91ÁÔĆć

How to write good alt text

Alt text is a short description that helps readers who cannot see an image understand the information it provides. It can be read by screen readers, read-aloud tools and refreshable braille displays. It is also displayed if the image cannot be loaded due to technical issues.

For research figures and images, good alt text describes the image in context. It should focus on the information the reader needs from the image, rather than describing every visual detail.

Where AI-generated alt text is provided, treat it as a starting point. Authors should review and edit it before publication to make sure it is accurate, meaningful and appropriate for the surrounding content.

Checklist for good alt text

Before approving or submitting alt text, check that it:

  • identifies the image type for example, a chart, map, diagram, microscopy image
  • describes the image’s purpose in context
  • includes any key results
  • avoids unnecessary visual detail
  • is accurate, with no invented, guessed or overstated details
  • does not repeat the caption

Common questions

What makes alt text useful?

Useful alt text communicates the purpose and meaning of an image in context.

  • It should help readers understand:
  • what information the image provides
  • why the image is relevant to the article, chapter or book
  • what key result, relationship, process, structure or finding the reader needs to take from it

Alt text should not try to describe every visual detail unless those details are relevant to the meaning.

There is often more than one acceptable way to write alt text. The aim is not to create a perfect description, but to provide a clear and meaningful description that gives readers access to the information the image provides.

Do I need to check alt text if it has been AI-generated for me?

Yes. Some manuscript proofing systems may suggest alt text for images and figures, but this still needs to be checked before publication.

AI-generated alt text can be useful, but it may not always capture the purpose, context or scientific meaning of an image.

It may:

  • misread labels, axes, values, legends or panels  
  • miss important visual relationships  
  • omit important information  
  • include details that are guessed, invented or overstated  
  • describe what the image looks like without explaining what it means

Author review is important because authors are best placed to check whether the description accurately reflects their chosen image and its role in the surrounding content.

When reviewing AI-generated alt text, edit it so that it reflects the figure, the caption and the surrounding discussion.

What is the difference between alt text, captions and surrounding text?

Alt text, captions and surrounding text work together, but they do different jobs. 

Caption 

A caption is visible to all readers. It may provide context, identify the image type, explain methods or give information needed to interpret the image. 

Alt text 

Alt text gives a concise description of the image for readers who cannot access it visually. It should not simply repeat the caption, although some overlap may be appropriate where the information is needed to understand the image. 

Surrounding text 

The surrounding text should explain important findings, especially where an image contains key data, trends, relationships or conclusions. 

Important information should not be available only inside an image. If a figure contains a key result, trend, comparison or conclusion, that information should also be available in the caption, surrounding text, long description or accessible data table.

Is grammar or text formatting important in alt text? 

Yes. Alt text should be clear, accurate and written in plain text. Use correct grammar, punctuation and terminology that matches the article, chapter or book. 

Avoid formatting such as bold, italics, superscript or subscript. Where needed, write symbols, numbers, units and mathematical expressions in a way that will be clear when read aloud. 

End the alt text with a full stop to ensure that screen readers pause before continuing to read other content. 

Does alt text differ by the type of image or figure? 

Yes. Different image types need different kinds of description. A chart may need the variables, axes and main trend. A map may need the area shown and spatial pattern. A medical image may need the modality, body part, view and key finding. 

Use the guidance below to check what to include for different image types.

What privacy or ethical considerations do I need to take into account?

Alt text should include only information that is relevant, appropriate and supported by the image and surrounding content. 

Do not identify people in photographs unless this is necessary, appropriate and consented to. Do not include patient-identifiable information, and take care with clinical, fieldwork, community or sensitive images.

Can I change alt text post publication? 

No. 91ÁÔĆć does not make  to alt text. To ensure descriptions are accurate, you must review AI-generated text carefully.

Guidance by image type

Different types of research images need different kinds of description. Use the sections below to check what to include.

Charts, graphs and plots

  • Name the chart or plot type where useful. Describe what is being compared or measured, and summarise the main trend, relationship, distribution or difference. 
  • Include axes or variables where they are needed to understand the image. Mention outliers, uncertainty, error bars or notable differences where these are important to the meaning of the figure. 
  • For complex charts or plots, such as heatmaps, box plots, scatter plots, volcano plots, survival plots or spectrograms, focus on the main pattern or finding rather than listing every data point.

Example pattern

Bar chart comparing [variable] across [groups], showing [main trend or difference].

Maps

  • Name the type of map where useful. Describe the area shown and explain what the map represents.  
  • Summarise the main spatial pattern, distribution, movement, concentration or relationship. 
  • Explain what colours, markers, regions or routes represent where these are important. 

