How can I calculate the amount of circles inside of a larger circle, without overlap?

Looking to have a user input two values: Large circle radius and small circle radius. I am trying to find the maximum amount of smaller circles that would fit inside the larger circle. I know how to do the inputs and the circle area calculation, however I am having trouble finding the best way to calculate "packing efficiency" since there is no specific formula for this problem, mathematically. If anyone can point me in the right direction, much obliged.