Alt Text Explained: What to Write, What to Skip, and Why

Today’s blog post explores how to write effective alternate text for everything from decorative images and photos to charts, flowcharts, diagrams, logos, and images of text. Alt text remains one of the most common accessibility failures in audits, and it is also one of the areas where well-meaning people often get it wrong.

Video Guide

The challenge starts with understanding what an image contributes to the document. Software can often recognize a person, bicycle, chart, or logo, but the person creating the content needs to determine why that image appears there and what information a reader needs from it.

That leads to the most important principle in alt text: Context, not the picture alone, determines the correct alternative text.

What Alt Text Really Is

Alternate text provides a text alternative that conveys the function or meaning of non-text content. Screen readers can read it aloud, browsers can display it when an image fails to load, and search engines and AI tools can use it to better understand content.

Automated tools can identify objects and visual elements, but they do not know the role an image plays in a specific document. The author has that context and needs to decide what information the alternative should communicate.

WebAIM offers a useful test: if you could not use the image, what would you put in its place? The words you would use to fill that gap can guide you toward effective alt text.

Alt Text Is an Accessibility Requirement

Alt text is a core accessibility requirement, and several legal and technical standards shape how organizations handle non-text content:

ADA Title II: The Department of Justice updated its rule in 2024 and requires covered state and local government entities to meet WCAG 2.1 AA, with compliance deadlines that depend on population size.
Section 508: Federal agencies and covered federal technology must meet applicable accessibility requirements based on WCAG standards.
Section 504: Organizations covered by federal disability nondiscrimination requirements, including many healthcare and education organizations, have digital accessibility obligations.
Colorado House Bill 21-1110: Colorado requires state and local government entities to make digital services, websites, documents, and other information technology accessible to people with disabilities.
WCAG 1.1.1, Non-text Content: Meaningful non-text content needs an appropriate text alternative.
WCAG 1.3.1, Info and Relationships: Content must preserve meaningful structure and relationships, which becomes especially important with tables, diagrams, and other complex visuals.
WCAG 1.4.5, Images of Text: Authors should use real text instead of images of text when they can present the information as text.

Missing or poor alt text appears frequently in accessibility audits, which makes this a practical issue as well as a compliance issue. Authors need a reliable way to decide what each image requires.

The Four Main Types of Images

A simple decision framework places most images into four categories:

Decorative images: These add visual interest without contributing information. Give them empty alt text or mark them as decorative where the authoring tool allows it.
Images of text or logos: Use the meaningful words as the alternative. Leave out descriptions of fonts, colors, and styling unless those details carry information.
Informative images: Describe the information the image contributes to the surrounding content. Let the image’s purpose determine what details belong in the description.
Complex images: Charts, graphs, diagrams, flowcharts, and infographics usually need a short alt text plus a longer description that makes the full information accessible.

Once you identify the category, you can choose the appropriate treatment. Complex images require the most attention because they often communicate data or relationships that one short sentence cannot fully capture.

Give Complex Images Two Layers of Information

Complex images such as charts and diagrams need two layers of accessible information. Short alt text should identify the visual and state its main takeaway in one concise sentence. A long description should provide the supporting data or relationships in an accessible table, structured list, or other appropriate format.

This approach gives screen reader users both the key message and the details they need to analyze the content. For example, instead of using “bar chart” as the alt text for a research figure, summarize the result in the short alt text and provide the complete data in an accessible table.

How to Write Alt Text for Line Graphs

A weak alt text such as “line graph” identifies the type of visual but leaves out its message. A useful short alt text tells the reader what is happening in the data.

Our example shows two series across seven years. Automated-only testing stays relatively flat, moving from 38% to 52%, while manual-plus-automated testing climbs from 17% to 74% and overtakes the automated-only series in 2023.

A short alt text could say:

Line graph showing automated-only testing staying relatively stable while manual-plus-automated testing rises from 17% to 74% and overtakes automated-only testing in 2023.

The long description should then provide both series, all seven years, and every value in an accessible table. Readers get the main trend first and can inspect the detailed numbers afterward.

Focus on the trend and the comparison. Details such as solid or dashed lines and line colors only belong in the description when they carry meaning that the reader needs.

