Alt Text Length: Context Over Character Counts

Accessibility checkers in learning management systems and content platforms often enforce rigid rules about alt text. Canvas, for example, caps alt text at 120 characters. That number isn’t backed by any WCAG guideline — it’s a product decision baked into the tool. Some claim screen readers stop announcing after 125 characters, but that’s not true today. The real guidance is messier and far more useful.

The consensus among accessibility practitioners is that there is no hard limit. Instead, the length should be driven by the image’s purpose and the surrounding context. What matters is whether the description serves the user’s needs, not whether it hits a specific count.

The Range of Reasonable Lengths

Different experts frame it differently:

  • Jake Archibald approaches length in terms of emotional impact — detail is good, but too much can obscure the point of the image.
  • Dave Rupert compares good alt text to short, succinct paragraphs — brief, but complete enough to convey the idea.
  • Carrie Fisher suggests a 150-character limit as a mental check — not because screen readers truncate text, but as a signal that the description might be overreaching.
  • Daniel Göransson argues that context determines which details are worth explaining, generally favoring conciseness.

There’s a common thread: the right length is a judgment call, and it varies with the content. Some images need a sentence; others need a phrase. Automation can’t reliably make that call, and neither can a fixed character cap.

Repetition Can Be the Right Answer

A useful example comes from writing alt text for a series of speaker headshots. As more speakers were added, the descriptions naturally began to sound alike. That feels like a failure of creativity, but it’s actually a faithful reflection of the visual experience. A sighted person scanning a page full of portraits sees the images blur together after a while. If a screen reader user hears repetitive descriptions, they’re getting an equivalent experience — not a deficient one.

That perspective reframes alt text writing as an exercise in parity, not just description. The goal is to mirror the visual experience, including its monotony, not to manufacture uniqueness where none exists in the source.

Tooling Reveals Its Limits

Platforms are improving how alt text is handled. For instance, Twitter now exposes UI that displays alt text to users:

Now if only there was more contrast between that text and the background… a11y is hard.

That’s a step forward in transparency, but it doesn’t solve the deeper problem. The core issue isn’t where alt text lives in the interface — it’s that any fixed rule about length misses the point. Context is king, and that’s exactly the kind of nuance that can’t be codified or automated.