Use four values for red, green, blue, and alpha: rgba(20, 80, 160, 0.5) makes a blue color 50% opaque. The modern equivalent is rgb(20 80 160 / 50%). Both forms are valid CSS; for a translucent background with opaque text, put alpha on the background color rather than applying opacity to the whole element.
How to write an RGBA color
The traditional syntax uses comma-separated red, green, blue, and alpha components:
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rgba(red, green, blue, alpha)
For example:
.notice {
background-color: rgba(20, 80, 160, 0.5);
}In this common numeric notation, each RGB channel is a value from 0 through 255. Alpha controls transparency: use a number from 0 to 1 or a percentage from 0% to 100%.
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0or0%: fully transparent.0.5or50%: half opaque.1or100%: fully opaque.
Alpha is not a 0–255 channel. If you omit alpha in an RGB color, it defaults to fully opaque. See MDN’s RGB function reference and the CSS Color Module Level 4 specification.
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Use modern slash-alpha syntax when you prefer it
CSS also allows alpha after a slash, with spaces separating the RGB channels:
.notice {
background-color: rgb(20 80 160 / 50%);
}This produces the same color and transparency as rgba(20, 80, 160, 0.5). In the modern form, do not use commas between the channels or combine comma-separated syntax with a slash. Although the modern form uses the name rgb(), it still supports alpha.
Keep text opaque while making a background translucent
Apply alpha to the background color itself, then set the text color separately:
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.card {
background-color: rgb(15 25 40 / 85%);
color: white;
}The background color is translucent, while the text remains opaque. Avoid setting opacity on .card when only its background should fade: element-wide opacity affects the element’s rendered contents too.
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Choose between RGBA, slash-alpha RGB, and alpha hex
| Format | Example | When it may fit |
|---|---|---|
| Comma-separated RGBA | rgba(0, 0, 255, 0.5) |
Alpha is explicit in the function name and familiar to many developers. |
| Modern slash-alpha RGB | rgb(0 0 255 / 50%) |
A readable choice when writing modern CSS color syntax. |
| Hex with alpha | #ff000080 |
Compact when your design values are already hexadecimal; the final byte encodes alpha. |
In eight-digit hex notation, the form is #RRGGBBAA: the last byte, AA, is alpha. 00 is fully transparent and ff fully opaque. For example, #ff000080 is an approximately half-transparent red. See MDN’s hex color reference.
rgba() has not been removed; it remains a valid legacy alias of rgb(). For newer CSS, slash-alpha syntax is another option. Check the browser compatibility information linked from MDN’s RGB reference if your project has a specific browser-support matrix.
Common mistakes and fixes
- Using 128 for half alpha: Alpha is a number from 0 to 1 or a percentage. Use
0.5or50%, not an 8-bit value. - Fading the text along with the background: Use alpha in
background-color, notopacityon the containing element. - Mixing syntaxes: Write legacy RGBA with commas, such as
rgba(20, 80, 160, 0.5), or modern RGB with spaces and a slash, such asrgb(20 80 160 / 50%). - Assuming transparent means a particular visible color: A transparent color lets what is behind it show through; the resulting appearance depends on the background beneath it.
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