Cob Floor Material Calculator

What this cob floor material calculator does

This tool estimates how much clay, sand, and straw you need for an earthen cob floor layer, based on floor area, layer thickness, and a clay:sand:straw mix ratio. It also converts straw mass into an approximate number of bales so you can plan deliveries and storage.

Use it for early design and material planning on natural building projects, including new cob floors, earthen subfloors under finish layers, and retrofit overlays. It is not a substitute for on-site test batches or structural engineering.

Key inputs and how to measure them

• Floor area (m²) – Measure the total interior floor surface you plan to cover with this layer. Exclude walls, thick built-in benches, and heavy masonry features that sit on foundations rather than on the cob layer.
• Thickness (cm) – Enter the compacted thickness of the cob layer, not the loose thickness when you first place it. For most cob subfloors this is in the range of 3–8 cm, while structural or leveling layers may be thicker.
• Clay:Sand:Straw ratio – Use a volume ratio such as 1:2:0.15. The first number is clay, the second is sand, and the third is straw. Numbers are separated by colons; the straw value can be a decimal (for example 0.1 = 10 percent of the solid ingredients by volume).
• Straw bale mass (kg) – Typical small square bales weigh about 18–23 kg. The default 20 kg is a reasonable mid-range value, but you should update this if your supplier’s bales are significantly heavier or lighter.

How the cob floor calculator works

The calculator uses the floor area and layer thickness to compute the total cob volume, then splits that volume into clay, sand, and straw using the ratio you provide. Finally, it multiplies each component volume by a typical bulk density to estimate masses, and divides the straw mass by a bale weight to estimate bale counts.

Step 1: Floor volume

If A is the floor area in square metres and t is the layer thickness in centimetres, the script first converts thickness into metres and calculates the raw volume:

Here V is the total cob volume in cubic metres (m³).

Step 2: Splitting the mix by ratio

The ratio string (for example 1:2:0.15) is interpreted as parts by volume:

• p_clay = 1
• p_sand = 2
• p_straw = 0.15

The calculator sums these to get the total number of parts:

p_total = p_clay + p_sand + p_straw = 3.15

Each component’s share of the volume is then:

V_component = V × p_component / p_total

For example, clay volume is V_clay = V × p_clay / p_total, sand volume is V_sand = V × p_sand / p_total, and straw volume is V_straw = V × p_straw / p_total.

Step 3: Converting volumes to masses

The calculator uses typical bulk densities for common cob ingredients:

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

Mass is then approximated by:

m_component = V_component × density_component

This yields clay and sand quantities in kilograms and straw in kilograms as well. The tool then divides the straw mass by the bale mass you supply to give an approximate bale count.

Step 4: Straw bales

If m_straw is the straw mass in kilograms and m_bale is the mass of one bale, the approximate number of bales is:

bales = m_straw ÷ m_bale

Because bales are not perfectly uniform, the result is an estimate, not a precise order quantity.

Interpreting your results

After you run the calculator, you will typically see:

• Total cob volume – The combined volume of clay, sand, and straw for the selected layer in cubic metres.
• Clay volume and mass – Useful for estimating how many trailer loads or bags of clay-rich subsoil you might need.
• Sand volume and mass – Helps you order sand by cubic metre or tonne, depending on how your supplier sells it.
• Straw volume, mass, and bale count – Lets you check whether a single pallet or truckload of bales will be enough.

Use these outputs as a planning baseline. On site you will still adjust water content, clay proportion, and straw density to match your particular materials and climate.

Worked example: cob floor for a small room

Imagine a 25 m² room where you want a 5 cm cob subfloor layer using a 1:2:0.15 clay:sand:straw ratio, and straw bales that weigh 20 kg each.

1. Total volume

Convert thickness to metres:

t = 5 cm ÷ 100 = 0.05 m

Calculate volume:

V = A × t = 25 m² × 0.05 m = 1.25 m³

2. Volume shares from the ratio

Total parts:

p_total = 1 + 2 + 0.15 = 3.15

Clay volume:

V_clay = 1.25 × 1 / 3.15 ≈ 0.397 m³

Sand volume:

V_sand = 1.25 × 2 / 3.15 ≈ 0.794 m³

Straw volume:

V_straw = 1.25 × 0.15 / 3.15 ≈ 0.059 m³

3. Mass estimates

Using the typical densities above:

m_clay ≈ 0.397 × 1,600 ≈ 635 kg

m_sand ≈ 0.794 × 1,500 ≈ 1,191 kg

m_straw ≈ 0.059 × 100 ≈ 5.9 kg

4. Straw bales

If each straw bale weighs about 20 kg, then:

bales = 5.9 ÷ 20 ≈ 0.3

That means a single small square bale is more than enough for this layer. In practice you would probably keep one or two spare bales on site to allow for wastage, test patches, and other small cob jobs.

