Find a container with a close enough size to that container that
  has a known value. If you have a 3 gallon ice tea container, use
  it. Fill it to 3 gallons. Pour it into your 13x12x29 contain as
  many times as it takes to fill it up. If it overflows, you've
  gone far enough. And Stop. Subtract that last one, and you have
  a good estimate for how much liquid it holds on a practical
  basis. "