Cob Building Calculator

This calculator multiplies three dimensions together -- Total Wall Length, Wall Height, and Wall Thickness -- into a single wall volume, then splits that volume into a fixed cob recipe: 25% clay-heavy subsoil, 60% sand and fine gravel, and the remaining 15% scaled into straw bales (about one bale per 3 cu ft of that straw share). Because volume is a straight product of all three dimensions, each one has an equal proportional pull on every material quantity -- pushing Wall Thickness from the default 18 in up to 24 in adds a third more volume and a third more of every material, the same as adding a third more length or height would.

The default 20 ft of wall at 8 ft tall and 18 in thick calls for 240 cu ft of cob, which breaks down to about 2.2 cu yd of clay soil, 5.3 cu yd of sand and gravel, and 12 straw bales. What it does not model is moisture content of the source soil, foundation stem-wall height (cob should sit above grade on a stone or concrete base), or the extra material lost to niches, curves, and sculptural shaping that cob is often chosen for.