Set the background-size property on the element that has the background-image. For a full-bleed hero or panel, begin with background-size: cover; it fills the element while preserving the image’s proportions, so part of the image may be cropped. Use contain when every part of the image must remain visible, accepting possible empty space. Add background-position, background-repeat and a fallback background-color to control the result.
The basic CSS
Here is a complete hero example:
.hero {
min-height: 28rem;
background-image: url("hero.jpg");
background-size: cover;
background-position: center;
background-repeat: no-repeat;
background-color: #1d2430;
}background-size changes the rendered size of a background image; it does not resize an ordinary inline <img>. Put the declaration on the same element that owns background-image. The initial value is auto auto, which uses the image’s intrinsic dimensions where available. See the MDN background-size reference.
The element must have a usable background area. A block with no height may collapse to the height of its content, making a background appear missing or unexpectedly small. Set a height, min-height, padding, or content-driven dimensions as appropriate.
Choose between cover and contain
cover: fill the whole area
cover scales the image proportionally until the entire background positioning area is covered. If the image and element have different aspect ratios, one axis extends beyond the area and is cropped. This is usually the right choice for banners, cards and hero sections where edge-to-edge coverage matters more than showing every pixel.
.hero {
background-size: cover;
background-position: center;
background-repeat: no-repeat;
}For example, a wide image in a tall mobile hero will lose some of its left and right edges. A tall image in a wide banner will lose some of its top and bottom. Cropping is expected behavior, not a loading failure.
contain: show the entire image
contain scales the image proportionally so all of it fits inside the area. The unused part of the element remains visible as the background color. Because backgrounds repeat by default, explicitly disable repetition when using contain unless tiling is intentional.
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.product-panel {
background-color: #f3f5f7;
background-image: url("product.png");
background-size: contain;
background-position: center;
background-repeat: no-repeat;
}This option suits logos, diagrams and product artwork that cannot be cropped. The MDN guide to resizing background images documents the practical difference between the two keywords.
Quick decision table
| Requirement | Value | Result and trade-off |
|---|---|---|
| Fill the element edge to edge | cover |
Preserves the ratio and crops one axis when ratios differ. |
| Keep every part of the image visible | contain |
Preserves the ratio but can leave empty bands; use no-repeat. |
| Set a rendered width while preserving ratio | 300px or 50% |
The other dimension is calculated as auto. |
| Set both rendered dimensions | 300px 200px |
Both dimensions are forced, so the image can distort. |
Use lengths, percentages and auto
You may provide one or two size values:
background-size: 300px;sets the width to 300 pixels and calculates the height automatically.background-size: 50%;sets the width to half of the relevant background positioning area and calculates the height automatically.background-size: 300px 200px;sets width and height explicitly. Use this only when distortion is acceptable or the dimensions match the source ratio.background-size: auto 240px;calculates the width from the image ratio while fixing the height.background-size: 100% auto;makes the image as wide as the positioning area and preserves its ratio.
A single value is interpreted as width, with height treated as auto. With two values, the first is width and the second is height. Lengths can be absolute units such as px or relative units such as rem; percentages are also valid.
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A percentage is relative to the background positioning area, normally the element’s padding box because that is the default background-origin. It is not automatically relative to the viewport. If you change background-origin, the reference area changes. With background-attachment: fixed, the reference area for percentage sizing is the viewport.
.banner {
background-image: url("banner.jpg");
background-size: 100% auto;
background-origin: padding-box;
}If a percentage looks unexpectedly large or small, inspect the element’s box dimensions and its background-origin and attachment settings.
Control cropping and placement
Move the focal point with background-position
When cover crops a subject, reposition the image instead of stretching it:
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.hero {
background-size: cover;
background-position: 70% center;
}Common values include center, top, bottom, left and right. You can combine keywords or use percentages, such as 20% 40%. Choose a position that keeps a face, product or headline-safe region inside the visible crop at the smallest supported viewport.
Stop accidental tiling
Background images repeat in both directions by default. Set background-repeat: no-repeat for a single image. Other valid patterns include repeat-x, repeat-y and space or round for specialized textures.
