The following instructions are for calculating earthbag tubing in a roundhouse. For calculating the number of bags, go to How Many Bags Do I Need?

First, determine the exterior diameter of the roundhouse. 20’ interior diameter is a good size for a small starter home, so let’s use 20’ as an example. Earthbag walls are typically 18” wide. That means the exterior diameter is 23’. (Check: 20’ + 18” + 18” = 23’).

Now we need to know the circumference of the roundhouse.
Circumference of a circle: C=πD
The diameter is 23’.
Put 23’ into the circumference formula: π(23’) = (3.14)(23’) = 72’ circumference

Now we need to determine how many courses of bags there are.
Typical wall height is 8’. Most earthbags are about 5” high after tamping.
Convert 8’ into inches: 8’(12”) = 96”
Divide: 96” ÷ 5” = 19 courses of bags.
Many earthbag buildings have two courses of gravel bags below ground: Add 2 courses.
Most earthbag buildings have a 6” high concrete bond beam: Deduct 1 course.
Total number of courses = 19 + 1 = 20 courses of bags
You could deduct one course or so to compensate for windows and doors. Most earthbag builders prefer to have some extra tubing and so they don’t deduct for these openings. (It’s good to have extra. Maybe some of your courses are less than 5” or you make a mistake, etc.)

So you need 20 courses of bags with a 72’ circumference.
Multiply: 20(72’) = 1,440’