What is the correct water-to-plaster ratio?

It depends on the plaster. Plaster of Paris uses roughly 70 parts water to 100 parts plaster (0.70:1 by weight). Harder gypsum plasters use less water: Crystacal R is 0.35:1 and Herculite No.2 is 0.42:1 (Saint-Gobain Formula datasheets), and Type IV dental stone is around 0.20:1. Less water gives a harder, denser, less porous cast; more water gives a softer, more porous one.

Is Jesmonite mixed the same way as plaster?

No. Jesmonite AC100 is a two-part acrylic-modified composite, not a water-gauged plaster. You mix 2.5 parts Base powder to 1 part AC100 Liquid (acrylic) by weight — never with plain water, which would give a weak, mis-cured cast. Add the powder to the liquid and mix until smooth. This calculator switches to the AC100 Base/Liquid split automatically when you select Jesmonite.

Should I add plaster to water or water to plaster?

For gypsum plasters, always add plaster to water — 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. (Jesmonite is the reverse: powder is added into the liquid.)

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 TypeGypsum plaster gauged with water, or Jesmonite AC100 (two-part composite)

PurposeRule-of-thumb water tweak for gypsum plasters (ignored for Jesmonite)

How We Calculate This

Wet pour weight

The mould volume (plus your overflow allowance) is multiplied by the wet-slurry density of the freshly mixed pour to give the total weight of water plus plaster needed to fill it.

Total Weight = Mould Volume (incl. overflow) × Wet-Slurry Density

Splitting water and plaster

That total weight is split using the water-to-plaster ratio for the chosen plaster (Plaster of Paris 0.70, Crystacal R 0.35, Herculite No.2 0.42, Type IV dental stone 0.20).

Plaster = Total Weight ÷ (Water Ratio + 1)

Water = Plaster × Water Ratio

Jesmonite AC100

Jesmonite is not water-gauged. The mixed weight (≈1.75 g/cm³ × volume) is split 2.5 parts Base powder to 1 part AC100 Liquid by weight, and the calculator labels the outputs accordingly.

Frequently Asked Questions

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Last updated: March 2026

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