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Use mm and cm directly on the CSS property that controls the element you want to size, then set the PDF sheet independently with wkhtmltopdf options. For example, width: 50mm creates a 50-millimeter CSS box, while --page-width 210mm --page-height 297mm defines an A4-sized page. Margins, scaling and the installed wkhtmltopdf build determine how that layout appears in the final PDF, so render and measure a representative page when physical accuracy matters.
Use metric units in the CSS itself
Millimeters and centimeters are CSS absolute length units. The CSS Values and Units specification defines 1mm as one tenth of 1cm. Put the unit on every declaration that represents a physical dimension; a bare number is not a millimeter or centimeter value.
.label {
width: 50mm;
height: 30mm;
margin: 1.2cm;
padding: 2mm;
border: 0.3mm solid #222;
}
This sizes the element in the document layout. It does not change the PDF paper. A class can therefore be 50mm wide on either an A4 page or a custom 100mm-wide page; the page geometry is a separate wkhtmltopdf setting.
Which CSS properties accept these units?
widthandheightfor the content box (or the border box when you usebox-sizing: border-box).padding,margin,top,right,bottomandleft.- Border widths, column gaps and other length-valued properties.
Remember that padding and borders add to the outer size under the default content-box model. If a label must remain exactly 50mm including its padding and border, use:
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.label {
box-sizing: border-box;
width: 50mm;
height: 30mm;
padding: 2mm;
border: 0.3mm solid #222;
}
How CSS millimeters map to pixels
WebKit’s historical absolute-unit convention treats 1in as 96 CSS pixels. Consequently, 1cm is 96/2.54, approximately 37.795 CSS pixels, and 1mm is 96/25.4, approximately 3.7795 CSS pixels. These conversions explain the layout model; they are not a guarantee that every printer or downstream wkhtmltopdf build will produce identical measured output.
| CSS value | CSS-pixel equivalent under the 96-pixel-per-inch convention | Typical use |
|---|---|---|
| 1mm | 96/25.4 (about 3.7795px) | Fine spacing, borders and labels |
| 1cm | 96/2.54 (about 37.795px) | Margins, cards and larger blocks |
| 1in | 96px | Reference for absolute-unit conversion |
Use the metric unit in your source rather than converting everything to pixels yourself. That keeps the intent visible and lets you change the page or component size without recalculating CSS values.
Set the PDF page separately
wkhtmltopdf provides a paper-size preset and explicit page dimensions. A4 is the documented default page size, but specifying it makes scripts self-documenting. The following command creates a 210mm by 297mm page:
wkhtmltopdf
--page-width 210mm
--page-height 297mm
input.html output.pdf
For a standard format, use the preset instead:
wkhtmltopdf --page-size A4 input.html output.pdf
Use explicit width and height for labels, tickets or other nonstandard sheets. Keep the page dimensions and the CSS component dimensions conceptually separate: page width is the available sheet, while a CSS width: 50mm is the element’s requested size inside that sheet.
Margins are another independent measurement
Set the four PDF margins explicitly when the printable area matters. The usage documentation describes these margin options in millimeters:
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wkhtmltopdf
--page-size A4
--margin-top 10mm
--margin-right 10mm
--margin-bottom 10mm
--margin-left 10mm
input.html output.pdf
If a block is 50mm wide and the page has 10mm left and right margins, the block is laid out within the remaining content area. Do not add the PDF margins to the element’s CSS width unless you intentionally want extra whitespace around that element.
A complete metric-size example
This document defines an A4 sheet, a 50mm by 30mm label, and a repeated grid. The page options belong to the command; the physical component dimensions belong to CSS.
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<style>
@page { margin: 0; }
html, body { margin: 0; padding: 0; }
.sheet {
width: 210mm;
min-height: 297mm;
padding: 10mm;
box-sizing: border-box;
}
.label {
box-sizing: border-box;
display: inline-block;
vertical-align: top;
width: 50mm;
height: 30mm;
margin: 0 3mm 3mm 0;
padding: 2mm;
border: 0.3mm solid #222;
font: 10pt Arial, sans-serif;
}
</style>
</head>
<body>
<main class="sheet">
<div class="label">Label 1</div>
<div class="label">Label 2</div>
<div class="label">Label 3</div>
</main>
</body>
</html>
wkhtmltopdf --page-size A4 input.html labels.pdf
If you use a full-width sheet wrapper, account for its padding with box-sizing: border-box; otherwise the padding is added outside the declared width.
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Control scaling: smart shrinking and DPI
Smart shrinking is enabled by default in the documented project usage. It lets WebKit scale a page so content fits, but the effective pixel-to-DPI ratio is then not constant. A layout that looks correct on screen can therefore measure differently when it barely fits or overflows.