Provide a color fallback
The background color remains visible wherever the image does not cover the area and behind transparent pixels. It also provides a useful fallback while an image loads:
.hero {
background-color: #202838;
background-image: url("hero.webp");
background-size: cover;
}Responsive background-size patterns
Full-width hero
.hero {
min-height: clamp(18rem, 60vw, 42rem);
background: #18202b url("hero.jpg") center / cover no-repeat;
}The shorthand places the color, image, position, size and repeat in one declaration. The slash separates background-position from background-size. Keep a minimum height so the hero does not collapse on narrow screens.
Image that spans a content column
.article-header {
background-image: url("header.jpg");
background-size: 100% auto;
background-position: top center;
background-repeat: no-repeat;
padding-block: 12rem 3rem;
}This preserves the image ratio while the padding supplies enough vertical space for the visible artwork and text.
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Different positioning on small screens
.hero {
min-height: 32rem;
background: url("wide.jpg") 65% center / cover no-repeat;
}
@media (max-width: 600px) {
.hero {
min-height: 24rem;
background-position: 35% center;
}
}Changing position is often better than using a distorted image. If mobile and desktop compositions are fundamentally different, use separate images with media queries or a responsive image strategy rather than forcing one crop to do both jobs.
Background images versus <img>
Use a background when the image is decorative, sits behind content, or should be cropped as part of a visual treatment. Use an <img> when the image is content that needs alternative text, intrinsic dimensions, captions, semantic meaning or independent loading behavior. background-size cannot resize an inline <img>; use CSS such as object-fit: cover or object-fit: contain for that element instead.
.card img {
width: 100%;
height: 14rem;
object-fit: cover;
object-position: center;
}Do not hide important text or information solely inside a background image. Ensure foreground text remains readable against the chosen image and fallback color.
Layering multiple backgrounds
You can combine a gradient overlay and an image. The first layer is painted on top of later layers, so put the gradient first:
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.hero {
background-image:
linear-gradient(rgba(0, 0, 0, .45), rgba(0, 0, 0, .45)),
url("hero.jpg");
background-size: cover, cover;
background-position: center, center;
background-repeat: no-repeat, no-repeat;
}Each layer receives a corresponding value. A single cover can be reused for both layers, but writing both values makes the pairing explicit. A gradient improves contrast without editing the source image.
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Debugging checklist
The image is not visible
- Verify that
background-imageis on the same element asbackground-size. - Inspect the image URL in the browser’s network panel. Check spelling, case, relative-path location and server permissions.
- Give the element a height,
min-heightor padding if it has no visible content height. - Check that another rule, shorthand declaration or pseudo-element is not overriding the background.
- Temporarily set a contrasting
background-colorto distinguish an unloaded image from a transparent image.
contain is tiled
Add background-repeat: no-repeat. Repetition is the default behavior and is independent of the contain scaling choice.
cover cuts off the subject
Adjust background-position, reduce the element’s extreme aspect ratio, choose a differently composed source image, or switch to contain if preserving the entire image is more important than filling the area.
A percentage has the wrong apparent size
Measure the element rather than the viewport. Review padding, background-origin and whether background-attachment: fixed changes the reference area to the viewport.
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Check for two explicit dimensions such as 300px 200px. Replace them with cover, contain, or one dimension plus auto to preserve the source ratio.
Performance, quality and accessibility considerations
- Serve an appropriately sized, compressed image.
background-size: coverchanges display scaling, not the downloaded file size. - Use modern formats where your delivery setup supports them, and avoid multi-megapixel backgrounds when a smaller source is visually sufficient.
- Keep text and controls in HTML so they remain selectable, responsive and accessible.
- Check crops at narrow, wide, short and tall viewport sizes. A centered desktop crop can remove the subject on a phone.
- Set a background color that maintains acceptable contrast while the image loads and in areas exposed by
contain. - If a background is purely decorative, do not add redundant alternative text. If the visual conveys essential information, use semantic HTML instead.
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Frequently asked questions
Frequently Asked Questions
Can I use background-size with a CSS gradient?
Yes. Gradients and images are background layers; give each layer its own size, position and repeat value when their settings differ.
Does cover change the original image file?
No. It only changes how the browser paints the image inside the element. The downloaded source file remains unchanged.
Why does my background look different after the browser window resizes?
The element’s aspect ratio changes, so proportional scaling and cropping change. Test representative viewport sizes and adjust the position or source composition.
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What is the default background repetition?
Backgrounds repeat by default. Use background-repeat: no-repeat when you want one non-tiled image.
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