CSS border-radius Demisitified

There were times, when, to get the borders around any element was a tough task, and this was usually done by stitching multiple background images. But the real pain was to maintain them.

Fortunately, CSS came up with a solution and thus a property called border-radius. It’s the shorthand property of the four well defined properties, which are:

  • border-top-left-radius
  • border-top-right-radius
  • border-bottom-right-radius
  • border-bottom-left-radius

The border-radius property takes the value(s) in any CSS units for all of the four corners.

  #box1 {
    border-radius: 10px 10px 10px 10px;

  /* equivalent to the following */
  #box2 {
    border-top-left-radius: 10px;
    border-top-right-radius: 10px;
    border-bottom-right-radius: 10px;
    border-bottom-left-radius: 10px;

border-radius property actually works as follows:

  • If only one value is set, then this applies to all of the four corners.
  • If two values are set, then the first one applies to top-left and bottom-right corners, the second one applies to top-right and bottom-left corners.
  • If three values are set, then the first one applies to top-left corner, the second one applies to top-right and bottom-left corners, the third one applies to bottom-right corner.
  • If four values are set, then they applies to top-left, top-right, bottom-right and bottom-left corners.

This property seems to be behaving much like border-color, but this is not the case actually. What is happening here is, the every value we set is set to x (horizontally) and y (vertically) radius of the final corner’s circle. Confused? The following example would help:

  #box2 {
    border-top-left-radius: 10px 10px; /* x=10px, y=10px */
    border-top-right-radius: 10px 10px; /* x=10px, y=10px */
    border-bottom-right-radius: 10px 10px; /* x=10px, y=10px */
    border-bottom-left-radius: 10px 10px; /* x=10px, y=10px */

Compare this #box2 rule to the previous example, and you would get it!

Any curious mind would ask, okay, now how would I set this using the shorthand version of the border-radius property? Nice question, I would say. Let’s try with the following example:

  #box2 {
    border-radius: 10px 10px 10px 10px 10px 10px 10px 10px;

Would this work? Nope! Not at all! This is now an invalid property value! To workaround this, the property has a different syntax, which is border-radius: x-radius(es) / y-radius(es) (notice the divide or forward slash symbol). Let’s see the following example:

  #box2 {
    * top-left-x=40px, top-left-y=20px;
    * top-right-x=50px, top-right-y=25px;
    * bottom-right-x=60px, bottom-right-y=30px;
    * bottom-left-x=70px, bottom-left-y=35px;
    border-radius: 40px 50px 60px 70px / 20px 25px 30px 35px;

This would literally result in following ugly looking box!

Let’s try some of the cool examples below:

  #box3 {
    * top-left-x=80px, top-left-y=30px;
    * top-right-x=30px, top-right-y=80px;
    * bottom-right-x=80px, bottom-right-y=30px;
    * bottom-left-x=30px, bottom-left-y=80px;
    border-radius: 80px 30px / 30px 80px;

  #egg {
    * top-left-x=50%, top-left-y=60%;
    * top-right-x=50%, top-right-y=60%;
    * bottom-right-x=50%, bottom-right-y=40%;
    * bottom-left-x=50%, bottom-left-y=40%;
    border-radius: 50% / 60% 60% 40% 40%;
    height: 200px;
    width: 144px;

  #oval {
    * top-left-x=50%, top-left-y=50%;
    * top-right-x=50%, top-right-y=50%;
    * bottom-right-x=50%, bottom-right-y=50%;
    * bottom-left-x=50%, bottom-left-y=50%;
    border-radius: 50%;
    width: 200px;
    height: 100px;

Results are:

Important: Changes to border-radius are expensive to calculate, especially on lower powered devices. So in a nutshell, try not to animate this values in CSS! You have been warned!

Thanks for reading till here. See you next time.


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