We often get questions from people wondering how many bags they need to have on hand for some particular project. Is there a formula to figure this out?
First you need to calculate the total square footage (or square meters) of the wall you intend to build. This can be done by simply multiplying the length times the height, and then to be precise, you can deduct the size of the windows and doors from this…but I usually don’t do this because it is better to over-estimate how many bags you might need, and also, you always need some partial bags to keep the brick pattern going.
If your wall is curved you’ll need to figure out the running length of the wall by either making a scale drawing and measuring the length that way, or in the case of a circle you can use the formula Length = 3.14 X the diameter for this.
Figuring out the surface area of a dome is obviously more complicated, partly because there are many shapes for domes. If the dome is close to hemispherical (not recommended for structural reasons) the formula would be 3.14 X the diameter squared. If the dome more closely resembles a cone, then the formula would be 3.14 X the radius X the radius plus the length from circle at the base to the top of the cone. Either of these formulas could be used to estimate approximately what the area of your dome might be, especially, if that total is then padded by not deducting openings, or simply rounding upward.
As far as bag coverage goes, it depends on the size of the bag how much square footage it will cover. I used what were labeled 50 lb. rice bags to build my house. These measured roughly 5″ X 20″ when laid in a wall and compacted. That is approximately .7 of a sq. ft. A nominal 100 lb. bag might actually be considerably larger than what I used, so you’ll need to run some tests to determine what kind of coverage to expect. The 50 lb. bags I used measured about 18″ X 30″ when laid flat.
Once you have figured out what the area of one bag might be after it is filled and tamped, then you can divide that figure into your total wall space to get the total number of bags needed. For instance, with the 50 lb. bags that I used, you would divide the area by .7 to get the total number of bags needed. In this case, for example, a 100 square foot wall would require roughly 143 bags.