Example pattern

Map of [area] showing [what is mapped], with [main distribution, concentration, route, boundary or movement].

Diagrams, workflows and relational figures

  • Name the type of diagram. Describe the key parts and how they are connected. Focus on the relationship, process, hierarchy, sequence or structure, rather than only the visual layout. 
  • This may apply to flow charts, Venn diagrams, decision trees, dendrograms, neural network diagrams, system architecture diagrams and similar figures.

 Example pattern

Flow diagram showing [process], moving from [first stage] to [final stage], with [key relationship, decision point or feedback loop].

Technical illustrations

  • For technical illustrations, describe the object, apparatus, structure or physical system shown. Focus on the key components, labels, orientation or spatial relationships that are needed to understand the image. 
  • This may apply to engineering drawings, blueprints, architectural plans, anatomical illustrations, apparatus illustrations, circuit layouts, product drawings or other technical images where the physical form or arrangement is important. 

Example pattern 

Technical illustration of [object, apparatus or structure], showing [main components] and their [position, relationship or function].

Medical images

  • State the imaging type and body part or subject. Where relevant, include the imaging plane or view, such as axial MRI or frontal X-ray, and any notable feature or finding that is important in context. 
  • If the image is used to demonstrate a condition, summarise the key finding. This may include whether the image is an X-ray, CT scan, MRI, ultrasound or other clinical image; which anatomical region is shown; and whether the view or plane is important for interpreting the image.

 Example pattern

Axial MRI scan of the brain showing [relevant feature or abnormality] in [region].

Microscopy and similar scientific images

  • Identify the subject, specimen or material, and include the imaging method or magnification where this is relevant.  
  • Describe the notable structures, pattern or comparison without attempting to catalogue every visible feature. 
  • This may apply to confocal micrographs, electron micrographs, fluorescence micrographs, optical micrographs and other scientific images. 

Example pattern

Fluorescence micrograph of [sample or tissue], showing [labelled structure or pattern] with [main feature or difference].

Equations, mathematical expressions and chemical structures

  • Where possible, equations, mathematical expressions and chemical structures should be provided as accessible text rather than as images. 
  • If an equation or expression is included as an image, describe it in the order it should be read aloud. Use words for symbols where needed, such as “equals”, “over”, “squared” or “to the power of”. 
  • For complex equations or chemical structures, provide a concise overview in the alt text and include fuller explanation in the caption or surrounding text. 

Example pattern

Equation reading: [spoken form of equation].

Photographs

  • Describe the subject, setting, action or condition shown, focusing on what is relevant to the content. 
  • Avoid unnecessary details about appearance, clothing, age, race, gender or other personal characteristics unless they are directly relevant and appropriate. 

Example pattern

Photograph of [subject] showing [relevant action, condition, object or setting].

Tables or text shown as images

  • Tables and text should usually be provided as accessible text, not as images. 
  • If an image of a table or of text cannot be avoided, summarise what it compares, lists or shows. If the full content is needed, provide it in an accessible table, caption, or surrounding text. 

Example pattern

Table comparing [variables or categories], showing [main comparison or finding].

Complex or multi-part images

  • For images that combine several elements, such as diagrams, plots, photographs and text, summarise the overall purpose first. Then identify the main parts or panels where this is needed to understand the figure. 

Example pattern

Multi-panel figure showing [overall purpose], including [main panels or image types] to compare [main finding or relationship].

Examples

Line chart comparing trends

Example line chart used for the alt text guidance

Alt text 

Line chart comparing 2023 and 2024 website visits from January to June, showing an overall upward trend. 2024 has higher visits than 2023 every month. Both lines increase over time, with 2024 reaching about 35K in June compared to 25K for 2023.” 

Why it works 

It explains what the lines represent, what is being compared, and the main trend.

Bar chart across groups

Example bar chart used for the alt text guidance

Alt text 

Bar chart showing customer satisfaction score across departments. Support is highest at 90%, followed by Marketing at 80%, Sales at 75%, IT at 70%, and Product is lowest at 65%. 

Why it works 

This avoids vague wording and gives the chart type, comparison and key finding. 

Describe areas by name and meaning

Example map used for the alt text guidance

Alt text 

Map of the UK highlighting the north west region showing the programme's focus area. 

Why it works 

This identifies the highlighted area by name and explains what the highlight means. 

Alt text that adds more than the caption

Example electrocardiogram trace with caption reading electrocardiogram trace, used for the alt text guidance.

Alt text 

Electrocardiogram trace showing a regular repeating heartbeat pattern with evenly spaced peaks and no obvious irregularity. 

Why it works 

This adds useful information from the image, rather than simply repeating the caption. 

Example images generated by AI for illustrative purposes only.