How to Write Alt Text for Flowcharts and Diagrams

Flowcharts communicate relationships, sequence, and direction. A description of rectangles, diamonds, and arrows tells a reader what the drawing looks like while leaving the actual workflow unclear.

A useful short alt text identifies the process and its key structure. For example:

Flowchart showing a four-step PDF remediation workflow with one decision point.

The long description should reproduce the workflow as an ordered or nested list. It should preserve the sequence, explain the decision point, and identify where each branch leads.

When a decision creates “yes” and “no” outcomes, include both paths. The reader needs enough information to follow the same process that the diagram communicates visually.

How to Write Alt Text for Pie Charts

Don’t describe the wedge colors. Focus on the meaningful information that comes from the categories and their values. The short alt text should state the chart’s main finding.

If “Images missing alt text” represents the largest category at 38%, the short description could say:

Pie chart showing images missing alt text as the largest accessibility issue at 38%.

The long description should then provide all categories and values in a table. Arrange the information in a useful order, such as by size or category, so the reader can understand the data without mentally reorganizing it.

This approach gives the reader direct access to the information encoded in the chart. Color descriptions should appear only when color itself communicates something essential.

Everyday Alt Text Cases

The same context principle applies to everyday images:

Decorative images: Give decorative swirls, flourishes, and other visuals that add no information empty alt text or mark them as decorative. If removing the image would not change the reader’s understanding, it likely does not need a description.
Informative photos: Describe the meaningful content concisely and let the surrounding context determine which details matter. For example: “A man using a white cane walks along a city sidewalk past storefronts.”
Simple charts: Even a basic chart needs accessible data. For a four-bar chart showing satisfaction rising from 42% in Q1 to 78% in Q4, the short alt text could say, “Quarterly satisfaction increased from 42% in Q1 to 78% in Q4.” Provide the individual values in a small data table beside or below the chart.
Images of text and logos: Provide the meaningful words rather than describing fonts or brand colors. For example, if a banner says “The Accessibility Guy: Your Access to Compliance,” use that wording as the alt text. When possible, use real text instead of an image of text.

The Same Photo: 3 Different Alt Texts

The same image can require different treatment depending on its role. If a photograph illustrates an article about the person shown, describe the person in a way that supports the article.

If an adjacent caption already provides the same information, empty alt text can prevent the screen reader from repeating it. If the image functions as a link to a biography, the alt text should communicate the link’s destination or purpose.

The image stays the same while its function changes. That is why authors need to write alt text in the context of the page rather than assigning one permanent description to an image file.

Common Alt Text Mistakes

Several mistakes appear repeatedly in accessibility reviews:

Using “image of” or “picture of”: Screen readers generally identify images automatically, so the description can begin with the meaningful content.
Using a file name: Text such as IMG_4927.jpg gives the reader no useful information.
Over-describing decorative images: Mark decorative content appropriately so readers can move through the page efficiently.
Giving charts generic labels: “Bar chart” or “line graph” identifies the format without communicating the takeaway or data.
Describing diagram shapes instead of relationships: Explain the process, sequence, connections, and branches represented by the visual.
Repeating a nearby caption: When a caption already provides the image’s information, empty alt text can prevent unnecessary repetition.

A Practical Way to Review Any Image

Use this quick review process to decide what an image needs:

Ask what the image contributes: If it adds visual interest without meaningful information, treat it as decorative.
Identify text or branding: If the image communicates words or brand identity, provide the meaningful text.
Describe informative images concisely: If the meaning fits in a short description, write alt text that reflects the image’s role in context.
Use two layers for complex images: For visuals with detailed data, sequences, relationships, or branches, provide a short takeaway plus an accessible table, list, or other structured description.
Keep the goal in mind: Give readers access to the same meaning and function the visual provides.

Strong alt text starts with context and gives every reader access to the meaning the image was meant to convey.

Let me be your champion for inclusion. I offer tailored solutions (and self-paced courses!) to ensure your documents meet and exceed compliance expectations. For more detailed insights, tutorials, and in-depth discussions on accessibility and related topics, don’t forget to check out my YouTube channel: The Accessibility Guy on YouTube. Subscribe for regular updates!