Image alt text: the complete guide for SEO and accessibility

Most site owners treat image alt text as a box to tick, if they fill it in at all. So images ship with no description, or a filename like “IMG_4471.jpg”, and two things quietly break: people using screen readers lose the content, and search engines lose the context they need to rank your images. Alt text fixes both in one short line. This guide covers what alt text is, how long it should be, how to write it well, how to add it on every major platform, and how it feeds both Google Images and the newer AI answers.

What is image alt text?

Image alt text (alternative text) is a short written description of an image, added in the HTML alt attribute of the <img> tag. It gives a text equivalent of the image for anyone or anything that cannot see the picture: screen readers read it aloud, browsers show it when an image fails to load, and search engines use it to understand what the image shows.

Here is what it looks like in the markup:

<img src="golden-retriever-puppy.jpg" alt="Golden retriever puppy playing with a red ball on grass">

That one attribute does three jobs at once. It carries the meaning of the image to visually impaired users, it acts as a fallback when the image breaks, and it hands search engines a plain-text description they can index. Google’s own image SEO documentation calls alt text the most important attribute for giving an image metadata, precisely because it serves users and crawlers in the same move.

Alt text vs image title vs image caption

People mix up three things that sit near an image: the alt text, the title attribute, and the caption. They are not interchangeable, and only one of them is doing real accessibility and SEO work.

Here is how they differ:

Alt text

Alt text is the description a screen reader announces and the text a browser falls back to when an image will not load. It is the only one of the three that accessibility standards actually require, so it should always be your priority.

Image title attribute

The title attribute appears as a tooltip when someone hovers a mouse over the image. It is optional, and most of the time you can skip it. Pointer users are the only ones who see it, and it carries almost no SEO value, so there is no need to fill it in for every image.

Image caption

A caption is visible body text, usually sitting just below the image, and everyone sees it. Captions tend to be among the most-read words on a page, so they are worth writing well, but they do a different job from alt text. You can have both: a caption for sighted readers and alt text for screen readers and crawlers.

Why image alt text matters

Alt text earns its place for four separate reasons, and only one of them is SEO. The other three are about real people, legal exposure, and what happens when your images break.

Accessibility and screen readers

The first job of alt text is accessibility. People with visual impairments use screen readers, which announce each image by reading its alt text aloud. With no alt text, the reader either says nothing useful or falls back to reading out the file name, which helps no one.

This is not a small audience. In a MozCon talk on the topic, SEO specialist Miracle Inameti-Archibong noted that missing alt text accounts for a large share of the accessibility errors found on typical homepages. A single missing description is a small gap. Across a whole site, it adds up to content that a slice of your audience simply cannot reach.

Legal and compliance requirements

Alt text is not just good manners. In many places it is a legal requirement. The Web Content Accessibility Guidelines (WCAG) are the international reference standard, and success criterion 1.1.1 on non-text content requires a text alternative for images at Level A, the most basic level of conformance.

Those guidelines sit behind real laws. In the United States, the Americans with Disabilities Act (ADA) and Section 508 both push organisations toward WCAG conformance, and the European Accessibility Act does the same across the EU. If your business serves those markets, alt text is part of staying compliant, not an optional nicety.

SEO and image search visibility

Search engines cannot see an image the way you do. They read the file name, the surrounding text, and the alt text to work out what the picture shows. Google states plainly that it uses alt text along with computer vision and the page content to understand an image’s subject.

Get it right and your images can surface in Google Images and in image packs, the rows of pictures that appear inside normal search results. For some queries, that is a real source of traffic you would otherwise miss.

When an image fails to load

Images break. A slow connection, a broken link, or a blocked host can all stop a picture from appearing. When that happens, the browser shows the alt text in the image’s place, so the reader still gets the point.

This matters most where the image carries information, like a product photo, a diagram, or a how-to screenshot. A blank box tells the reader nothing. A line of alt text keeps the page usable even when the visuals fail.

How alt text affects SEO

Alt text supports your SEO in two ways. It reinforces what a page is about by adding another relevant, keyword-aware description that matches your content, and it makes your images eligible to rank in image search. It works best alongside a descriptive file name and, where useful, a caption, so search engines see the same signal from several angles.

