Product image alt text is where your spec diagram quietly stops existing. The photo loads, the measurements sit right there in 48-point type, and every reader that is not a pair of human eyes — screen reader, search crawler, AI answer engine — gets a blank.
Baymard Institute's e-commerce accessibility benchmark puts a number on how common that is: 55% of e-commerce sites do not use alt text to communicate the core content of informational images, 64% embed text directly inside images, and 9% present complex information as an image only. If you sell furniture, hardware or building materials, that last 9% is almost certainly you, because the size chart is a JPEG.
Below are six alt-text patterns pulled from real listing galleries, why each one fails, and what to write instead.
What product image alt text actually has to do
Product image alt text is the text alternative stored in an image's alt attribute — the words a screen reader announces in place of the picture, and the only description of that image available to any system that cannot look at it.
It has two audiences, and most sellers only budget for the first one:
| Reader | What it uses alt text for | What it gets when alt text is wrong |
|---|---|---|
| Screen reader user | The entire content of the image | Silence, or a filename read letter by letter |
| Search crawler | Image context and page topic | No signal; the image ranks for nothing |
| AI answer engine | The only machine-readable version of what the picture shows | Whatever your HTML says, which may be nothing |
| Your own PIM or feed export | Image captions pushed to marketplaces and catalogues | Blank fields your channel partners fill in badly |
The W3C's Web Accessibility Initiative is blunt about the underlying problem: when text is baked into an image, "it is good design practice to use actual text that is styled with CSS rather than image-based text presentation," and where an image of text is genuinely unavoidable, the text alternative must contain the exact same text shown in the image. A dimension printed inside a JPEG is not product data. It is a picture of product data, and nothing but a human eye can read it.
There is now a legal edge to this in Europe. The European Accessibility Act — Directive (EU) 2019/882 — lists e-commerce services in Article 2(2)(f) as a covered service, and Member States have applied it since 28 June 2025. Microenterprises providing services are exempt under Article 4(5), with a microenterprise defined in Article 3(23) as fewer than 10 persons and annual turnover or balance sheet total not exceeding EUR 2 million. If you are bigger than that and you sell to EU consumers, the size chart JPEG is no longer only a conversion problem.
Anti-pattern 1: the filename
alt="IMG_4821.jpg". alt="sofa-3-final-v2-USE-THIS". alt="chair_dim_01".
This is what you get when images are exported from a folder and uploaded in bulk with nobody writing a caption. On a page with a missing alt attribute, most screen readers fall back to reading the file name — so the customer hears "I M G underscore four eight two one dot j p g" and moves on.
The fix is not clever. It is a sentence describing the specific product in the specific shot.
| Instead of | Write |
|---|---|
alt="IMG_4821.jpg" |
alt="Oak dining table, 180 cm long, seen from the front" |
alt="chair_dim_01" |
alt="Dining chair dimension diagram: seat height 46 cm, overall height 88 cm, seat width 45 cm" |
Anti-pattern 2: the keyword dump
alt="sofa couch settee 3 seater sofa cheap sofa modern sofa living room sofa buy sofa online".
Google's own image guidance calls this out directly: alt text is "the most important attribute when it comes to providing more metadata for an image," and stuffing it "results in a negative user experience and may cause your site to be seen as spam." The example Google publishes as good is four words long: alt="Dalmatian puppy playing fetch".
There is a second cost nobody mentions. A screen reader reads the whole string, at the user's reading speed, with no way to skim. Nine repeated keywords is nine seconds of noise before they learn anything about your product.
Write the one true description. The keyword you actually want to rank for is in it naturally, once.
Anti-pattern 3: the size chart that exists only as a picture
This is the expensive one, and it is the default across furniture, apparel and building materials: a full measurement table exported as a 1200 px PNG and dropped into the gallery.
The W3C treats this as a complex image, and says a two-part text alternative is required — a short description to identify the image and point to the long description, and a long description that is "a textual representation of the essential information conveyed by the image," available to everyone rather than hidden.
In practice, "available to everyone" is the whole answer. Do not try to cram twelve measurements into an alt attribute. Publish the table as an actual HTML table on the page and let the alt attribute point at it:
alt="Size chart for the Larsen sofa range; the same measurements are listed in the table below"
Then put the table below. It costs you nothing — a marked-up table is indexable, quotable by an AI answer engine, translatable, and it scales on a phone, which a 1200 px PNG of 9-point type does not. Reference pages like standard sofa dimensions exist as text for exactly this reason. If you also want the measurements on the image itself for the shopper who never scrolls, keep both. The image and the table are not competing; the image without the table is the problem.
Anti-pattern 4: describing the photo instead of the numbers
alt="grey fabric sofa in a bright living room" — on a dimension diagram.
This is the subtlest failure, because the sentence is technically accurate. It describes a picture. It just does not describe this picture, whose entire reason for existing is that it carries five measurements the product description does not.
Two rules that cover almost every case:
- If the image's job is to look good, describe what it looks like.
- If the image's job is to carry information, the alt text carries the information — the numbers, the units, and what each number measures.
The point at which people get this wrong is when the same product has both. A hero shot and a dimension diagram of the same sofa need completely different alt text, and pasting the hero's alt onto the diagram throws away the only image on the page that answers "will it fit."
Anti-pattern 5: a paragraph of alt text on a decorative image
The background texture, the divider graphic, the brand flourish at the top of the A+ module. Someone writes alt="beautiful natural oak wood grain texture background" for all of them, and now the screen reader user hears three sentences of scenery between every real product image.
Decorative images take an empty alt attribute — alt="" — which tells assistive technology to skip the image entirely. That is different from omitting the attribute, which makes the image an unlabelled unknown that gets announced anyway. Empty and missing look identical in your CMS and behave in opposite ways.
