Some folks
like to fill their 14" x 26" bags to 60 lbs each with
filled, tamped dimensions of:

- 12" wide,
- 18" (1.5 feet) long, and
- 4" high.

For calculation purposes, you can estimate the number of
60 lb bags you'll need by

halving
the bag count in the column to the left. If
you're estimating a wall that's 20 square feet, you'll
need either 80 thirty-pound bags or 40 sixty-pound bags.
Give or take.

The advantage of heavier bags is that you will need fewer
bags. The disadvantage is that they're much harder to
handle, especially as your walls rise.

Lighter bags = more bags = less labor.

Heavier bags = fewer bags = more labor.

It'll be to your advantage to fill your bags as
consistently as possible to keep your courses straight
& to streamline your production with assembly-line
precision (see our

Techniques
page). Sure, you can be organic and free-form (eyeballing
the filled bag, guessing at the weight), but you risk
introducing small errors that can grow exponentially, and
you also risk work slowdowns caused by futzing around.

Calculating
how many bags you'll need for something like a wall is to
simply figure how many square feet the face of the
construction will be (H x L). A very basic rule of thumb
is 4:1 - four bags for every square foot.

If you're
considering building something curved or circular,
then calculating how many 12" long filled bags you'll need
will start with this formula:

where
d=diameter and

pi
= 3.14. Multiplying these will give you your
circumference. Since 30 pound bags are 1 ft long, this
figure will give you your approximate per-course bag
count. (A "course" = a single horizontal row.) If you're
using 60 lb. bags (18" long or 1.5 feet long), then you'll
need to divide your circumference by 1.5 to arrive at the
number of bags needed.

A 10-foot diameter round structure will have a
circumference of 31.4 feet. You'll need 31.4 thirty-pound
bags. For 60-pound bags, you'll need to divide 31.4 by 1.5
to arrive at 20.9 (21) filled bags per course.

As to height: if your tamped bags are 3" high, you'll need
four courses for one vertical foot of wall height.
So 8 foot high walls, will require 32 courses of 31.4
bags per course = 1005 bags. (Fewer bags, of course, if
you're planning on having doors or windows.) 60-pound bags
(4" high, 3 courses per vertical foot) will require 24
courses, so 24 x 21 = 504 bags.

Establish whether your diameter is inner or outer; it'll
make a difference, considering that your walls are about
14" thick after plastering.

You can lay out & control your circles & arcs by
driving a pin or pole in the center of your building site,
and then using either string or a lightweight pole as a
pivoting compass.

Again, if you're filling your own bags to different
dimensions (or using different sized bags), you may have
to make corresponding changes to the formulas
provided.

### Arcs and sectors

It also might be helpful to know how to calculate the

arc
or

sector
of a circle. An arc is a segment of the circumference -
you'd use this to, say, figure the width of a doorway or a
window. A sector is a pie-shaped chunk of the circle
starting at the center and going out to the circumference.
It can be defined by its angle.

By determining the length of your arc in feet, you can get
an idea of how many bags you'll need - or (in the case of
a doorway) how many bags you

won't
need. Here's a formula:

x/360 [d * pi] =
number of bags

where x is the degree of your sector, d=the diameter of
your circle and

pi
= 3.14. Multiply these together. 60 lb bags? Divide by
1.5.

Example 1: You have a structure with a diameter of 10 feet
and (see above) a circumference of 31.4 feet, requiring a
total of 31.4 thirty-lb. bags. You know the angle of your
sector is 30 degrees (x). So

pi
* 10 * 30 = 942. Divide this by 360 and you have 2.6 feet
(the length of your arc - or, more practically, the width
of your dome's door or window). Subtract 2.6 bags (the
door) from 31.4 bags (the total needed) and you arrive at
28.8 bags for each course where there'll be a gap for the
doorway. Do this for all your doors & windows when
calculating how many bags you can subtract from the total
bags you'll be needing.

Example 2: A quarter-circle retaining wall (say, a
flower bed in the corner of your yard) has a sector of 90
degrees and a radius of six feet. Your full circle would
have a diameter of 12 feet; 12 *

pi
would give you a circumference of 37.68 feet. Divide
this by four (remember, it's a quarter circle) and you
have an arc of 9.42 feet. Your courses will require 9.42
(say, 9.5) end-to-end 30 lb. bags, or (for sixty-lb bags,
dividing by 1.5), 6.28 bags.

Play with this. You'll get the hang of it. Or you can
email us at

info@nmdirtbags
and we'll see if we can help.

### Why include the weight?

Bag weight is included for several reasons, not the least
to give you food for thought regarding the importance of
preparing your foundation. Without a good foundation,
differential settling and compaction of the ground can
cause everything from minor to catastrophic problems with
your structure. The critical importance of understanding
this, of establishing a good foundation, and of
maintaining a high degree of quality control as your walls
rise cannot be overstated.

Gravity exerts its force straight downwards. The sheer
weight of the bags pressing straight down over time can
help consolidate your project's structural
integrity...

if
your foundation is rock-solid, and

if
you carefully maintain vertical walls with a level and/or
a plumb bob. Click

here
for illustration.

If a 10 foot long, 8 foot high wall (weighing some 9,000
lbs) is

not
perfectly vertical - that is to say, if the mass is being
directed downward at an

angle
- then the resulting instability can "creep" over time and
result in cracking plaster, sticking doors, or - worst
case - total structural collapse.

Conversely, if you're building a dome or an arch (doorway,
window), then you'll want to offset or stagger your bags
to direct the weight/mass to your needs. It's not our
scope here to describe this here. We highly recommend Kaki
Hunter & Donald Kiffmeyer's indispensible

*Earthbag
Building*.

If you're considering building a habitation, it's
mandatory here in New Mexico to enlist the services an
architect or a structural engineer to sign off on your
plans. Elsewhere, it's still a good idea. Doing so
will not only ensure peace of mind, but may be an
important or essential factor in securing a building
permit and home insurance.

We hope to be posting more on this site on the how-to's of
building structures & relevant codes.