The effect is contextual, not magic. Google leans on the words around the image as much as the alt text itself, which is why a description that fits the surrounding content beats one stuffed with keywords. If you want the wider picture on this, our guide on how search engines work sets out how crawlers read a page.

Is alt text a Google ranking factor?

Yes, with a sensible caveat. Alt text is a genuine ranking factor for Google Image Search, where it is one of the main signals Google uses to match an image to a query. For ordinary web search, Google’s John Mueller has said alt text is counted as part of the on-page text, so it feeds general relevance rather than acting as a standalone lever.

The practical takeaway is simple. Write alt text for people first, keep it accurate and specific, and the SEO benefit follows. Treat it as a keyword slot and you risk the keyword stuffing that Google’s own guidance warns can be flagged as spam.

How long should alt text be?

There is no hard character limit, but a good working rule is to keep alt text to around 125 characters, or roughly one sentence.

That number comes from how screen readers handle the alt attribute: some cut it off at about that length, so anything past it may never be read. Google takes a similar line, advising you to be descriptive but not overly long rather than setting a fixed cap.

If an image genuinely needs more explanation than a sentence, like a detailed chart, the extra detail belongs in the surrounding body text or a linked long description, not crammed into the alt attribute. Keep the alt text short and let the page do the rest.

How to write effective alt text: best practices

Good alt text is specific, short, and written for a person who cannot see the image. The practices below turn that into a repeatable checklist.

Be specific and descriptive

Describe what is actually in the image, using plain, concrete words. “Dog” tells the reader almost nothing. “Golden retriever puppy playing with a red ball on grass” lets them picture it. Include the details that matter for the context, and leave out the ones that do not.

Keep it concise

Aim for a single, clear sentence. Once you drift past roughly 125 characters you risk a screen reader cutting you off, and you are probably adding detail the reader does not need. Say what the image shows, then stop.

Front-load the most important information

Put the key point first. Screen reader users can move on before the description finishes, so the opening words should carry the essential meaning. Lead with what the image is or does, then add supporting detail if it helps.

Don’t start with “image of” or “picture of”

Screen readers already announce that the element is an image, so “image of a red bicycle” makes the reader hear “image” twice. Drop the preamble and start with the description itself.

Use keywords naturally

Alt text is a fair place to include a relevant keyword, but only where it genuinely fits the image. Match it to the picture and the surrounding content, the same way you would approach any keyword research. Forcing a keyword in, or stuffing several, hurts the reader and can be flagged as spam by Google.

Decorative images: use an empty alt

Not every image needs a description. Purely decorative images, like a background flourish or a divider, add nothing to the meaning of the page. For these, use an empty alt attribute so screen readers skip them:

<img src="divider-flourish.png" alt="">

Note the difference between an empty alt and a missing one. An empty alt=”” tells the screen reader to ignore the image on purpose. A missing alt attribute leaves the reader to guess, and many will read out the file name instead. If an image is decorative, say so with an empty attribute rather than leaving it off.

When the surrounding text already describes the image

Even when your caption or body text already explains the image, still add alt text. A screen reader user may not read the two together, and the alt text can be tuned to the image’s specific purpose, which often differs from the broader point of the paragraph around it.

Complex images: charts, graphs and infographics

A chart or infographic carries more information than a sentence can hold. Give it a short alt text that names what it is, then provide the full detail in the nearby body text or a linked long description. For example, a sales chart might have alt text of “Bar chart of quarterly online sales, described below”, with the actual figures written out in the paragraph beneath. Heavy images also add weight to a page, so keep an eye on your page speed when you use a lot of them.

Good vs bad alt text: examples by image type

The best way to learn alt text is to see it done badly, then well. Each type of image calls for a slightly different approach, so here are seven common ones with a weak and a strong version of each.

Simple object or product photo

A bare label wastes the slot. Name the object and the detail that matters.

• Weak: alt=”shoe”
• Strong: alt=”White leather trainer with a chunky sole, side view”

Action or scene

Describe what is happening, not just what is present.