Typical cob floor mixes and where this layer sits

Cob floors often use slightly different ratios depending on whether the layer is structural, leveling, or a refined finish coat. The calculator is most suitable for the bulk structural or subfloor layer, but you can also use it as a rough guide for other layers by adjusting the ratio and thickness.

Above the earthen layers, many builders apply drying oils (such as linseed or tung oil) and sometimes waxes to harden and seal the surface. Those finishes are not covered by this calculator, but you may wish to add a buffer when ordering materials for test patches and extra coats.

Why use cob floors?

Cob floors appeal to natural builders, architects, and homeowners who value low embodied energy and a warm, tactile interior surface. Properly designed and protected from moisture, cob floors can last for decades with modest maintenance.

• Low embodied energy – Most of the mass comes from sand, local subsoil, and straw rather than high-temperature industrial products.
• Thermal mass – Earthen floors store heat from the sun or from radiant heating systems and release it slowly, smoothing temperature swings.
• Comfort – When finished and oiled correctly, cob floors can be smooth yet slightly forgiving underfoot.
• Repairability – Localized damage is often repairable with matching cob and refinishing, rather than replacing an entire floor.

This calculator supports those benefits by helping you size your mixes and coordinate deliveries, especially when working with volunteers or small crews.

Assumptions and limitations

The estimates from this calculator depend on several simplifying assumptions. Keep these in mind when planning your project:

• Volume-based ratios – The ratio you enter is interpreted by volume, not by weight. This matches common on-site practice with buckets or shovels but does not account for variations in density between materials or suppliers.
• Typical densities – The tool uses fixed, typical bulk densities for clay, sand, and straw. Real-world materials can be significantly heavier or lighter depending on moisture content, grain size, and compaction.
• Uniform thickness – The floor is treated as a flat slab of constant thickness. Slopes toward drains, thickened areas at thresholds, and local buildup are not calculated separately.
• No waste factor – The calculator does not automatically add extra for mixing losses, trimming, or test batches. It is usually wise to add 5–15 percent to the calculated volumes when ordering materials.
• Dry vs. wet volume – Shrinkage as the cob cures and moisture evaporates is not explicitly modeled. The volumes should be read as approximate wet-mix volumes.
• Non-structural guidance – The tool is intended for planning and ordering materials, not for structural design or code compliance. For heavily loaded floors, unusual soil conditions, or building code questions, consult an experienced designer or engineer.

Because of these limitations, you should always verify your chosen mix with test patches on site before committing to large pours or final finish layers.

Practical tips for using the results on site

• Translate volumes into your actual mixing units. For example, if one wheelbarrow holds about 0.06 m³, then a 1.25 m³ total mix would be roughly 21 wheelbarrow loads.
• For bucket mixing, decide how many litres your bucket holds and convert cubic metres into bucket counts (1 m³ = 1,000 litres).
• Mix small test batches using the same clay, sand, and straw you will use on the floor. Adjust the ratio slightly if test patches crack excessively or feel weak once dry.
• Record the bucket or wheelbarrow recipe that works best and scale it up using the calculator’s total volumes as a guide.

Frequently asked questions

How accurate is this cob floor material estimate?

The estimates are generally good enough for planning and ordering, provided your materials are close to the assumed densities and your floor thickness is reasonably uniform. However, natural materials vary, and on-site technique has a large influence, so it is safer to treat the outputs as approximate and add a contingency.

Can I use this calculator for earthen subfloors only?

You can use it for subfloors, structural cob layers, or thicker earthen leveling coats by entering the appropriate thickness and ratio. Very thin finish coats or highly specialized mixes may require different assumptions than the defaults used here.

What if my clay or sand density is different?

If you know your material densities, you can compare them with the typical values described above to judge whether the mass estimates are likely to be high or low. At present the interface does not expose density inputs to keep it simple, so consider that the calculator’s mass outputs are based on typical ranges rather than measurements from your specific site.