I got a customer wants some lava rock put down... even though I hate the stuff. I need to cover 130 square feet at 2 inches thick. I know how to calculate mulch, not sure how to calculate rock? I was looking at getting some .5 cu ft bags and doing it with them. The bag says it covers 6 square feet (doesn't say how deep) so I would need approximately 22 bags. I've read that lava rock will cover 180 square feet per ton at 3" deep. Using that it seems like I would need more than 22 bags (484 lbs) even at 2" instead of 3"? Need some help here?