• Weak: alt=”person on bike”
• Strong: alt=”Woman in a yellow jacket cycling through a city park in autumn”

Chart or graph

Summarise the finding, not the shape. The takeaway is what the reader needs.

• Weak: alt=”bar chart”
• Strong: alt=”Bar chart showing online sales rising 30% from Q1 to Q2 2025″

Logo

Name the brand, and add the purpose if the logo is doing a job like linking home.

• Weak: alt=”logo”
• Strong: alt=”Roots Digital logo”

Icon or button

For a functional image, describe the function, not the picture. The reader needs to know what it does.

• Weak: alt=”magnifying glass”
• Strong: alt=”Search”

Linked image

When an image is a link, the alt text works like anchor text. Describe where the link goes.

• Weak: alt=”click here”
• Strong: alt=”Read our SEO services page”

Person or team photo

Name who is in the shot and, where it matters, what they are doing.

• Weak: alt=”man”
• Strong: alt=”Ian Ong, Marketing Director at Roots Digital, presenting at a workshop”

Common alt text mistakes to avoid

Most alt text problems come down to the same handful of habits. Watch for these.

Using the file name or a generic label

Leaving the alt text as “IMG_4471.jpg” or “image1” gives the reader nothing. It is one of the failures WCAG specifically calls out. Always replace the default with a real description.

Over-explaining

Starting with “this is an image of…” or narrating every visual detail slows a screen reader user down. Get to the point and keep it to what matters.

Keyword stuffing

Packing the alt attribute with keywords, like “SEO, SEO services, SEO agency Singapore”, reads badly and can be treated as spam. Use one relevant term at most, and only where it fits.

Leaving important images with no alt text

An informative image with no alt text is a real gap. Product shots, diagrams, and charts all carry meaning that is lost without a description. These are the images to prioritise.

Writing alt text for purely decorative images

The opposite mistake is describing images that do not need it. A decorative divider with alt text just adds noise for a screen reader user. Give these an empty alt=”” instead.

How to add alt text on every major platform

Where you type the alt text depends on your platform, but the field is always there. Here is where to find it on the six most common ones. Platform interfaces change often, so if a menu has moved, look for a field labelled “alt text” or “alternative text” near the image settings.

WordPress

WordPress has an alt text field built into the editor, and both major SEO plugins add checks on top of it.

Adding alt text in the block editor

Select the image block in your post or page. In the settings sidebar on the right, under the block settings, you will see an “Alternative text” field. Type your description there. You can also set it in the Media Library: click an image and fill in the “Alt Text” field on the right.

Editing alt text with Rank Math

Rank Math does not replace the WordPress alt field. Instead, its content analysis checks whether your focus keyword appears in at least one image’s alt text on the page, and flags it if none of your images have alt text at all. You still write the alt text in the block or Media Library field.

Editing alt text with Yoast SEO

Yoast works the same way. Its analysis checks whether images on the page carry an alt attribute containing your focus keyphrase, and warns you if the keyphrase is missing or, on image-heavy pages, over-used. The field you type into is still WordPress’s own.

Shopify

In your Shopify admin, go to Products and click the product you want to edit. Click a product media item to open its preview page, then click the alt text field and enter your description, and Save. Shopify’s help documentation confirms the maximum length is 512 characters, though it recommends 125 or fewer. Images that live in your theme, like homepage banners, get their alt text in the theme editor instead.

WooCommerce

WooCommerce runs on WordPress, so it uses the same Media Library alt text field. Open a product image in the media library and fill in the “Alt Text” field. Because product images all show a specific item, give each one a clear, unique description rather than leaving any blank.

Wix

Click the image in your editor, then click the Settings icon. Enter your description in the field labelled “What’s in the image?” and press Enter to save. Remember to publish the site so the change goes live. Gallery images get their alt text through the Media Manager.

Squarespace

Squarespace handles alt text a little differently. For an image block, the caption or filename field can double as the alt text depending on your version and settings, so check Squarespace’s current guidance for your setup and write that field as a proper description rather than a bare filename.

