Border-radius 50% or 100%

Anything more than the radius defaults to the actual radius. By definition a radius is the same in all directions, defining a circle. In other words, half of the diameter. 50%. The browser understands this.

Some authors choose to write 100%, but their reasoning is unclear, at least to me. It just seems to make the browser do more work calculating what the true radius is (in order to prevent curves from overlapping). Even if we use px units the diameter of the circle, the browser will calculate it down to half of that. For your own sanity, go with what seems most reasonable: 50%.

I agree with Roy on this. 50% should be used to make a circle. More info on this(which comes down to what Roy said): http://jessica-eldredge.com/2014/09/07/border-radius-50-or-100-percent/