What is the mathematical formula to convert a circle to square millimeters.
Well then no conversion needs to be done since your radius is already in mm. Just simply find the area.
This may help: if you had a square with dimensions 5 metres by 5 metres, then the area would be 25 metres squared or square metres (The term 'square' refers to the fact that you're dealing with a concept in 2 dimensions). You wouldn't say the area is 25 metres.