Webflow

Select the image element and open its settings. You will find an “Alt Text” field where you can write a custom description, or choose “Decorative” for images that carry no meaning, which sets an empty alt attribute. You can also set alt text once in the Assets panel so it travels with the image wherever you place it, and bind it to a CMS field for dynamic collections.

Alt text on social media

Alt text is not just for your website. The major social platforms all have a built-in field for it, and they each auto-generate a guess you should replace with your own. The exact steps and character limits differ, so here is where the field sits on each.

Instagram

When creating a post, edit your photo and tap “Advanced settings” near the bottom, then “Write alt text” under accessibility. Instagram caps alt text at roughly 100 characters, so keep it to one tight sentence.

X (Twitter)

Attach an image to your post and tap or click the “ALT” badge on the image. Enter your description, which can run up to 1,000 characters, and save. Each image in a multi-image post needs its own alt text.

LinkedIn

From a browser, create a post and attach your image, then click the “Alt text” button below the image. LinkedIn supports up to 120 characters. Note that you add it before posting: you cannot edit alt text once the post is live, and the field is browser-only.

Facebook

Upload your image and open the photo editing screen, then choose “Alternative text” from the menu. Facebook offers auto-generated text, but pick the custom option and write your own, since the automatic guess is often vague.

Using AI to generate alt text, and how to QA it

AI tools can now write alt text for you, and several are built into platforms or sold as plugins. For a large back catalogue of images with no descriptions, they are a genuine time-saver. But they work best as a first draft, not the final word.

The reason is context. An AI model usually sees only the pixels, so it can describe what is in the image without knowing why you used it. It might label a chart accurately yet miss the finding you wanted readers to take away, or name the wrong product, or write something too vague to help.

So treat AI alt text as a starting point you review. Before publishing, check that each description captures the image’s purpose in context, names things correctly, and stays concise. It is worth writing alt text by hand for your most important images, like hero shots and anything where a wrong description would mislead someone. For bulk, low-stakes images, an AI pass with a human review is a fair trade.

Alt text and AI Search visibility

Alt text is starting to matter beyond classic image search. AI Overviews and chat-based search read the same signals search engines always have, and image understanding is part of that. A clear, specific description helps an AI system work out what your image shows and how it fits the page, which can only help when these systems decide what to surface or cite.

The advice does not change, which is the useful part. Write accurate, specific, context-aware alt text for people, and you are also giving AI systems the clean description they rely on. There is no separate trick for AI Search here. Good alt text is the same good alt text.

Tools to audit and fix missing alt text

You cannot fix what you cannot see. If your site has more than a handful of images, use a tool to find the ones missing alt text rather than checking each by hand. Here are four types worth knowing.

Browser inspect tools

The quickest spot check needs no software. Right-click any image, choose “Inspect”, and look at the <img> tag for the alt attribute. If it is missing or empty where it should not be, you have found a problem. This is fine for a page or two, not a whole site.

Screaming Frog SEO Spider

Screaming Frog crawls your whole site and lists every image, with filters for missing alt text and for alt text over a set length. It is a fast way to get a complete list of gaps across hundreds of pages in one pass.

Site audit crawlers

Full SEO platforms like Semrush, Ahrefs, and Moz include image checks in their site audits, flagging missing or duplicate alt attributes alongside your other on-page issues. If you already use one for SEO, the alt text report is built in.

Accessibility scanners

Accessibility-focused tools like WAVE, Lighthouse, and axe test against WCAG and will flag images with no text alternative. These catch alt text problems in the wider context of accessibility, which is useful if compliance is your main driver.

Final thoughts

Alt text is a small detail that quietly does a lot. It opens your content to people who use screen readers, keeps your pages usable when images fail, and gives search engines and AI systems the context they need to understand and surface your images. None of it takes long, and the habit compounds across a whole site.

The rule to remember is to write for the person who cannot see the image, be specific, and keep it short. Do that and the SEO benefit follows on its own. If you would rather hand the whole job over, our SEO services and SEO content services cover image optimisation as part of a wider on-page strategy. Found this useful? Share it with someone whose images could use the help.

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