Inspired by the following xkcd:
Click to go back to this original comic on xkcd.

How many playpen balls would it take to fill your apartment?

The suggestions given below are just that--feel free to enter nonsensical values for fun and profit. Feedback can be directed to relsqui on the xkcd forum or IRC channel, or relsqui@chiliahedron.com

Metric version available here, courtesy Berge.

Packing efficiency: %
The theoretical best possible packing density is about 74%, and the theoretical worst (in a space whose dimensions are multiples of a ball's diameter) is about 60% before you stop actually filling the space or start ignoring gravity. Don't believe me? Figure it out yourself.

I recommend excluding bathrooms for sanitary reasons.

If you don't have screen windows, you probably want to stay under sill level … or get screen windows.

Floor area to cover: ft.²
Depth of balls: ft.
Radius of each ball: in.
Price of each ball: $

It would take balls and cost dollars to fill your apartment with playpen balls.