The Tool Desk
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The direct answer: design for pixels, not DPI
If an image is displayed at 600 CSS pixels wide, a 600-pixel-wide source is a natural match on a 1× display. A 1,200-pixel-wide source can supply roughly twice as many pixels for a 2× display, such as a high-density phone or laptop screen. Those are planning examples, not a rule to double every file.
What matters in a browser is:
- Intrinsic dimensions: the raster width and height stored in the file.
- Rendered size: the width and height assigned by CSS or HTML, measured in CSS pixels.
- Device-pixel ratio: the ratio of physical display pixels to CSS pixels. MDN defines
window.devicePixelRatioas the ratio of resolution in physical pixels to resolution in CSS pixels. - Transfer cost: the bytes needed to download and decode each candidate.
A JPEG exported as “72 DPI” and the same pixels exported as “300 DPI” normally render at the same CSS size. DPI (dots per inch) and PPI (pixels per inch) describe physical density; a CSS pixel is an abstract layout unit whose physical size changes with display hardware and zoom.
How to calculate a suitable source size
1. Measure the largest rendered slot
Inspect the layout at every important breakpoint and record the maximum CSS width. Include content columns, cards, hero images and full-bleed sections. If a content image can reach 760 CSS pixels, use 760 as the baseline slot rather than the width at which the author uploaded it.
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2. Choose the density you intend to support
Multiply the slot width by the target DPR. A 600 CSS-pixel slot planned for 1× and 2× displays suggests candidates near 600 and 1,200 image pixels wide. If you also support 3× phones, a 1,800-pixel candidate may be useful, but only when the extra detail justifies its download and storage cost.
3. Generate several candidates
Do not force every visitor to download the largest source. Create a practical ladder such as 320, 640, 960, 1,280 and 1,920 pixels, then map those files to the widths at which the design actually renders. The exact ladder depends on your breakpoints, image subject and compression settings.
4. Preserve the aspect ratio
Scale height from the original aspect ratio unless the design intentionally crops the image. If a component uses a fixed ratio, create that crop deliberately instead of stretching a source with CSS.
Use responsive HTML so the browser can choose
Width descriptors with srcset and sizes
For the same composition at different display sizes, use width descriptors. The number in each w descriptor must match that file’s intrinsic pixel width. The sizes attribute tells the browser how wide the image is expected to be in the layout.
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src='photo-960.jpg'
srcset='photo-320.jpg 320w,
photo-640.jpg 640w,
photo-960.jpg 960w,
photo-1280.jpg 1280w,
photo-1920.jpg 1920w'
sizes='(max-width: 700px) 100vw, 760px'
width='760'
height='507'
alt='A cyclist riding beside a lake'>On a viewport narrower than 700 CSS pixels, the browser estimates a slot close to the viewport width. Above that breakpoint it estimates a 760-pixel slot, then considers DPR and available candidates. An inaccurate sizes value can cause an unnecessarily large download or a soft image.
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Density descriptors for fixed-size variants
If the rendered CSS size is fixed and you only need a small set of density choices, use 1x and 2x descriptors instead:
<img
src='avatar-1x.png'
srcset='avatar-1x.png 1x, avatar-2x.png 2x'
width='160'
height='160'
alt='Profile photograph'>Do not mix width descriptors and density descriptors in the same srcset list. Use width descriptors when the layout slot changes; use density descriptors when the CSS size is essentially fixed.
Art direction with <picture>
Resolution switching supplies the same image at different sizes. Art direction is different: a narrow layout may need a tighter crop or a different composition. Use <picture> with media conditions when the image itself must change.
<picture>
<source media='(max-width: 600px)' srcset='portrait-crop-480.jpg 480w, portrait-crop-800.jpg 800w' sizes='100vw'>
<img src='wide-1200.jpg' srcset='wide-800.jpg 800w, wide-1200.jpg 1200w' sizes='(max-width: 1000px) 100vw, 1000px' width='1200' height='675' alt='Mountain valley at sunrise'>
</picture>Why 72 or 96 DPI is not a web-image requirement
Image editors often ask for a DPI value because print workflows calculate a physical size from pixels and density. HTML layout does not use that metadata to decide an image’s CSS width. CSS defines 1dppx as 96dpi as a resolution-unit conversion, but that does not mean you should export every web file at 96 DPI.
