While I was designing Icon for my client I realise as it is true in website design as it is in Icon Design you can work in odd numbers,
When you have even numbers like 14px Square or 16px Square you can easily fit a round box inside it but in 15px Square Icon it’s really hard to manage…
Here is The example…
Odd Pixels with 15px by 15px
Even Numbers of Pixels:
10px by 10px, with 1px of rounded corners it fails to generate illusion of circular when viewed at actual size or at maximum size
16px by 16px, with 1px of rounded corners it fails to generate illusion of circular when viewed at actual size or at maximum size
20px by 20px, With 20px by 20px with radius of more than 3 pixels it creates perfect rounded corners for square or rectangle & it does create illusion or circular when viewed at actual size
It seems you need to have minimum of ‘3 Pixel’ of Round Curve to look fine, finished & crisp on screen
It is just my experience hope it help to you to….
Thanks & Regards
Mandar Apte