Cob House Material Calculator
Estimate clay, sand, straw, and water quantities for a hand-built cob wall using the classic 40 / 35 / 15 / 10 volumetric mix proportions.
How this works
Wall volume is computed from wall geometry with the door / window openings subtracted:
wallVol = (wallLength × wallHeight − openingsArea) × wallThickness
clay = wallVol × 0.40
sand = wallVol × 0.35
straw = wallVol × 0.15
water = wallVol × 0.10 (litres = m³ × 1000)The proportions sum to 1.0, so the four materials add back to the net wall volume exactly.
Worked example
A 30 m of perimeter wall, 2.7 m tall, 0.45 m thick, with 6 m² of openings:
- Net wall area = 30 × 2.7 − 6 = 75 m²
- Wall volume = 75 × 0.45 = 33.75 m³
- Clay (40%) ≈ 13.50 m³
- Sand (35%) ≈ 11.81 m³
- Straw (15%) ≈ 5.06 m³
- Water (10%) ≈ 3,375 L
Sources
- Vernacular cob mix proportions (Smith / Evans / Smiley canon, west-coast North American practice)
FAQ
What exactly is cob and how is it different from adobe?
Cob is a hand-shaped mix of clay-rich subsoil, sand, straw, and water that is built up in monolithic wet courses without forms. Adobe is the same family of materials but cast into sun-dried bricks first and laid up like masonry. Cob walls are sculpted in place; adobe walls are stacked. Cob can curve and corbel freely, which is why traditional cob homes often have organic, rounded shapes.
Why are the proportions 40% clay, 35% sand, 15% straw, 10% water?
These are the volumetric proportions used in classic English and west-coast North American cob practice. Clay is the binder, sand is the bulk aggregate that prevents shrinkage cracks, straw is the tensile reinforcement, and water is the workability agent. Real subsoils vary, so always test a small batch first — if it cracks badly when dry, add sand; if it crumbles, add clay.
How thick should a cob wall be?
Load-bearing cob walls are typically 450 to 600 mm thick at the base, tapering slightly as they rise. The default of 0.45 m matches the most common modern cob house specification. In high-seismic regions like the Himalayas, consult a structural engineer — cob has poor tensile capacity and benefits from a continuous ring beam at the top of every storey.
How long does cob take to dry and how much does it shrink?
A 450 mm cob wall typically takes 2 to 4 months to dry fully in good weather, and shrinks 1 to 3 percent linearly as it dries. Plan the build so walls have a full summer to cure before the first wet season. The roof should be installed early (cob loves a good hat) so the walls dry evenly without rain damage.
Is the openings area subtracted before computing the wall volume?
Yes. The calculator computes net wall area as length × height − openings, then multiplies by thickness. This is the conservative way to estimate material — you do not pour cob in the door or window holes. If you have not finalised opening positions, leave the openings field at zero and treat the result as the gross requirement.
Can I substitute the straw with another fibre?
Yes — local long-fibre straw (rice, wheat, barley) is traditional, but hemp shiv, jute, or coconut fibre work too. Avoid short or chopped fibres; they do not bridge cracks effectively. Whatever fibre you use, treat it as a 15 percent volume input and inspect the test patch before committing to a full batch.