I have a problem and would like a practical solution please.

I have a toddler playpen which measures 90cm * 90cm * 30cm (height) and I would like to fill it with those plastic balls to play (5.5cm diameter). Now I have searched and found this formula (C = 2πr = πd = 9) and after using this then multiplication of the sides and so, I came to the amount of 45,287 balls which I find rather too high to be true. Perhaps it is calculating the whole area by pieces but dunno.

Can someone provide the correct formula and also calculate it in advance?

Thanks a lot