A ping pong ball is 1.5" in diameter which equals about .0010 (and change) cubic feet which equals about .0076 gallons. Since I needed to displace 280 gallons to raise a 2400 lb boat, this would equate to 36,601 ping pong balls. Mind you, this is a calculation with tiny numbers and a good bit of error. It also does not take into consideration the additional displacement required to overcome the weight of the ping pong balls. But 40,000 should get you in the ball park....if you can FIT 40,000 in the hull.