Plaster Mix Calculator — Plaster to Water Ratio

It depends on the plaster. Manufacturers quote a "consistency": the parts of water by weight per 100 parts of plaster. Fine casting Plaster of Paris is about 67 parts water per 100 (roughly 1.5:1 plaster:water). Saint-Gobain Formula specifies Herculite No. 2 at 100/42 (2.38:1) and Crystacal R at 100/35 (2.86:1) — the harder, denser plasters take less water, not more. Always sprinkle plaster onto the water and never the reverse.

Always measure by weight using digital scales. Volume measurement of the powder is unreliable because plaster density varies with how compressed it is. This calculator sizes the batch so the poured slurry exactly fills the mould: it splits the cavity volume between water and dry powder (water plus plaster-powder volume equals the cavity volume, using a hemihydrate solid density of about 2.7 g/cm³), then sets the plaster weight from the consistency. That is roughly 0.44-0.65 g of water per cm³ depending on the plaster, not a fixed figure — harder, lower-consistency plasters use less water per cm³. Weigh plaster in grams and water in millilitres (1 ml water = 1 g).

Plaster of Paris sets in roughly 10-15 minutes, Herculite No. 2 in 15-20 minutes and Crystacal R in 20-30 minutes; dental stone is faster at 8-12 minutes. Warm water accelerates the set and cold water slows it. A pinch of salt speeds setting; a little vinegar or citric acid retards it. These are working-time guides — always check your batch.

Crystacal R is the strongest common art plaster (Saint-Gobain quote a high compressive strength at 100/35). Herculite No. 2 (100/42) is a strong mid-range choice. Standard Plaster of Paris is the weakest and cheapest. Over-watering any of them — adding more than the quoted consistency — increases porosity and reduces strength, so stick to the manufacturer figure. For the highest strength use Jesmonite or resin instead.

Sprinkle the plaster evenly across the water surface until small dry islands stop sinking. Let it slake (absorb) for 1-2 minutes — do not stir during this time. Then mix gently from the bottom with a flat tool. Do not whisk, as that traps air. Tap or vibrate the container to bring trapped bubbles to the surface before pouring.