How to write alt text that actually helps
Not "describe the image" — that's true but useless advice. Here's what to actually type into the box.
Alt text has two real audiences — a screen reader describing the page to someone who can't see it, and a search engine that can't see it either — and both need the same thing: a plain, accurate, reasonably short account of what the image is or does. Not keywords. Not "image of". What it actually is. (Auditing a whole page rather than one image? The on-page SEO checklist is the wider list.)
Content images versus decoration
What to actually type
Describe what a sighted visitor gets from looking at it — specific enough to be useful, short enough to stay readable. "Product photo" tells a screen-reader user nothing; "matte blue ceramic mug, 350ml, with a side handle" tells them what's actually for sale. A commonly used practical ceiling is around 125 characters. Beyond that, six habits separate useful alt text from filler:
Skip "image of" The screen reader already announces it's an image — those words are pure overhead, repeated on every single one.
An image that's also a link describes the action "Search" beats "magnifying glass icon" for a search button: what clicking it does is what a screen-reader user needs in order to decide whether to click.
A chart describes the finding, not the shape "Bar chart showing monthly sales rising from €2,000 in January to €9,500 in June" is useful; "a chart with six bars" is not.
Don't repeat the caption If visible text next to the image already says everything, let the alt text add something the caption doesn't or defer to it — otherwise a screen reader reads the same sentence twice in a row.
Drop the opinion words "Beautiful" and "stunning" describe your reaction, not the image — they only make sense to someone already looking at it.
Don't force the keyword in If the image genuinely is what the keyword describes, an honest description already contains it; if it isn't, forcing it in just makes the text less accurate.
Don't forget to translate it too
That text needs to be in the alt, word for word (or close to it) — a screen reader can't read pixels, so if "20% off this weekend" only exists as part of the image, a visitor who can't see it never finds out. It's a strong reason to keep real text as real text wherever you can, rather than baked into a graphic.
It's one contributing signal among many, and a fairly minor one on its own — but a page riddled with missing alt text is usually a symptom of a page that got rushed in other ways too, so the correlation with weaker pages is real even where the direct causation is small. How a page's overall SEO score weighs it against the other 17 checks is its own guide if you want the full picture.
Describe it the way you'd describe it out loud to someone on the phone who's asking what's on the page — specific enough that they'd recognise it, short enough that you wouldn't keep going for a full paragraph. If you can't manage even that, it's often a sign the image itself isn't earning its place on the page.
Every image that carries content does; purely decorative ones are typically excluded rather than penalised, provided they're actually marked as decorative rather than just left blank by accident. A page with genuinely no content images at all usually passes this check by default — there's nothing to fail.
Keep the alt text itself short — a one- or two-sentence summary of what it shows overall — and put the full detail in visible text near the image instead: a caption, a paragraph, a table, whatever suits the content. Alt text is meant to be a quick, equivalent description, not a place to transcribe an entire infographic line by line.