What Is Alt Text? — Free Tool
Alt text describes an image for people who cannot see it. Learn the length platforms allow, how to write a description that is actually useful, and why alt text written for algorithms fails the people it exists for.
A short written description of an image, read aloud by screen readers. It makes posts accessible to blind and low-vision users and gives platforms extra context about what the image contains.
What it is
Alt text is a short written description of an image, attached to the image rather than shown in the post. Screen readers speak it aloud, so it is how a blind or low-vision follower knows what your photo shows. It also appears when an image fails to load.
How it is measured
Most platforms have a field for it. On Instagram it is under advanced settings when composing, X and LinkedIn offer an add description button on the image, Facebook lets you override its automatic text, and Mastodon prompts for it before you can post. Allowances run to a few hundred characters and in some places over a thousand, but length is not the target. Describe the subject, what is happening, and any detail the post depends on. If there is text in the image, put the text in the description, because that is the part automatic tools miss most often and the part that usually carries the message.
Commonly misunderstood
The habit worth naming is writing alt text for the algorithm. People fill the field with keywords and hashtags, which produces a stream of disconnected words in a blind user's ear: useless as a description and plainly rude, since the field exists for them. It also does very little for reach. The second mistake is starting with image of, which a screen reader has already announced. The third is skipping it on carousels, where each slide needs its own description and only the first tends to get one.
When it matters
Every image post. It takes fifteen seconds, it is the accessibility baseline, and in a growing number of jurisdictions public sector accounts are required to do it.
Features
- Plain definition of alt text and who is actually reading it
- Where to add it on Instagram, X, LinkedIn, Facebook and Mastodon
- How to describe an image well: subject, action, relevant detail, text in the image
- The keyword-stuffing habit that makes alt text useless and insulting at the same time
- Links to related terms so alt text sits next to captions and image formats
Frequently asked questions
Who reads alt text?
Screen reader users, primarily blind and low-vision people, who hear it spoken. It is also shown when an image fails to load, and platforms read it for context, but the first group is the reason it exists.
How long should alt text be?
A sentence or two. Platforms allow more, often several hundred characters, but a listener has to sit through every word, so describe what matters and stop.
Should I put keywords in alt text?
No. Stuffing keywords into a description makes it useless to the person hearing it and does not meaningfully help ranking. Describe the image.
Do I need alt text if the caption describes the image?
If the caption fully describes it, you can keep alt text brief or mark the image decorative where the platform allows. Usually the caption says something different, so write both.
Does auto-generated alt text count?
It is better than nothing and often wrong. Automatic descriptions miss text in the image, brand names and the point of the photo, so overwrite them on anything that matters.
