# Why do ping pong balls float?

If they float in water, then it is because the density of a ping pong ball is lighter than the density of water. The density of water is just below 1, and the density of a ping pong ball is much less than 1.

### How many ping-pong balls can fit in a 12'X12' box if 25 ping-pong balls cover the bottom of the box?

625 A ping-pong ball is a 40mm diameter sphere, which has a volume of 33.5 square centimeters (2.04 in^3). If we assume the ping pong balls naturally pack themselves in Hexagonally Closest Packed (HCP) arrangement, then we have 12^3 inches divided by 2.04in^3, times the 74% packing efficiency of HCP, which gives us about 625 ping pong balls.