Alt description SEO: why your image descriptions affect search rankings
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Alt description SEO: why your image descriptions affect search rankings
Google's crawlers cannot see images. They read text. Every time a Googlebot visits your product page, it encounters the alt attribute on your images and uses that text to understand what the image depicts — and by extension, what the page is about. For e-commerce stores where product images dominate the page content, this makes alt description SEO one of the highest-leverage technical tasks available.
How Google's crawlers actually process the alt attribute
When Googlebot crawls a page, it encounters image tags in the HTML. The alt attribute is the primary signal the crawler reads to interpret image content. Google has published guidance on this through Google Search Central: alt text helps Google understand "the subject matter of the image" and contributes to how the page is indexed for both standard web search and Google Images.
The alt attribute sits directly in the HTML source:
<img src="product.jpg" alt="Navy blue wool blazer, slim fit, two-button front" />
Googlebot reads this as plain text. It feeds into the page's topical relevance signals alongside the <title>, headings, and body copy. On a product page that has a single short product description and six images, the alt texts on those six images may represent the majority of indexable text content.
There is also the title attribute — and this is where many store owners get confused. The title attribute creates a tooltip on hover. Search engines do read it, but it carries far less weight than alt. Google Search Central explicitly states that the alt attribute is the correct place for image descriptions. The title attribute is for supplementary information, not accessibility or primary SEO signal.
Google Images as a traffic channel
Google Images is frequently overlooked in e-commerce SEO strategy. According to data from Jumpshot and later analyses of search behaviour, image search drives roughly 22% of all web searches. For product categories like clothing, jewellery, home décor, and food, the proportion is higher — visual search is a natural behaviour in these categories.
A product page with accurately described alt text is eligible to surface in Google Images results. A page with missing or generic alt text — "image001.jpg", "product", "photo" — is effectively invisible to this channel. For a store with 2,000 product images and no alt text, the potential traffic uplift from fixing this is substantial.
The difference between alt and title: a practical comparison
| Attribute | Where it appears | Who reads it | SEO weight |
|---|---|---|---|
alt | Not visible; announced by screen readers; shown if image fails to load | Screen readers, Googlebot, Bingbot | High — primary image signal |
title | Appears as a tooltip on mouse hover | Sighted users who hover; assistive tech (secondary) | Low — supplementary only |
aria-label | Not visible; ARIA attribute | Screen readers | Not a direct SEO signal; improves accessibility |
src filename | Not visible to users | Googlebot (minor signal) | Minor — descriptive filenames help marginally |
The alt attribute is where both accessibility and SEO converge. This is why fixing alt text has a dual return: it simultaneously satisfies WCAG 1.1.1 and improves image search indexability.
How alt descriptions affect page-level topical relevance
Search engines assess topical relevance — how thoroughly a page covers its subject — as part of their ranking algorithms. A product page for a hiking boot that includes alt texts describing "waterproof hiking boot", "Vibram outsole detail", "ankle support construction", and "removable insole" is signalling far richer topical coverage than a page where all four images have alt="" or blank attributes.
Google's documentation notes that descriptive alt text "helps Google better understand" the page. This is particularly important for pages that are thin on text — which describes a significant number of e-commerce product pages, where much of the information is conveyed visually through the images themselves.
A practical consequence: if your competitors have detailed alt text and yours do not, their product pages have more indexable text signal on the same topical queries. Over thousands of pages, this compounds.
A practical formula for e-commerce alt descriptions
The goal is a description that would allow someone who cannot see the image to understand exactly what it shows. For product images, a reliable formula is:
[Key descriptor] + [product type] + [distinguishing feature] + [context if relevant]
Applied to real examples:
- "Handmade ceramic coffee mug, matte black glaze, 350 ml capacity" — not "mug"
- "Silver hoop earrings, 40 mm diameter, sterling silver" — not "earrings product photo"
- "Men's running shoes, grey and orange, mesh upper, size UK 10" — not "shoes image"
Avoid keyword stuffing. Google Search Central explicitly warns against "filling alt attributes with keywords" rather than descriptions. An alt text of "buy cheap coffee mug online ceramic mug best price" is not a description — it is spam, and it will not rank or serve accessibility. The test is whether the description is genuinely useful to someone who cannot see the image.
Keep alt text under 125 characters where possible. Screen readers typically pause at around 125 characters, and descriptions beyond that point may be truncated in the announcement.
What WCAG 2.1 AA requires alongside the SEO case
WCAG criterion 1.1.1 requires that every non-decorative image has a text alternative that conveys equivalent information. For e-commerce, this means product images need alt text that describes the product with enough detail for a blind shopper to make an informed decision.
The SEO requirement and the accessibility requirement are functionally identical for product images — a good alt text serves both. The difference lies in edge cases: a decorative banner image that carries no product information should have alt="" for accessibility (telling screen readers to skip it), but an empty alt attribute also means no SEO signal from that image. For purely decorative images, that is the correct trade-off.
For informative product images, there is no trade-off. Good alt text is simultaneously good accessibility and good SEO.
Run a free WCAG scan on your storefront to see exactly where you stand — then start free to fix image alt text at scale.