Earthbag Dome Material Calculator

Plan an earthbag dome with clearer material assumptions

Earthbag and superadobe projects are attractive because the core materials can be simple: bags or tubes, fill soil, barbed wire between courses, and a dome geometry that naturally creates strength through compression. The hard part is not the idea. The hard part is turning a sketch into a shopping list that is realistic enough to guide deliveries, labor planning, and site logistics. This calculator helps with that early-stage estimating step. You enter the dimensions you control, and the page converts them into a rough quantity estimate for the dome shell itself.

The tool is intentionally practical. It does not try to replace detailed engineering or on-site mockups. Instead, it answers the question most builders ask first: if this dome is this wide, this thick, and built with bags of roughly these filled dimensions, how much wall volume am I creating, how many bag runs might that consume, how many courses will I stack, and how much barbed wire should I expect to use between those courses? Those are the numbers that affect hauling, staging, crew time, and budget.

Everything on this page uses metric units. Interior diameter and average bag length are entered in meters. Wall thickness, filled bag width, and filled bag height are entered in centimeters. The script converts the centimeter values to meters before doing any volume math, so it is worth pausing for a moment to verify your units before you click calculate. Most surprising outputs come from a unit mismatch rather than a broken formula.

What each input means in plain language

Interior diameter is the open span inside the dome, measured from one inner wall face to the opposite inner wall face. If you are starting from a floor plan, this is usually the easiest dimension to trust. Wall thickness is the thickness of the dome wall itself, from the inside face to the outside face. Thicker walls increase the shell volume quickly, because you are enlarging the whole dome shell, not just adding a little material around the edges.

Bag width when filled and bag height when filled describe the actual packed shape of the bags in the wall, not the empty bag dimensions printed by a supplier. The filled size matters because it determines how much volume one bag run holds and how tall each course becomes after tamping. Builders often discover that the tamped dimensions are smaller than the loose-fill dimensions, so field measurements from a test bag are more useful than catalog measurements here.

Average bag length is the average length of one filled bag run as you expect to place it in the wall. If you plan to work with long continuous tubes, you can still use this field as an average placed length for estimating. The calculator is not trying to model every seam or cut. It is trying to give you a reasonable average bag volume so you can estimate total quantity.

How the calculator turns dimensions into quantities

The model treats the dome as a hemispherical shell. First it computes the inner radius r from the interior diameter. Then it computes an outer radius R by adding wall thickness. The wall material is the volume between those two hemispherical shapes. That shell-volume estimate is the heart of the calculator because nearly every other output follows from it.

Once the wall volume is known, the script multiplies it by an assumed bulk soil density of 1700 kilograms per cubic meter to report an approximate fill mass. That mass is not a code value and it is not a statement about the exact performance of your mix. It is simply a planning conversion so you can think in terms of truckloads, buckets, and handling effort instead of abstract cubic meters alone.

Next, the calculator estimates the volume of one average bag run from the filled bag width, filled bag height, and average bag length. It divides the total wall volume by that bag volume to estimate the number of bag runs. Because you cannot place a fraction of a real bag run in procurement planning, the displayed bag estimate rounds up to the next whole number.

Course count is estimated by dividing an approximate dome height by filled bag height. This page uses the interior radius as a practical dome-height approximation, then rounds up to the next whole course. The barbed wire estimate assumes two strands between courses and uses an average circumference to keep the estimate fast and readable. In other words, the wire output is a useful planning number, not a precise spool-cutting schedule for every ring.

General model notation

The page also keeps a general notation view because many readers like to see how a practical estimator fits the broader idea of a function that maps inputs to outputs. These two MathML blocks are general-purpose, but they still describe what happens here: several measured inputs are normalized and combined into one result or a set of results.

In the dome context, those weighting or conversion terms include the unit changes from centimeters to meters, the geometric constants from the hemispherical shell formula, and the density factor used to translate cubic meters of wall fill into approximate kilograms of soil. Seeing the general structure can help when you sanity-check the result. If a small change in wall thickness produces a much bigger change in soil volume than you expected, that is not a bug. Cubic geometry amplifies that change.

