New Mexico Dirtbags

Number of 30#, 12"x12"x3" Bags Weight @ 30 lbs each
Approx square feet, in profile (12" length x 3" height)

Calculations  click here for basic formulas

How many bags are you going to need?

This page is just a guide. For our baseline, we're using a 14"x26" bag about 1/2 filled, averaging about 30 lbs, with tamped (finished) dimensions of:
  • 12" wide,
  • 12" (1 foot) long, and
  •  3" high.
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.

Bag Height

With a tamped bag height of 3" (30 lb. bags), then you'll need four bags stacked atop one another (four courses) for each vertical foot. With 60 lb. bags, you can achieve a tamped height of 4" each which calls for only three courses per vertical foot. 

You can alter your bag height & weight by adding or lessening the fill in your bags, but the bag width will be largely unchanged. Your only gains are in the length and height of the bags.

Linear projects (walls, etc.)

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.
Take, for example, a wall 5 feet high by 10 feet long. Your area would be 5' x 10' = 50 sq. feet.

Taking this figure, you can do one of 3 things:
  • Use the 4:1 guideline (four bags for every square foot). Fifty times four = 200.

  • Use the chart to the left as a guide (which uses our dimensions of 12" x 12" x 3"). Looking up 50 sq. feet in the 3rd column shows that you'd need 200 bags. 

  • Divide your square footage by 0.33 square feet, which is arrived at by calculating the side profile of a 12"L x 3"H bag. (50 / 0.25 = 200 bags).

Round structures

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:
d * pi  = number of bags
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 sectorssector

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.

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1 bag 30 lbs. 0.25 sq ft.
2 bags 60 lbs. 0.5 sq ft.
3 bags 90 lbs. .75 sq ft.
4 bags 120 lbs. 1 sq ft.
5 bags 150 lbs. 1.25 sq ft.
6 bags 180 lbs. 1.5 sq ft.
7 bags 210 lbs. 1.75 sq ft.
8 bags 240 lbs. 2 sq ft.
9 bags 270 lbs. 2.25 sq ft.
10 bags 300 lbs. 2.5 sq ft.
20 bags 600 lbs. 5 sq ft.
40 bags 1200 lbs. 10 sq ft.
60 bags 1800 lbs. 15 sq ft.
80 bags 2400 lbs. 20 sq ft.
100 bags 3000 lbs / 1.5 tons 25 sq ft.
200 bags 6000 lbs / 3 tons 50 sq ft.
400 bags 12,000 lbs / 6 tons 100 sq ft.
600 bags 18,000 lbs / 9 tons 150 sq ft.
800 bags 24,000 lbs / 12 tons 200 sq ft.
1,000 bags 30,000 lbs / 15 tons 250 sq ft.
1200 bags 36,000 lbs / 18 tons 300 sq ft.
1400 bags 42,000 lbs / 21 tons 350 sq ft.
1600 bags 48,000 lbs / 24 tons 400 sq ft.
1800 bags 54,000 lbs / 27 tons 450 sq ft.
2000 bags 60,000 lbs / 40 tons 500 sq ft.
4000 bags 120,000 lbs / 60 tons 1000 sq ft.
6000 bags 180,000 lbs / 90 tons 1500 sq ft.
8000 bags 240,000 lbs / 120 tons 2000 sq ft.
10,000 bags 300,000 lbs / 150 tons 2500 sq ft.
12,000 bags 360,000 lbs / 180 tons 3000 sq ft.
14,000 bags 420,000 lbs / 210 tons 3500 sq ft.
16,000 bags 480,000 lbs / 240 tons 4000 sq ft.
18,000 bags 540,000 lbs / 270 tons 4500 sq ft.
20,000 bags 600,000 lbs / 300 tons 5000 sq ft.
Number of Bags Weight @ 30 lbs each (average bag) Approx square feet (length x height)