Clara Amfo and mathematician Dr Hannah Fry use numbers to explore Katy Perry's career, calculating the maths behind the music to discover how her success adds up.

How many copies did her first album as a Christian rock artist sell? Why is she the most followed person on Twitter? And just how much money did it take for Katy to become the highest earning music star in 2015?

We speak to the number experts and those who have helped shape her career including award winning producer Glen Ballard, music executive Tim Devine and the man who first playlisted 'I Kissed a Girl' on the radio. Show less