What is the correct water-to-plaster ratio?

It depends on the plaster type. Plaster of Paris uses roughly 70 parts water to 100 parts plaster (0.7:1 ratio by weight). Harder plasters like Herculite use less water (0.4:1). Less water produces a harder, denser result. More water produces a softer, more porous result.

Should I add plaster to water or water to plaster?

Always add plaster to water, never the other way around. Sift the plaster into the water and let it soak (slake) for 1-2 minutes before mixing. This reduces air bubbles and gives a smoother, stronger cast.

How do I avoid air bubbles in plaster?

Mix gently — avoid whipping or fast stirring. After mixing, tap the container on the table to release bubbles. When pouring, pour in a thin stream from one corner and let it flow across the mould. Vibrate the mould gently after pouring.

How do I know when plaster is ready to pour?

After mixing, plaster goes through stages: thin (like cream), then thicker (like yoghurt), then sets. Pour at the cream stage for detail work, or at the yoghurt stage for quicker setting. If it starts to feel warm, it is about to set — pour immediately.

ArtCalcs

Plaster Mixing Calculator

Calculate water-to-plaster ratio and volume needed for casting.

Mould Volume

Internal volume of mould in millilitres

Plaster TypeType of plaster or casting material

PurposeIntended use affects water ratio

How We Calculate This

Weight Calculation

Total Weight = Mould Volume x Plaster Density

Plaster Amount

Plaster = Total Weight / (Water Ratio + 1)

Water Amount

Water = Plaster x Water Ratio

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Always verify quantities before purchasing materials.