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For ordinary website images, you generally do not need to set a special DPI value. Choose image pixel dimensions to match the space the image occupies in the page, then offer larger alternatives when you want it to look sharp on high-density displays. Browsers lay out images in CSS pixels; DPI metadata is not a reliable way to control their on-screen size.
Why DPI usually does not determine a web image’s size
DPI means dots per inch; PPI means pixels per inch. Both describe how many image or printer dots fit into a physical length, which matters when preparing an image for print. A website, however, normally lays out an image using CSS pixels, not a physical measurement in inches.
A CSS pixel is a layout unit. It does not guarantee a fixed physical size on every screen: the relationship between CSS pixels and physical display pixels varies by device and can also be affected by zoom. As a result, changing an image’s DPI metadata—such as changing it from 72 to 300—usually does not make it appear larger or sharper in a web layout. The image’s actual pixel dimensions and the size at which the page displays it matter more.
CSS defines a conversion of 1 dppx to 96dpi for its resolution units, and its default image resolution is one image pixel per CSS pixel. That is a rule for interpreting CSS resolution units; it is not an instruction to save every web image with 96 DPI metadata.
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What to consider instead: slot size and display density
Rendered size
Start with the maximum width at which the image will appear in the page layout, measured in CSS pixels. Account for responsive breakpoints: an image may occupy nearly the full width on a phone, then sit in a narrower column on a desktop, or the reverse. The page’s layout—not the DPI field in the file—sets the displayed size.
Device pixel ratio
A device pixel ratio (DPR) describes the ratio of physical display pixels to CSS pixels. MDN Web Docs defines the browser’s devicePixelRatio value as the ratio of physical-pixel resolution to CSS-pixel resolution for the current display device. When DPR is above 1, a source image with more pixels than the CSS-sized slot may appear sharper.
As a planning example, a 600 CSS-pixel-wide slot needs about 1,200 image pixels across to supply a 2x candidate. That is an example of multiplying slot width by target density, not a rule to double every image. Whether the extra detail is useful depends on the display density you intend to support, the image, and the bandwidth cost.
Actual pixel dimensions and file size
Pixel dimensions are the raster image’s real width and height. If an image is displayed at 320 CSS pixels on a 1x display, a 320-pixel-wide source is a natural match; at 2x, a 640-pixel-wide candidate can supply more detail. Larger candidates generally contain more data, but the exact file-size difference depends on the image and its encoding. Sending a very large source to a small display slot can waste bandwidth without a visible benefit for that viewer.
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How to choose image dimensions
- Measure the layout slot. Identify the image’s maximum rendered width in CSS pixels at the relevant responsive breakpoints. Use the width the image will actually occupy, not the width of the full browser window if the image is in a column.
- Choose the density you want to support. For each slot, decide whether to supply a 1x source, a higher-density source such as 2x, or several alternatives. Multiply the slot width by the target DPR to estimate the pixel width of a candidate. Treat the result as a planning guide, not a universal minimum.
- Keep proportions and composition in mind. If the image is the same picture at different sizes, provide size alternatives. If a phone needs a different crop or composition than a desktop, use art direction rather than merely serving a larger copy of the same crop.
- Balance clarity with transfer cost. Avoid delivering a source far larger than the displayed slot and density require. Compare the resulting file sizes and formats for your actual images; there is no universal byte saving because content and encoding differ.
- Reserve layout space. When the intrinsic dimensions are known, set the image’s
widthandheightattributes. The browser can use their ratio to reserve space before the image loads, helping avoid layout shifts. This is useful layout behavior, separate from DPI.
Use responsive markup to offer suitable candidates
Rather than forcing one oversized image on every visitor, HTML lets you offer alternatives. The browser can use the candidate list, the expected rendered size, and display density to choose an appropriate source. MDN’s responsive-image guidance distinguishes resolution switching—the same image at different sizes—from art direction, where the image or crop changes for a context.
Width descriptors with srcset and sizes
Use width descriptors when candidates have different intrinsic widths. The number in a descriptor such as 640w must match that file’s actual pixel width. Pair width descriptors with an accurate sizes attribute so the browser can estimate the slot width before choosing a candidate.
