The APR formula is incredibly difficult to get to grips with. (I've got a maths degree and I definitely don't like it!):
https://en.wikipedia.org/wiki/Annual_percentage_rate#European_Union
If you google 'APR calculator' you'll find several sites which claim to be able to give you a figure but, since none of them deal with all
seven variables that make up the formula, none will be really accurate. However a very rough figure is 'approaching 20,000%'