Cob Floor Material Calculator

How to use

Start by measuring the floor area you actually plan to cover with this cob layer. For a simple rectangular room, multiply length by width. For irregular rooms, break the plan into smaller rectangles or triangles, calculate each area, and add them together. Use square metres for the final number. If there are built-in masonry features, thick hearths, or other elements that do not sit on the cob layer, subtract those areas so you do not overestimate materials.

Next, enter the compacted thickness in centimetres. This is important. Freshly placed cob can look thicker before tamping, smoothing, and drying. The calculator assumes the thickness you enter is the finished thickness of the layer you want to build. For many subfloors and structural earthen layers, that may be several centimetres thick. For a finer finish coat, the thickness may be much smaller. If your floor varies in depth, use an average thickness or calculate separate zones.

The ratio field expects a volume ratio in the form clay:sand:straw, such as 1:2:0.15. The first number is the clay-rich portion, the second is sand, and the third is straw. These are not percentages unless they happen to add up that way. They are simply parts of a recipe. A ratio of 1:2:0.15 means that for every 1 part clay, you use 2 parts sand and 0.15 parts straw by volume. This mirrors how many builders work on site with buckets, tubs, or wheelbarrows.

Finally, enter the mass of one straw bale in kilograms. Bale weights vary more than many first-time builders expect. A dry, tightly packed bale from one farm may weigh much more than a looser bale from another. The default value of 20 kg is a reasonable planning assumption for a small square bale, but if your supplier gives you a more accurate number, use that instead. The bale estimate is especially useful when you are deciding whether to buy a few bales locally or arrange a larger delivery.

After you press the calculate button, the result area shows the total volume of cob and the estimated mass of clay, sand, and straw. The straw line also includes an approximate bale count. Treat the output as a planning estimate rather than a promise. Natural building works best when you combine a good estimate with a small test batch, a moisture check, and a little contingency for waste and adjustments.

Formula

The calculator follows a simple sequence. First it converts thickness from centimetres to metres. Then it multiplies area by thickness to get total volume. After that, it divides the total volume among clay, sand, and straw according to the ratio you entered. Finally, it multiplies each component volume by a typical bulk density to estimate mass. The straw mass is divided by bale weight to estimate the number of bales.

If A is floor area in square metres and t is thickness in centimetres, the thickness conversion is:

The total cob volume is then:

Suppose your ratio is written as three parts: p_clay, p_sand, and p_straw. The total number of parts is:

p_total = p_clay + p_sand + p_straw

Each ingredient gets a share of the total volume:

V_component = V × p_component / p_total

So if the ratio is 1:2:0.15, the total parts are 3.15. Clay gets 1/3.15 of the total volume, sand gets 2/3.15, and straw gets 0.15/3.15.

To estimate mass, the script uses typical bulk densities:

• Clay-rich subsoil: about 1,600 kg/m³
• Sharp sand: about 1,500 kg/m³
• Loose straw in the mix: about 100 kg/m³

The mass relationship is:

m_component = V_component × density_component

For straw bales, the final step is:

bales = m_straw ÷ m_bale

These formulas are intentionally straightforward. They are designed for planning, not for modeling every detail of moisture content, compaction, shrinkage, or local material variation. That simplicity is useful because it matches how many real projects are organized: estimate first, test second, refine on site.

What the inputs mean in practice

The floor area field is the easiest to understand, but it still deserves care. If your room is 5 m by 4 m, the area is 20 m². If the room has alcoves, curved edges, or a central masonry feature, sketch the plan and break it into smaller shapes. A careful area measurement can save you from ordering too much sand or discovering halfway through the job that you are short on clay.

The thickness field has the biggest effect on total volume. Doubling thickness doubles the amount of material. That means a small change from 4 cm to 6 cm is not minor; it increases the total volume by 50 percent. If you are unsure, measure the intended build-up at several points and average them. On uneven substrates, it can be smarter to estimate separate layers rather than forcing one average to do all the work.

The ratio field controls the character of the mix. More sand generally reduces shrinkage and can improve dimensional stability, while clay acts as the binder and straw adds tensile reinforcement and texture. The right balance depends on your soil, aggregate grading, intended layer, and finish expectations. This calculator does not tell you which ratio is best. Instead, it helps you scale a ratio you have already chosen or want to test.

The bale mass field matters because straw is often purchased in discrete units rather than by kilogram. A result of 0.4 bales does not mean you can buy less than one bale; it means one bale should be enough for that layer, with some left over for test patches or repairs. A result of 3.2 bales means you should think in practical terms and round up, especially if the bales are inconsistent or if some straw will be used elsewhere on the project.

Example

Imagine you are building a cob subfloor in a small room with an area of 25 m². You want the compacted layer to be 5 cm thick, and your test batches suggest a ratio of 1:2:0.15 by volume. Your straw supplier says the small square bales weigh about 20 kg each. Here is how the estimate works.

First convert thickness to metres:

t = 5 ÷ 100 = 0.05 m

Then calculate total volume:

V = 25 × 0.05 = 1.25 m³

Now add the ratio parts:

p_total = 1 + 2 + 0.15 = 3.15

Split the total volume by those shares:

V_clay = 1.25 × 1 / 3.15 ≈ 0.397 m³

V_sand = 1.25 × 2 / 3.15 ≈ 0.794 m³

V_straw = 1.25 × 0.15 / 3.15 ≈ 0.059 m³

Convert each volume to mass using the typical densities:

m_clay ≈ 0.397 × 1600 ≈ 635 kg

m_sand ≈ 0.794 × 1500 ≈ 1191 kg

m_straw ≈ 0.059 × 100 ≈ 5.9 kg

Finally, estimate straw bales:

bales = 5.9 ÷ 20 ≈ 0.30

That result tells you the room needs about 1.25 cubic metres of total cob, with roughly 635 kg of clay-rich material, 1,191 kg of sand, and only a fraction of one small bale of straw. In real life, you would still buy at least one bale and probably keep extra material on hand for test patches, edge details, and inevitable site losses. The example also shows why sand often dominates the delivered weight even when the recipe sounds balanced in parts.

