The border-radius property is used to round the corners of an element.

It is a shorthand property for the longhand properties: border-top-left-radius, border-top-right-radius, border-bottom-right-radius, and border-bottom-left-radius.

The border-radius can take either one to four values, or eight values separated by a slash ‘/’, with one to four values on each side of the slash.

1. If there is no slash, then: it can take one, two, three, or four values.

   border-radius: [radius value] [radius value]? [radius value]? [radius value]?; // "?" indicates value is optional
                               

   • If four values are given, then they specify the radii of each of the four corners equally; that is, the first value will specify both the x and y radii of the top-left corner, so it will be a circular curve. The second value will specify the radii for the top-right corner, the third value will specify the radii for the bottom-right corner, and the fourth value will specify the radii of the bottom-left corner.
   • If three values are given, the first specifies the radii of the top-left corner, the second specifies the radii of the top-right and bottom-left corners, and the third one specifies the radii of the bottom-right corner.
   • If two values are given, then the first specifies the radii of the top-left and bottom-right corners, and the second value specifies the radii of the top-right and bottom-left corners.
   • If one value is given, then it specifies the radii of the four corners equally.

   For example, the following:

   border-radius: 1em 3em 2em;
                                   

   is equivalent to:

   border-top-left-radius:     1em;
   border-top-right-radius:    3em;
   border-bottom-right-radius: 2em;
   border-bottom-left-radius:  3em;
                                   

   In each of the above cases, when no slash is used, the four corners of the element will be rounded and their curvature will be a perfect circle.

   

2. If there is a slash, then: it can take up to eight values—one to four values on each side of the slash.

   border-radius: [top-left horizontal radius] [top-right horizontal radius]? [bottom-right horizontal radius]? [bottom-left horizontal radius]? / [top-left vertical radius] [top-right vertical radius]? [bottom-right vertical radius]? [bottom-left vertical radius]?
   

   The values specified before the slash set the horizontal radius of the corners in the same order specified previously. The values specified after the slash set the vertical radius of the respective corners.

   For example, the following:

   border-radius: 2em 1em 4em / 0.5em 3em;
                               

   is equivalent to:

   border-top-left-radius:     2em 0.5em;
   border-top-right-radius:    1em 3em;
   border-bottom-right-radius: 4em 0.5em;
   border-bottom-left-radius:  1em 3em;
                               

   In this case, the corners of the element will be rounded and their curvature will be elliptical.

   

Trivia & Notes

If the element has a background color or a background image, the background will also be clipped to the border radius specified.

However, sometimes the background color of an element may “leak” outside the curvature of the border when it is rounded. To fix that, you can use the background-clip property and set its value to padding-box.

.element {
    border-radius: 30px;
    background-clip: padding-box;
}
                

If an element has a border image, the border image is not clipped to the curve of the rounded corners.

The content of the element may overflow the element on the corners that are rounded if the element does not have enough padding to “push” the content inwards. (See image below)

This example adds appropriate padding, so that the contents do not overflow the corners. Note that there is no border, but the background will still have rounded corners.

.element {
    background: black;
    color: white;
    border-radius: 1em;
    padding: 1em;
}
                

Also, The area outside the curve of the border edge does not accept pointer events on behalf of the element.

Moreover, corner curves must not overlap: When the sum of any two adjacent border radii exceeds the size of the border box, browsers must proportionally reduce the used values of all border radii until none of them overlap. You can learn more about how this works in this excellent talk by Lea Verou about the border-radius property.

Percentage values are perfect for creating full circular or elliptical shapes. Using border-radius: 50%; will result in a full circular shape on an element whose height and width are equal.

If the element’s height and width are not equal, the resulting shape would be an ellipse.