The experimental CSS image-resolution property is not a normal fix for web images; MDN’s compatibility summary reports no browser support at the time documented. Set the rendered width with CSS, supply adequate pixels, and use responsive candidates instead.
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Prevent layout shifts while the image loads
Declare width and height attributes when the intrinsic dimensions are known. Browsers can calculate the aspect ratio and reserve space before the file arrives, reducing content movement. This is independent of DPI and remains useful even when the image is styled with width: 100%.
.article-image {
display: block;
width: 100%;
height: auto;
}
.card-image {
aspect-ratio: 4 / 3;
object-fit: cover;
}For responsive components whose crop changes, set an intentional aspect ratio and use object-fit rather than relying on an accidental height.
Balance sharpness, format and download size
- Photographs: JPEG or WebP commonly provides smaller files than a lossless format at comparable visual quality. Choose the format your browser and delivery pipeline support.
- Flat graphics and transparency: PNG, WebP or another format suited to sharp edges may preserve text and alpha better than a heavily compressed photograph.
- Vector artwork: SVG scales without a DPI setting, though embedded raster images inside an SVG still have pixel limits.
- Compression: judge quality at the actual rendered size. A 3,000-pixel source that is never displayed above 600 CSS pixels can waste bandwidth.
- Decoding and memory: very large images consume more memory even when displayed small, which can matter on mobile devices.
Generate candidates from a high-quality master, but avoid shipping a candidate that is dramatically wider than the slot and target DPR. Responsive markup is a delivery strategy, not a substitute for sensible source dimensions.
Testing on real layouts and densities
- Open browser developer tools and inspect the image’s rendered CSS width at mobile, tablet and desktop breakpoints.
- Check the selected resource in the Network panel. Confirm that the file’s intrinsic width is close to the slot multiplied by the device DPR, without being needlessly larger.
- Test at browser zoom levels and on a high-density display. Zoom changes the relationship between CSS and physical pixels, so a single screenshot at 100% is not comprehensive.
- Inspect text, fine lines and edges at the final display size. If they are soft, verify the selected candidate and its intrinsic dimensions before changing compression.
- Measure transfer size and loading priority. A sharper source that delays the main content may be a worse experience than a slightly smaller candidate.
Troubleshooting common image problems
The image is blurry even though its DPI says 300
Check the file’s pixel width and the browser’s rendered CSS width. A 300-DPI label cannot add pixels to a small raster. Supply a larger candidate and verify that srcset and sizes allow the browser to select it.
The browser downloads the largest file every time
Inspect sizes and the width descriptors. A missing or overstated sizes value makes the browser assume a wider slot. Ensure each descriptor matches the actual intrinsic width and that the markup is not replaced by a script with a fixed URL.
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The image looks stretched or cropped unexpectedly
Compare the source and container aspect ratios. Use matching dimensions, an intentional <picture> crop, or an explicit object-fit: cover rule. Do not change DPI metadata; it does not correct geometry.
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The page jumps when images appear
Add accurate width and height attributes or reserve space with a known aspect ratio. Confirm that CSS is not overriding one dimension in a way that destroys the ratio.
Fine detail disappears after optimization
Compare the compressed file at its real CSS size, not only at 400% zoom. Raise quality or choose a more suitable format for that subject, then regenerate the density candidates so a low-quality file is not selected for a high-DPR slot.
A high-DPI test device still receives a small source
Check the current DPR, zoom level, effective sizes calculation and cache. A browser may legitimately choose a smaller file when network conditions or the calculated slot make it sufficient.
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Bottom line
Set web images by pixel dimensions and rendered CSS size, not by a DPI label. Estimate the largest slot, multiply by the DPR you need to support, publish accurate srcset/sizes candidates, and reserve layout space with dimensions. That approach delivers sharp images where needed without sending every visitor an oversized file.
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Should an image’s DPI metadata be removed before uploading?
It is optional for browser sizing. Removing nonessential metadata can reduce file bytes, but it will not change the CSS dimensions or visual sharpness of the raster.
Do CSS background images use the same DPI rules?
Yes. The browser scales a background according to its CSS box and available pixels; use responsive image techniques or media queries when different sizes or crops are required.
Does an SVG need 1× and 2× versions?
A purely vector SVG normally scales without density-specific raster versions. Any bitmap embedded inside it still needs enough intrinsic pixels for the size at which it is displayed.
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