<img
src="photo-640.jpg"
srcset="photo-320.jpg 320w, photo-640.jpg 640w, photo-1200.jpg 1200w"
sizes="(max-width: 640px) 100vw, 600px"
width="600"
height="400"
alt="A description of the image">In this example, the browser is told that the image may occupy the full viewport width up to 640 CSS pixels and is expected to occupy 600 CSS pixels above that breakpoint. Adjust the values to match the real layout and use files whose intrinsic widths match their descriptors. The dimensions in the markup preserve a 3:2 aspect ratio in this example; use the actual image ratio rather than copying these numbers blindly.
Density descriptors for a small set of fixed-size variants
If an image has a known fixed display size and you only need density variants, descriptors such as 1x and 2x can express those alternatives:
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<img
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srcset="avatar-1x.png 1x, avatar-2x.png 2x"
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alt="Profile portrait">This suits a fixed-size image better than a layout whose width changes substantially across breakpoints. Do not mix width descriptors such as 640w with density descriptors such as 2x in the same srcset candidate list.
Use <picture> when the image itself should change
When a narrow layout needs a different crop or composition, use <picture> with source conditions. Use srcset and sizes to choose among sizes of the same image; use art direction when the appropriate image differs by context. Avoid treating art direction as a DPI adjustment: it changes the image served, not the meaning of its resolution metadata.
Common DPI and image-resolution mistakes
- Saving everything at 72 DPI or 96 DPI as a supposed web requirement. Neither value is a universal setting that determines a web image’s rendered size. Prioritize pixel dimensions and CSS layout.
- Changing DPI metadata to fix a blurry image. If the image looks soft, check whether the source has enough actual pixels for its CSS slot and target DPR. A metadata edit does not add image detail.
- Using one huge source for every device. A larger source can provide detail on high-density screens, but may send more bytes than a smaller slot needs. Offer suitable candidates where responsive delivery is appropriate.
- Writing inaccurate width descriptors. A descriptor such as
1200wrepresents a 1,200-pixel-wide source. The number is not the intended CSS display width. - Leaving out or misjudging
sizes. With width descriptors,sizestells the browser how wide the image is expected to render. Describe the real layout rather than assuming the browser can infer it from an arbitrary candidate list. - Using
image-resolutionas a normal fix. MDN describes this CSS property as experimental and reports no browser support in its compatibility summary at the time documented. Do not rely on it for routine web-image sizing.
Check what the page actually renders
To diagnose an image that appears soft, too large, or unexpectedly cropped, inspect the rendered CSS width and height, the image’s intrinsic pixel dimensions, the selected responsive candidate, and the display’s DPR. Also check whether the browser is showing the expected crop and whether the page’s sizes value matches its layout. Those checks separate an image-resolution problem from a layout or source-selection problem.
For visual QA across pages or viewport sizes, a screenshot can help you inspect the rendered result. ScreenshotNeo is a website screenshot API and MCP server, not a DPI converter or image-sizing rule; a screenshot shows how the page rendered, while the pixel dimensions and responsive markup determine which image source is served. See ScreenshotNeo for the service.
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Practical decision table
| Situation | What to set or provide | What not to rely on |
|---|---|---|
| Ordinary web image, one layout size | Choose pixel dimensions suited to the CSS slot and desired display density. | A particular DPI metadata value as the way to set on-screen size. |
| Same image at responsive widths | Provide size candidates with accurate width descriptors and a realistic sizes value. |
One unnecessarily large source for every viewport. |
| Fixed-size image needing density alternatives | Use a small set of density candidates, such as 1x and 2x. |
Mixing density and width descriptors in one candidate list. |
| Different crop needed on different layouts | Use art direction with <picture> and source conditions. |
Assuming a higher-resolution copy of the same crop solves a composition issue. |
| Image causes content to jump while loading | Declare known width and height so the browser can reserve aspect-ratio space. |
Changing DPI metadata to address layout movement. |
Frequently Asked Questions
Should I use 72 DPI or 96 DPI for a website image?
Neither is a universal web-image requirement. Set suitable pixel dimensions and size the image through the page layout.
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Not by itself. Sharpness depends on actual image pixels relative to the rendered CSS size and display density.
How many pixels wide should a web image be?
Estimate the largest CSS-pixel slot it will occupy and multiply by the density you want to support; use responsive candidates when slot sizes vary.
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