One judgement call worth making explicitly: a scale-reference shot is not decorative. A photo of your product next to a standard A4 sheet is doing informational work, and its alt text should say so.
Anti-pattern 6: one alt text, twelve gallery images
Bulk-edit tools make this trivially easy. Every image on the listing gets alt="Larsen 3-seat sofa", so a screen reader user hears the same five words twelve times and never learns that image 4 is the dimension diagram, image 7 is the fabric close-up, and image 9 is the packed carton.
Number them by function, not by position. Here is the template we hand to people who list dimensional products:
| Image role | Alt text pattern |
|---|---|
| Hero | {product name}, {finish/colour}, {view} |
| Dimension diagram | {product} dimension diagram: {measurement 1}, {measurement 2}, {measurement 3} |
| Scale reference | {product} shown beside {reference object} for scale |
| Detail / material | Close-up of {material/joint/finish} on {product} |
| Installed / in situ | {product} installed in {context}, showing {clearance or fit detail} |
| Packed carton | Packed carton for {product}: {L} x {W} x {H} {unit}, {weight} |
| Size chart | Size chart for {range}; same measurements listed in the table below |
| Decorative | alt="" |
The 60-second audit
Run this on your five best-selling listings before you run it anywhere else.
- No alt attribute contains a file extension (
.jpg,.png,.webp) - No two images on the same listing share identical alt text
- Every dimension diagram's alt text contains at least one number and a unit
- Every size chart image is accompanied by the same data as an HTML table
- Decorative images have
alt="", not a missing attribute and not a description - No alt attribute repeats a keyword more than once
- Alt text is written in the language of the storefront, not only in English
- Measurements in the alt text match the measurements in the spec table — check, they often do not
That last one catches more errors than the rest combined. Once a number is drawn onto an image, it stops being connected to the record it came from, and it stays wrong through every spec revision. The same failure shows up in feeds: see product dimensions structured data for the fields that go unfilled for identical reasons.
What this has to do with getting cited
Search engines have indexed alt attributes for two decades. What changed is who else reads them. An AI answer engine assembling a response about "sofa depth for a small apartment" cannot open your JPEG. It can only work from the text in your HTML — the product description, the spec table, and the alt attributes. Everything you drew onto the image and nowhere else is invisible to it.
That is the practical argument for the discipline above, independent of accessibility law and independent of screen readers: the measurements you publish as text are the measurements you can be quoted on.
FAQ
How long should product image alt text be?
Long enough to convey what the image shows, short enough to be heard in one breath. For a hero shot that is usually 5 to 12 words. For a dimension diagram it is longer, because the numbers are the content. If you find yourself past roughly two lines, the image is a complex image and needs the two-part treatment — a short alt plus the full data published as page text.
Does alt text help SEO?
Yes, but as context rather than as a ranking lever you can pull. Google describes alt text as the most important source of metadata about an image, and stuffing it with keywords is explicitly listed as spam behaviour. Write it for a person who cannot see the image; the SEO benefit is a side effect of doing that well.
What alt text should I write for a size chart image?
A short identifying sentence that points to the real table — for example, "Size chart for the Larsen sofa range; the same measurements are listed in the table below" — and then actually publish the table in HTML. Trying to fit a whole measurement grid into an alt attribute produces something no screen reader user can follow and no crawler can parse into rows.
Do I need alt text on decorative product images?
They need an empty alt attribute, alt="", which is not the same as leaving the attribute out. Empty tells assistive technology to skip the image. Missing makes it an unknown that gets announced, usually by filename.
Does the European Accessibility Act apply to my online store?
If you provide e-commerce services to EU consumers and you are not a microenterprise, yes — e-commerce services are listed in Article 2(2)(f) of Directive (EU) 2019/882, applicable since 28 June 2025. The microenterprise exemption in Article 4(5) covers service providers with fewer than 10 persons and annual turnover or balance sheet total of EUR 2 million or less. Check your own national transposition for the details, because enforcement and penalties are set at Member State level.
Why do my dimension labels get cut off before anyone can read them?
That is a separate problem with the same root — information living only in pixels. Marketplace galleries crop to their own ratios, and labels parked near the edge disappear; product image aspect ratio covers the safe-area rules per channel.
Sources & References
- Baymard Institute — Use "ALT" Text to Communicate the Core Content of "Informational" Images (55% of Sites Don't)
- W3C Web Accessibility Initiative — Images Tutorial: Images of Text
- W3C Web Accessibility Initiative — Images Tutorial: Complex Images
- W3C Web Accessibility Initiative — Images Tutorial: Decorative Images
- Google Search Central — Google Images best practices, including alt text guidance
- EUR-Lex — Directive (EU) 2019/882 (European Accessibility Act): Article 2(2)(f), Article 3(23), Article 4(5), Article 31
- European Commission — European Accessibility Act: covered products and services
Fixing this once instead of once per image
Every anti-pattern above is the same failure wearing a different hat: a measurement was drawn into a picture and never written down anywhere a machine could read it. You cannot fix that with better alt-text habits alone, because the numbers you are describing were typed over the photo by hand and are already drifting from the spec.
The version that holds up is to treat the dimension layer as data rather than artwork. Measure against the real edges of the product so the number is the measured one, not an eyeballed one; keep the annotation layer live and editable instead of flattening it into a JPEG; then export two things from the same source — the labelled image at each channel's spec size, and the measurement list as text you paste into the spec table and the alt attribute. That is the difference between measured annotation software and an AI image generator that produces a plausible-looking number next to your product: one of them can tell you what it measured, the other invented it. If you are rebuilding your listing assets anyway, the spec sheet is the right place to make the numbers live, because it is the document both the buyer and your own catalogue read from.