The ALT Quality Score: full methodology
Every rule and every threshold, published. No AI, no black box.
The ALT Quality Score rates the alt text of a page from 0 to 100 using published, deterministic rules: missing scores 0, generic 20, too short 40, duplicate 60, too long 70 and correct 95 to 100. The page score is the simple average, decorative images excluded, and a page with zero issues scores exactly 100.
How the score is computed
The same engine runs in the free online checker and in the WordPress plugin. Every image goes through three steps.
Normalize
The alt text is trimmed, lowercased and stripped of accents and punctuation. This is what makes detection reliable: “Photo” and “photo” become the same thing.
Classify
Each image runs through the rules in order and the first match wins: decorative, missing, generic, too short, duplicate, too long, then correct. One category per image, no exceptions.
Average
The page score is the simple average of the image scores, decorative images excluded. A page with zero issues scores exactly 100. Grades follow: A from 90, B from 80, C from 70, D from 60, E from 50, F below.
The scoring table
Seven statuses, two byte thresholds (8 and 125) and a fixed score for each category.
| Status | Rule (first match wins) | Score per image |
|---|---|---|
| Decorative | role="presentation" or aria-hidden="true" | excluded from the average |
| Missing | empty after normalization | 0 |
| Generic | dictionary term, known pattern or lone word containing a digit | 20 |
| Too short | under 8 bytes | 40 |
| Duplicate | identical alt on 2+ images of the page | 60 |
| Too long | over 125 bytes | 70 |
| Correct | everything else (+5 if ≥ 8 bytes, +5 if ≥ 2 words) | 95–100 |
How each status is detected
Missing
The attribute is absent, or its value contains only whitespace once normalized. The most damaging issue: search engines have nothing to index and screen readers skip the image entirely. Scored 0.
Generic
Three detectors work together: a multilingual dictionary covering 7 languages (“image”, “photo”, “logo”), 32 universal patterns such as camera file names and screenshot titles, and a heuristic that flags a lone word containing a digit. Scored 20.
Too short, too long
Under 8 bytes, a description rarely describes; over 125 bytes, screen readers may truncate. Good alt text usually runs 5 to 15 words. Scored 40 and 70.
Duplicate
The same normalized alt on two or more images of the page, a classic on product grids. Search engines cannot tell the images apart. Scored 60.
Why there is no AI in the loop
A score you cannot reproduce is an opinion. The ALT Quality Score is fully deterministic: same page, same rules, same result, every time. The thresholds are published, no image is sent to any external service and you can verify any classification by hand.
That choice also keeps the tool free and fast, in the checker as in the plugin. Our benchmark of 2,449 images is built on these exact rules.
See your score now
Paste any URL in the free checker and get your grade in about ten seconds. On WordPress, the free plugin scores your entire media library.
Alt text audit and bulk management for WordPress. Quality score, bulk fixing, accessibility. Built in France by Lyode.
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