Total Material Cost
$51
Cob Volume
3.7 cu ft
Pizzas
2
Outer Dia.
30"
Estimated Material Cost
$51
Total Cob
3.7 cu ft
Pizza Capacity
2 pizzas
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Result
Dome volume calculated from hemisphere geometry: outer radius = 15" (11" inner + 4" wall). Cob volume = outer hemisphere - inner hemisphere + floor slab. Materials split per 1:3:0.5 ratio.
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Result
Larger dome with 5" walls and ceramic fiber insulation. Higher material cost due to premium insulation at $8/cu ft vs $2/cu ft for perlite.
A standard cob mix uses a 1:3:0.5 ratio of clay to sand to straw by volume. A medium 22-inch oven with 4-inch walls needs about 4–6 cubic feet of cob mix total: roughly 1 cu ft clay, 3 cu ft sand, and 1 small bale of straw.
| Oven Size | Inner Diameter | Total Cob | Pizza Capacity |
|---|---|---|---|
| Small | 16" | ~3 cu ft | 1 pizza |
| Medium | 22" | ~5 cu ft | 2 pizzas |
| Large | 28" | ~10 cu ft | 3 pizzas |
| Extra Large | 36" | ~18 cu ft | 5 pizzas |
For a family of 4, a 22-inch interior diameter oven fits 2 pizzas at once and is the most popular size for home builds. It heats up in about 2 hours and retains heat for baking bread afterward. A 28-inch oven is better for entertaining groups of 8+.
Insulation dramatically improves heat retention. An uninsulated cob oven loses heat in 1–2 hours; an insulated one holds baking temperature for 4–6 hours. Perlite-cement mix (5:1 ratio) is the most common DIY insulation, applied 3–4 inches thick over the dome.
A cob oven takes 3–5 days of active work spread over 2–4 weeks of drying time. Day 1: build the base. Day 2: sculpt the sand form and apply the thermal layer. Days 3–4: add insulation and rain shell. Allow 1–2 weeks drying between layers and before the first firing.
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Last Updated: Mar 20, 2026
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