PDF contrast: check text and form fields against WCAG
Drop in a PDF and the tool measures the contrast ratio of every text run against the background it actually sits on, plus the borders and labels of form fields. You get a verdict for level AA, a list of colour pairs, level AAA for information and a page preview with everything below the threshold marked.
This is an automatic, indicative measurement: the tool calculates the contrast ratio for the text and form fields it can read from the file. It is not an accessibility audit and not a certification of conformance with WCAG, PDF/UA or any legislation; it does not assess document structure, alternative text, reading order or content. Text converted to outlines, scans and the content of images stay outside the measurement, and CMYK colours and ICC profiles are converted to screen values. The absence of elements below the threshold does not mean the document is accessible. Confirm the result in the environment your readers use; the decision to publish the file is yours. The rules for using the Tools are set out in the Terms of Service.
Measurement result
Document
Colour pairs
| Sample | Colours | Ratio | Class | Result | Elements | Notes |
|---|
Page preview
Preview legend: a red frame is an element below the AA threshold, orange is an estimate on a non-uniform background, green is an element that passes, and a grey dashed frame is a place left out of the verdict.
This is an automatic, indicative measurement: the tool calculates the contrast ratio for the text and form fields it can read from the file. It is not an accessibility audit and not a certification of conformance with WCAG, PDF/UA or any legislation; it does not assess document structure, alternative text, reading order or content. Text converted to outlines, scans and the content of images stay outside the measurement, and CMYK colours and ICC profiles are converted to screen values. The absence of elements below the threshold does not mean the document is accessible. Confirm the result in the environment your readers use; the decision to publish the file is yours. The rules for using the Tools are set out in the Terms of Service.
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Contrast ratio and the WCAG thresholds: 4.5:1, 3:1 and 7:1
A contrast ratio compares the lightness of two colours: the text and the background it sits on. It is derived from the relative luminance of both colours in sRGB and falls between 1:1 (two identical colours) and 21:1 (black on white). WCAG 2.1 success criterion 1.4.3 requires at least 4.5:1 for normal text and 3:1 for large text at level AA, while criterion 1.4.6 raises this at level AAA to 7:1 and 4.5:1 respectively. Large text means a type size from 18 pt, or from 14 pt when bold; in a PDF the point of the standard is the same point as the point of the document, so nothing has to be converted. Criterion 1.4.11 adds a separate 3:1 threshold for non-text elements, which covers the boundary of a form field, a border or a fill alike, the very thing that tells a user where to click and type. Thresholds are compared on the raw value, while the ratio on screen is truncated down to two decimals so that 4.499 never looks like a passing 4.5.
What this tool measures in a PDF
The text colour is read straight from the page content stream, together with the type size, the weight and the colour space. The background colour comes from a sample of the page rendered the way a reader displays it: under the letters there may be a rectangle, a photo, a gradient or a field fill, so reading a colour out of the file is not enough on its own. From forms the tool collects field boundaries (the 3:1 threshold), field fills and the colour and type size from the default appearance of the field text. When a field carries its own stored appearance, the image sample decides, and the colours declared in the file only serve for comparison and for reporting a discrepancy. The boundary of a field may be the border just as well as the fill: it is enough that one of them reaches 3:1 against the page, and a field with no visible boundary at all is reported as a failure. A read-only field is measured as well: it conveys content, so the same 4.5:1 threshold applies to it, and greyed-out text in such a field is one of the most common causes of inaccessible forms. A section background or a coloured band has no threshold of its own, because contrast always concerns a pair of text and background; only a letter placed on that band produces a measurement. CMYK colours and colours with an ICC profile are converted to screen values using the same formula as the preview, so in print the outcome will depend on the profile and the substrate. The boundary of a read-only field is not assessed, because criterion 1.4.11 excludes inactive components; the text in such a field is held to the threshold like any other.
What the tool cannot measure, and why this matters
Text converted to outlines stays outside the measurement, and so do scans and screenshots: to the file they are a drawing or an image with no letters to read. We list such pages separately instead of pretending they are fine. Encrypted files, text covered by another object and text drawn in invisible mode, for example a layer recognised from an image, are outside the measurement too. Transparent text is not the same as invisible: it can be seen on the page, so we measure it from a sample of the page image and show the colour that is actually visible. With transparency the visible colour can end up one lightness level away from the value on paper, because the preview rounds the colour compositing; the difference moves no threshold. The preview and the measurement cover the appearance stored in the file: the coloured field highlight and the red outlines of required fields that a reader adds only while the form is being filled in are not part of the document and are not measured. Checking contrast is both practical and a legal matter: documents published by public sector bodies are covered by the national digital accessibility law implementing the EU Web Accessibility Directive, in Poland the Act on the digital accessibility of websites and mobile applications of public sector bodies, which points to WCAG at level AA, and since 28 June 2025 the European Accessibility Act has also covered market services: e-commerce, banking, transport and e-books, and therefore the invoices, contracts, terms and forms sent out as PDFs. Beyond the law there is plain legibility: on a poor screen, in bright sunlight and in greyscale printing, text that is too light simply disappears.
