The honest answer is "as short as it can be while still doing its job". That sounds like a dodge, so this post makes it concrete: where the popular numbers come from, which of them hold up, and what a good length looks like for a photo, a screenshot and a chart.
The 125-character rule is a myth
Many guides say screen readers stop reading alt text after 125 characters. They do not. Accessibility consultant Eric Eggert looked into the claim in There is no character limit for alt text (archived copy), drawing on screen reader tests by Terrill Thompson and his own: none of the screen readers cut the text off. JAWS splits long alt text into chunks and reads them all, which may be how the idea started.
So 125 is not a technical limit. It is a reasonable rough size for a short description, and nothing more.
Why shorter is still better
Length matters for a different reason: alt text is hard to listen to when it runs long.
- You cannot skim it. Alt text is plain text with no headings or paragraphs. Adrian Roselli shows in Long Alt that it has no structure to move around in, which makes long descriptions hard to follow.
- You cannot easily pause it. The Nielsen Norman Group points out in Alt Text: What to Write that screen reader users cannot pause and resume in the middle of alt text without going back to the start, and that people skip alt text that does not seem useful right away.
- The start does the most work. The W3C's images tutorial advises putting the most important information at the beginning. If the point is in the last sentence, a listener who has moved on misses it.
The guidance that follows from this is consistent. NN/g suggests alt text for a simple image should not be "much longer than around 150 characters". The W3C asks for "the most concise description possible of the image's purpose" and suggests a separate long description when more than a short phrase or sentence is needed.
A practical length for each kind of image
Rather than counting characters, think in sentences.
Simple photos: one or two sentences
Most photos have one main point. Say it first, then add the detail that matters for this post.
- Too short: "Dog."
- Good: "A grey whippet asleep on a sofa, wrapped in a knitted blanket with only its nose showing."
- Too long: "A beautiful, cosy and heartwarming photo of my adorable grey whippet, who is called Pepper and is four years old, lying fast asleep on our new green velvet sofa in the living room on a cold Sunday afternoon, wrapped up in a blue and white knitted blanket that my grandmother made, with just the tip of her nose poking out."
The good version is about 90 characters. The long one adds opinion ("beautiful", "adorable"), backstory the image does not show, and detail nobody needs. If the dog's name matters, it belongs in the post.
Images with people or a scene: two or three sentences
Group photos, events and places usually need a little more. Lead with what is happening, then who and where.
- Good: "Six volunteers in matching orange vests planting saplings along a riverbank. Two of them are carrying a young tree between them, and a sign behind reads 'Riverside restoration, phase two'."
That is two sentences and still under 200 characters. The quoted sign carries information a sighted reader would get, so it stays.
Screenshots and text-heavy images: as long as the text needs
When the image is mostly text, such as a screenshot of a post, a quote card or a recipe, the text is the content. Here the right length is set by what someone needs to read, not by a target.
Short text should be quoted in full. Long text, such as a page of terms, can be summarised with the key lines quoted. The platform's limit is the real ceiling here, and it varies a lot: see the alt text character limits for each platform. The guide to alt text for screenshots goes into detail.
Charts and graphs: the takeaway in a sentence or two
A chart's alt text should give its message, not every data point.
- Good: "Line chart of monthly visitors in 2026. Visits rise steadily from about 4,000 in January to 11,000 in September, with a dip in August."
If readers need the full data, put it in a table or text next to the chart, as the W3C's guidance on complex images suggests. More on that in alt text for charts and graphs.
How to make long alt text shorter
If your description keeps growing, run through these cuts:
- Remove "image of" and "photo of". The screen reader already announces an image. Do keep the type when it matters, such as "Screenshot" or "Bar chart".
- Remove what the post already says. If your post reads "Our new office in Leeds", the alt text does not need to say "our new office in Leeds" again. Describe what it looks like.
- Remove opinions. "Stunning", "beautiful" and "amazing" tell the listener how you feel, not what the image shows.
- Remove detail that does not serve the post. A photo shared to show a new haircut does not need the brand of the chair.
- Move the main point to the front. Even if you keep the length, a listener who stops after one sentence should have what matters.
When short is too short
The opposite mistake is common too. "Chart", "Screenshot", "Me" and "Sunset" tell a listener almost nothing. A good test is the one Eric Eggert gives: if someone described the image to you over the phone using only this text, would you picture something close to it? If not, add the detail that would get you there.
Automatic alt text is a common source of this. When you do not write your own, Instagram fills in a description from object recognition, and Facebook's automatic version starts with "Image may contain:" followed by what it detected. That names objects but rarely says what the image is about.
The short version
- No screen reader truncates alt text at 125 characters.
- For most photos, one or two sentences and roughly 150 characters is plenty.
- Put the most important part first.
- Text-heavy images can be longer, within the platform's limit.
- Complex charts need a short alt text plus a longer description somewhere readers can see it.
AltKit writes to the same rules: usually two or three sentences for a photo, more for a text-heavy image, and never over 1,000 characters. If you want a starting point to edit, try the web creator, and read the result against the checklist in Good alt text before you post.