Mould Volume Calculator — Silicone & Plaster Moulds

Standard wall thickness is 10-20 mm for silicone moulds. Thin walls (10 mm) work for small, simple shapes. Complex forms with undercuts need 15-20 mm. Very large moulds may need 25 mm plus a rigid mother mould.

Silicone rubber costs £35-50 per kg in the UK. Its high price is offset by durability — a good silicone mould lasts 20-50 castings. Plaster moulds are cheap but limited to 1-5 uses. Consider cost per casting, not per mould.

Plaster of Paris is the cheapest at about £2/kg. It works well for simple shapes and waste moulds (broken off after one use). Jesmonite at £8/kg is good for larger piece moulds. Alginate (£25/kg) is for life casting.

This calculator estimates volume as 70% of the bounding ellipsoid — a good average for organic sculpture shapes. For precise measurement, submerge the object in water and measure displacement.

Flexible moulds (silicone, latex) need a rigid outer shell (mother mould) to maintain shape during casting. Make the mother mould from plaster bandage or fibreglass. Two-part mother moulds allow easy demoulding.