Calculate turning diameter

I am building a robot and want to calculate the turning diameter for a given angle of my wheels. 

I tried googling it but cannot find to seem the formula. 

So for example I have a wheelbase of 10cm and my front wheels are angled at 20 degrees. How big will my diameter be? 
I also want to understand what the effect is if I increase/decrease the wheelbase and front/rear axes.

Thanks.