Pottery Bisque Kiln Load Calculator
Calculate how many pottery pieces fit in your kiln for bisque firing based on kiln size, piece dimensions, and stacking rules.
Bisque firing basics:
Bisque firing is the first firing of raw (greenware) clay, typically to cone 06–04 (1830–1940°F / 999–1060°C). Unlike glaze firing, bisque pieces CAN touch each other and can be stacked or nested because there is no glaze to fuse them together.
Kiln loading rules for bisque:
- Pieces may touch each other (no glaze to stick)
- Pieces can be stacked inside each other (bowls nested, cups stacked rim-to-rim)
- Leave 0.5–1 inch (1.3–2.5 cm) between pieces and kiln walls for heat circulation
- Leave 1 inch between pieces and kiln elements (electric kilns) to prevent hot spots
- Do not block peepholes or thermocouple ports
- Place heaviest pieces on the bottom shelf
Usable kiln space formula:
Usable volume (cu in) = (Kiln diameter - 2) × (Kiln diameter - 2) × π/4 × (Kiln height - 1)
For square kilns: (Width - 2) × (Depth - 2) × (Height - 1)
The subtracted inches account for wall clearance and lid clearance.
Packing efficiency:
In practice, you cannot fill 100% of the usable space — pieces are irregular shapes with hollow interiors. Typical packing efficiencies:
- Bowls and cups (nestable): 40–55% of usable volume
- Plates and tiles (stackable): 50–65% of usable volume
- Vases and tall forms: 25–35% of usable volume
- Mixed load: 35–45% of usable volume
Worked example:
An 18" wide × 18" deep × 18" high electric kiln:
- Usable volume = (18-2) × (18-2) × (18-1) = 16 × 16 × 17 = 4,352 cubic inches
- Average mug volume (with handle): ~60 cubic inches
- Mixed load efficiency: 40%
- Effective volume: 4,352 × 0.40 = 1,741 cubic inches
- Estimated mugs: 1,741 ÷ 60 = ~29 mugs
Shelf considerations:
Each kiln shelf plus posts takes about 1.5–2 inches of vertical space. A kiln with 3 shelves loses ~6 inches of stacking height. Factor this into your calculation.
Weight limit:
Most kiln shelves hold 30–50 lbs safely. A typical greenware mug weighs 0.75–1.5 lbs. Don’t exceed shelf weight limits even if you have space — shelves can crack or warp under excessive load during firing.