ArtCalcs.uk
Clay Weight Calculator — Solid & Hollow Sculpture
Estimate the weight of clay needed for a solid or hollow sculpture.
How We Calculate This
How Clay Weight Calculation Works
Wet weight = Clay volume × wet density
The calculator works out the volume of your chosen shape from its outside dimensions, subtracts the hollow interior when you give a wall thickness, then multiplies by the wet plastic density to get the wet weight.
The bone-dry weight is the wet weight minus the water of plasticity (about 20% of the mass for a plastic clay body), since drying only removes water. The fired weight is the bone-dry weight minus the loss on ignition — the chemically bound water, carbon dioxide and organics driven off in the kiln (roughly 5-8% of the dry mass). Because firing can only remove mass, the fired weight is always lower than the bone-dry weight, giving a total wet-to-fired loss of roughly a quarter (about 25%). Linear shrinkage changes the finished size but not the mass, so it is reported separately.
Frequently Asked Questions
Wet clay weighs about 1.7-1.9 g/cm³ depending on type. Stoneware is heavier than earthenware. A 20 cm solid sphere of stoneware weighs about 8 kg wet. Hollow forms use significantly less clay.
Total linear shrinkage from wet to fired is roughly 6-8% for earthenware, 11-13% for stoneware and 12-16% for porcelain. It happens in two stages — drying then firing — with each contributing about half. Because it is linear, a 12% rate means a piece loses around 32% of its volume, so always scale your dimensions up when planning a finished size.
Solid clay pieces over about 3 cm thick can crack or explode during firing as trapped moisture turns to steam. Hollowing to 1-2 cm wall thickness reduces weight and prevents kiln disasters.
For most kilned work, walls should be 10-15 mm. Very large pieces can go to 20 mm. Thin walls (under 8 mm) may crack or warp. Air-dry clay can be thinner (5-8 mm) since it is not fired.
Buy 20-30% more than calculated to account for waste, test tiles and mistakes. Clay is sold in 10-12.5 kg bags in the UK. For a first project, start with one bag and track your usage.
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Last updated: February 2026
All calculations are estimates. Always verify quantities before purchasing materials.