For a controlled layout, test with smart shrinking disabled:
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wkhtmltopdf
--disable-smart-shrinking
--page-size A4
input.html output.pdf
Disabling it can preserve the intended proportional relationship, but it can also produce overflow or additional pages. There is no universal best setting: choose between automatic fit and more predictable scaling for the document you are generating, then inspect the resulting PDF.
The documented --dpi option defaults to 96. Its documentation also notes that it has no effect on X11-based systems. A DPI flag alone is therefore not proof of exact physical output across operating systems, patched Qt builds or downstream packages.
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- Define the geometry in CSS. Put
mmorcmon every width, height, margin, padding and border that has a physical requirement. - Choose the paper geometry. Use
--page-sizefor a standard format, or both--page-widthand--page-heightfor custom stock. - Set PDF margins deliberately. Keep them separate from the CSS dimensions of your labels, cards or forms.
- Decide how scaling should behave. Start with the default smart shrinking, then compare a render using
--disable-smart-shrinkingif the layout must retain a controlled scale. - Record the environment. Save the wkhtmltopdf version, build and command-line options with the source HTML.
- Measure a representative PDF. Check a page containing the smallest and largest critical dimensions. If the physical size is contractual, measure the generated PDF or a printed sample instead of assuming the CSS value is a certification.
Or skip the browser setup
If your goal is a rendered page image or PDF rather than a locally managed wkhtmltopdf process, ScreenshotNeo provides a single HTTP request. Its API can return PNG, JPEG, WebP or PDF, and its options include page dimensions, margins, full-page capture, waiting rules, custom CSS and JavaScript, resource blocking and other capture controls. See the ScreenshotNeo documentation for the parameter reference.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webp
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
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Troubleshooting metric layouts
The declaration has no effect
Check that the property has a unit and that the selector actually matches the element. width: 50; is invalid for a normal length; use width: 50mm;. Also check for a later rule, inline style or stylesheet that overrides it.
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Everything is smaller than expected
Smart shrinking may be scaling the page to fit. Render once with --disable-smart-shrinking, compare the page count and inspect overflow. Check that the PDF margins and any wrapper padding have not reduced the available content width.
The element is clipped or moved to another page
Verify the sum of the element width, margins, borders and padding against the page’s usable width. A 210mm page with left and right margins has less than 210mm available for content. Reduce the CSS footprint, change the page width, or revise the grid instead of relying on shrinking.
The measured size changes between machines
Record the exact wkhtmltopdf version and build. Patched Qt/WebKit packages can differ, and the documented DPI behavior varies on X11-based systems. Compare the same HTML, fonts, assets, page options and scaling flags, then measure the output again.
Images or web fonts change the layout
Make sure assets are reachable by the rendering process and have stable intrinsic dimensions. A late-loading image or font can change line wrapping and therefore the physical position of later elements. Use fixed CSS dimensions for critical media and render only after the required resources are available in your application.
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Practical limits of “exact” millimeters
CSS gives you a precise layout unit, but the final artifact passes through WebKit layout, wkhtmltopdf scaling, PDF generation and possibly a printer’s own non-printable area. The available documentation defines the units and controls; it does not certify identical physical output for every downstream build. For forms, die lines, labels or regulatory documents, treat the CSS value as the design specification and validate a generated PDF (and, when relevant, a printed sample) with the build you will deploy.
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FAQ
Can I mix millimeters and centimeters in one stylesheet?
Yes. They are both CSS absolute units, so a margin in centimeters can surround a component sized in millimeters. Keep the unit choice consistent with the measurements your design team uses.
Should I use --page-size A4 or explicit dimensions?
Use the preset when the document targets a named standard. Use --page-width and --page-height when the stock is custom or when a script must make the exact sheet geometry explicit.
Does setting --dpi 96 guarantee a 1:1 physical result?
No. The option’s documented default is 96, and its effect is limited on X11-based systems. Smart shrinking, the wkhtmltopdf build and the final print path still need to be controlled and verified.
Frequently Asked Questions
Can a CSS millimeter value control the PDF paper size?
No. CSS units size content boxes. Set the sheet independently with wkhtmltopdf’s page-size preset or explicit page-width and page-height options.
Why does disabling smart shrinking sometimes create extra pages?
Without automatic fitting, content keeps its requested scale. If its total width or height exceeds the usable page area, it can overflow or paginate instead of being reduced.
What should I keep for reproducible PDF generation?
Store the source HTML/CSS, wkhtmltopdf version and build, page and margin options, scaling flags, fonts and assets, then remeasure a representative output after environment changes.
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