An informative image needs a text alternative conveying its relevant information. A decorative image should usually avoid interrupting reading with an unnecessary description. An image used as a link needs an understandable purpose.
There is no universal rule requiring every alternative text to contain an exact number of characters. Be as concise as possible without losing essential information. Put longer explanations where readers can find and read them.
Start by explaining why the image is in the document. A photograph might demonstrate an assembly step, identify a building or create atmosphere. The same image can therefore need different treatment in different contexts. Alternative text should support the current task, rather than inventory everything visible.
Suppose an instruction shows two sockets and a cable. “Two black sockets and a white cable” describes appearance but not assembly. If the purpose is to show where the cable connects, the description needs that instruction. Verify the correct socket against the image and surrounding text; do not guess from familiar products.
Read the paragraph, caption and alternative text together. Repeating the same explanation three times makes listening laborious. Moving an important instruction into visible text can benefit more readers. The image alternative can then be shorter and convey what still needs explaining.
Before
A red door in a grey wall.
After
Accessible entrance on the courtyard side. The door opener is to the right of the door.
Why this works In visitor instructions, access matters more than door colour. Include only details supported by the image and checked background information.
An organisation chart may contain names, departments and reporting relationships. A floor plan may contain dimensions, doors and routes. Determine which relationships readers need to investigate. A sentence naming the image type does not convey them.
One solution combines a short description with a longer structured explanation in the document. A chart may use a table for exact values and a paragraph for its main pattern. A flowchart may use a numbered description of decisions and destinations. Choose a representation suited to the information, rather than a fixed character limit.
Also establish how readers find the longer explanation. Place it coherently or provide a clear reference. “See the table” is inadequate when six tables have no names, or the table itself is an untagged image.
An automated description can be a draft. Check numbers, directions, names and claims the image does not actually show. Never let a model invent a ramp, trend or safety feature because it seems plausible.
Be particularly careful with causes, emotions, identity and safety. A chart can show an increase without explaining its cause. A photograph can show a person without establishing their name. Include only what the description needs and the evidence supports. Ask the document owner about uncertainty rather than writing a confident guess.
For a chart, short alternative text can identify its subject and point to a nearby explanation. A data table can provide exact values. Readers need to understand the finding and inspect details when those details matter.
Sources
Examples are our own unless stated otherwise. These guides distinguish legal requirements, standards and practical advice. Passing an automated check is not a complete assessment of document accessibility.