If you already have a bag and a "5 gallon" bucket, you can dump the bag into the bucket: the first ring near the top is the level of 5 gallons. Estimate how many gallons you have and then there are 7.481 gallons per cubic foot. In order to convert from mass or weight to volume, you need some density conversion. These conversions aren't readily available, necessarily.

About 0.5 cubic foot per the Home Depot website.

I need to make about 40 cu. ft. of "gritty mix" so I need about 13 cu. ft. each of:

• Gran-I-Grit (grower size) or Cherry Stone #2 Poultry Grit
• Turface MVP or Napa #8822 floor-dry
• Pine bark fines.

So do I need to buy 26 50-lb bags of Gran-I-Grit or Cherry Stone #2 Poultry Grit

It's going to be somewhere around that. Why don't you call the manufacturer? Or buy a bag of each and measure it?

How are you going to mix that much? Yikes!

I have about 50 containers of plants. I've measured the size of each container (allowing for its shape).

I need about 40 cubic feet of gritty mix to fill all of the containers so I'll need about 13 cubic feet of each gritty-mix ingredient.

I'd rather buy all of the bags at the same time rather than having to make multiple shopping trips.

The manufacturer of Turface MVP says that one 50-lb bag contains 1.3 cu. ft. so I will need to buy 10 bags.

1/2 TON of Turface MVP = 13 cubic feet?

General rule is that one cubic yard of loose consistently sized material, which is 27 cubic feet, weighs 3000 pounds. This includes soil, sand, gravel, etc. Clearly lots of organic material causes it to weigh less and wet material weighs more. Turface has lots of pores so weighs a bit less also. MVP is the cheap junk so has lots of different sizes and lots of dust so will weigh more than the more refined material such as Pro League. Tiny pieces and dust fit in between the larger peices, adding weight without additional volume. I'm guessing that 1000 pounds of Turface MVP is probably about 13 cubic feet.

Most potting soil has a density somewhere around 20 pounds per cubic foot.

For comparison, regular dirt has a density of about 76 pounds per cubic foot.