see it — not to literally catalogue its visual contents. “Image of a plumber” tells a screen reader user almost nothing useful about why that image is on the page. “Fully licensed plumber Dave Chen inspecting a hot water system” tells them what a sighted visitor would actually take away from looking at it.

What alt text should do, and what it commonly does instead

Purpose	Poor alt text	Better alt text
Convey what the image communicates	“image1.jpg” or blank	Describes the image’s actual content and purpose
Add information, not redundancy	“Photo of a plumber” (visitor’s caption already says this)	Alt text that adds what the caption doesn’t repeat
Match the image’s function on the page	A logo linking home described as “logo”	“Return to homepage” — describing the link’s function, not just the image
Skip purely decorative images	A meaningless filename or “decorative image” read aloud	Empty alt text (alt="") so a screen reader skips it silently

Why “describe what a sighted person sees” is the wrong instruction for alternative text

A common but flawed rule of thumb is “describe exactly what’s in the image.” This produces alt text that’s technically accurate and practically useless. It ignores why the image is on the page. An icon of an envelope next to a phone number and an address isn’t meaningfully described as “envelope icon.” It’s better described by its function: “email address.” If the surrounding text already states the email address in full, it may need no alt text describing the icon’s function at all. The information is already available in text.

The functional test: what would be lost if this image disappeared?

A more reliable approach than “describe what’s shown” is asking what information or function would be lost for a sighted user if the image vanished. Write alt text that restores exactly that. Take a product photo on an ecommerce page: what would a customer lose without seeing it? Probably a sense of the product’s actual appearance, colour or key visual feature — which the alt text should attempt to convey in words. Take a purely decorative border graphic instead: nothing meaningful would be lost, so it should have empty alt text.

Purely decorative images need empty alt text, not a description

An image with no informational purpose — a background pattern, a spacer, a purely aesthetic flourish — should be given empty alt text (alt=""). This tells assistive technology to skip it silently, rather than announce “decorative image” or read out a meaningless filename. Both of those add noise without adding information. Deciding an image is decorative is itself a judgement call. The test above — what would be lost without it — is the way to make that call correctly.

Alt text for images that are also links

Where an image functions as a link — a logo that returns to the homepage, an icon that opens a menu — the alt text should describe the destination or function of the link, not the visual appearance of the image itself. “Company logo” tells a screen reader user what’s pictured but not what clicking it does. “Return to homepage” tells them exactly what they need to decide whether to click it.

Complex images, PDFs, charts, diagrams and infographics

A chart or diagram usually cannot be adequately described in a short alt text attribute alone. The practical approach is a concise alt text summarising the image’s overall point. Pair it with the same information presented in accessible text form nearby — a data table alongside a chart, for instance — rather than attempting to cram a full description into the alt attribute.

Where this connects to copywriting more broadly

Alt text is written content. It is most naturally produced by whoever is writing the rest of a page’s copy, briefed specifically on this distinction between literal description and functional purpose — covered more broadly on who writes the words. Treating alt text as a copywriting task, not an afterthought a developer fills in at the end, produces meaningfully better results.

Why AI-generated alt text is a starting point, and one example that is not a finished answer

Some tools now offer automatically generated alt text using image-recognition technology. This can be a useful starting point for a large existing image library, but it typically describes literal visual contents rather than the image’s functional purpose in context — precisely the distinction this page argues matters most. Treat AI-generated alt text the same way you’d treat auto-generated video captions, covered elsewhere in this cluster: a draft to review and correct, not a finished, reliable answer to publish unchecked.

How alt text length should be judged

There’s no fixed correct length for alt text — the test is whether it conveys the necessary information as concisely as the image warrants, not whether it hits a particular word count. A simple icon might need only two or three words; a complex product photo showing texture, colour and scale might reasonably need a full sentence. Padding alt text with unnecessary detail to seem thorough is as much a failure of the functional test as leaving it too sparse to be useful.

Alt text, screen readers and SEO — a genuine overlap, with a caution

Alt text is also read by search engines as a signal about an image’s content. That has led some content teams to treat it primarily as a keyword-insertion opportunity rather than a genuine accessibility description. Writing alt text stuffed with keywords rather than an accurate functional description serves neither purpose well. A screen reader user gets a confusing, unnatural description. Search engines increasingly discount alt text that reads as manipulative rather than descriptive. The two purposes align well when alt text is written accurately and specifically in the first place.

What to do next to convey a screen reader description that works

Audit your site’s existing images against the functional test — what would be lost without this image — and rewrite alt text that merely restates a visible caption or describes an icon’s appearance rather than its function. What that discipline costs as part of a copywriting engagement is covered on website copywriting.