Worked example with realistic values

Suppose you are sketching a dome with an interior diameter of 4.0 m, a wall thickness of 40 cm, a filled bag width of 35 cm, a filled bag height of 15 cm, and an average bag length of 1.0 m. The calculator converts the three centimeter values to 0.40 m, 0.35 m, and 0.15 m. The inner radius is 2.0 m and the outer radius is 2.4 m.

Using the shell formula above, the wall volume comes out to about 12.20 m³. Multiplying by the default density assumption gives roughly 20,735 kg of soil. The bag volume estimate is 0.35 × 0.15 × 1.0 = 0.0525 m³ per bag run, so the dome shell needs about 232.3 average bag runs. The page rounds that to 233 bags for planning. With a 2.0 m dome-height approximation and a 0.15 m course height, the course estimate becomes 14. The average-circumference wire estimate works out to about 387.0 m.

That example is useful because it teaches the shape of the problem. A modest change in diameter or wall thickness can move the total wall volume by several cubic meters, while a change in bag height may leave volume unchanged but significantly alter the number of courses and the barbed wire total. If you are trying to reduce labor rather than soil volume, bag height may be the variable that deserves the most attention.

How sensitive is the estimate to dome diameter?

Diameter is one of the strongest drivers because it changes the whole shell geometry. The comparison below keeps wall thickness and bag dimensions the same as the worked example while changing only the interior diameter. The numbers are approximate, but they show the pattern clearly.

Notice that the estimate does not rise in a simple one-for-one way. Because the shell formula depends on cubes of radius, increasing diameter stretches the whole volume faster than many first-time builders expect. That is exactly why a quick estimator is useful before you lock in a footprint that looks only slightly larger on paper.

How to interpret the result on a real project

The result panel should be read as a planning summary. Soil volume tells you roughly how much compacted wall fill the shell contains. Estimated bags is a first-pass count of average placed bag runs. Courses tells you how many layers you will likely stack if your filled bag height is close to reality. Barbed wire length helps with procurement and spool planning. These numbers are most valuable when you compare scenarios rather than treating one run as a final answer carved in stone.

A good workflow is to calculate a baseline design, then run a conservative and an aggressive scenario. For example, if you are not yet sure whether the tamped bag height will be 14 cm or 16 cm, run both. If the course count changes meaningfully, you have learned something important about labor and wire needs. Do the same with bag length if your crew may place shorter runs around openings or tighter curves.

You should also add your own project allowances outside the calculator. Real builds may need extra fill for test bags, trimming losses, foundation transitions, buttresses, bond beams, plaster keying, door and window detailing, and spare materials for learning curves. Some projects will subtract large openings from the shell estimate; others will add back reinforcing details that consume material anyway. The calculator gives you a disciplined starting point so those judgment calls can happen on top of a clear baseline.

Assumptions and limits worth remembering

This page assumes a hemispherical-style dome shell with roughly constant wall thickness. It does not model apses, stem walls, buttresses, skylight rings, earth berms, varying soil density, moisture changes, or local reinforcement rules. It also does not subtract doors and windows. For that reason, the most reliable way to use it is as a comparative design tool and a rough purchasing guide, not as a structural sign-off document.

If your bag dimensions come from a manufacturer, verify them with a test fill. If your soil mix includes gravel, clay, or stabilizer in a way that substantially changes density or compaction, treat the soil-mass estimate as an order-of-magnitude number and adjust with field knowledge. If code, insurance, or engineering review matters on your project, keep this calculator in the estimating lane and verify the final design with qualified local expertise.

Used that way, the calculator becomes genuinely helpful. It turns a dome sketch into material consequences, shows which inputs matter most, and gives you better questions to ask before work starts. That is often the difference between a design that is merely inspiring and a design that is actually buildable.