How to interpret the result

The total volume is your broadest planning number. It helps you think about mixer size, wheelbarrow trips, labor time, and whether the work can be completed in one mixing session or should be split into stages. If you know the capacity of your wheelbarrow or mortar tub, you can convert the cubic metre result into a more intuitive count of loads.

The clay and sand masses are useful because suppliers often sell these materials by weight or by truckload. If your sand yard quotes by tonne, the mass estimate gives you a quick way to compare the calculator output with the supplier’s unit. If your clay-rich subsoil is on site, the mass estimate still helps you understand the scale of the job and whether screening, stockpiling, or moisture conditioning will be significant tasks.

The straw estimate is usually the smallest mass but can still matter a lot for workability and crack control. Because straw is light, the bale count often looks surprisingly low. That does not mean straw is unimportant. It simply reflects the low density of straw compared with mineral ingredients. If the result is less than one bale, that is normal for smaller rooms or low-straw mixes.

Remember that the calculator output is best used as a baseline. Many builders add a waste allowance of 5 to 15 percent depending on the complexity of the room, the experience of the crew, and how much testing remains. If you are ordering materials from a distance, rounding up is usually safer than trying to hit the exact estimate.

Typical cob floor mixes and where this layer sits

Cob floors are not all the same. A thicker base or structural layer may use a somewhat different balance than a finer leveling coat or finish layer. This calculator is most useful for the bulk earthen layer where clay, sand, and straw are all meaningful parts of the mix. You can still use it for other layers, but the assumptions become less exact as the mix becomes more specialized.

Many finished earthen floors are later treated with drying oils and sometimes waxes. Those finishing materials are outside the scope of this calculator, but they are part of the full project budget and schedule. If you are planning a complete floor system, think of this tool as the mix estimator for the earthen body of the floor rather than the final sealer planner.

Why builders use cob floors

Cob floors attract people who want natural materials, thermal mass, and a surface that feels handmade rather than factory-produced. When detailed correctly and protected from persistent moisture, an earthen floor can last for many years and can often be repaired locally instead of replaced wholesale. That repairability is one reason natural builders continue to value earthen systems even in modern projects.

There are also practical reasons to estimate materials carefully. Cob floors are heavy, and the mineral ingredients dominate the logistics. A room that seems modest on paper can still require a substantial amount of sand and clay. Good estimating helps you coordinate labor, staging, and drying time. It also helps you compare the environmental and financial cost of different design choices, such as a thicker layer for more thermal mass versus a thinner layer for faster drying and lower material use.

Limitations and assumptions

This calculator is intentionally simple, which makes it fast and useful, but it also means you should understand its limits. The ratio is treated as a volume ratio, not a weight ratio. That matches common site practice, yet real materials vary in density and moisture. A damp clay-rich soil can behave very differently from a dry screened subsoil, and one sand source may pack more tightly than another.

The density values used for clay, sand, and straw are typical planning values. They are not laboratory measurements from your site. Because of that, the mass outputs are approximate. The calculator also assumes a uniform floor thickness. If your floor slopes to a drain, thickens at thresholds, or includes built-up areas around embedded heating or transitions, the estimate will be less precise unless you calculate those zones separately.

Another limitation is that the tool does not automatically add waste, shrinkage, or extra material for test patches. Earthen work nearly always benefits from a margin. Small losses happen during mixing, moving, trimming, and cleanup. Drying behavior can also lead you to adjust the recipe after your first trial area. For those reasons, many builders round up the result and keep a little reserve material available.

Finally, this is not a structural design tool and not a code-compliance checker. It does not evaluate subfloor support, moisture management, insulation strategy, radiant heating details, or finish compatibility. Use it for estimating and planning, then confirm the actual mix and assembly with test batches and project-specific guidance.

Practical site tips

Once you have the result, convert it into the units your crew actually uses. If one wheelbarrow holds about 0.06 m³, then a 1.25 m³ mix is roughly 21 wheelbarrow loads. If you mix by bucket, remember that 1 m³ equals 1,000 litres. A 12-litre bucket recipe can be scaled up surprisingly accurately if everyone on site uses the same bucket and fills it consistently.

It is also wise to make a small test batch with the exact clay, sand, and straw you plan to use. Let it dry, inspect cracking, and check hardness. If the test patch shrinks too much, you may need more sand or a different aggregate grading. If it feels weak or dusty, the clay content or finishing method may need adjustment. The calculator gives you the scale; the test patch gives you confidence.

Frequently asked questions

How accurate is this cob floor material estimate?

The estimate is usually good enough for planning and ordering, especially when your floor thickness is reasonably uniform and your materials are close to the assumed densities. Natural materials vary, though, so it is smart to add a contingency and confirm the recipe with a test batch.

Can I use this calculator for earthen subfloors only?

No. You can also use it for thicker leveling layers or other cob-based floor layers as long as the ratio and thickness you enter match the layer you are estimating. It is less suitable for very thin specialty finish coats with unusual aggregates or additives.

What if my clay or sand density is different?

The mass outputs may be somewhat high or low if your actual materials differ from the typical densities used here. In that case, rely more heavily on the volume result and use local supplier information to refine the order quantity.

Should I add extra material?

Usually yes. Many builders add 5 to 15 percent for waste, test patches, edge work, and small adjustments during mixing. The right margin depends on your experience level and how complex the room is.