What is the correct 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.

Should I measure by weight or volume?

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 estimates the water from the mould volume (about 0.67 g of water per cm³ for casting plaster), then sets the plaster weight from the consistency. Weigh plaster in grams and water in millilitres (1 ml water = 1 g).

How long does plaster take to set?

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.

What plaster is strongest for casting?

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.

How do I mix plaster without bubbles?

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.

ArtCalcs

Plaster Mix Calculator — Plaster to Water Ratio

Work out the correct plaster-to-water ratio by weight for casting and mould making.

Volume Needed

Internal volume of the mould cavity to fill. Tip: fill the mould with water, then measure that water — 1 ml = 1 cm³.

Plaster TypeRatios shown are plaster:water by weight, from each manufacturer's consistency.

How We Calculate This

How Plaster Mix Calculation Works

This uses the standard potter's water-first volume method, anchored on each plaster's manufacturer consistency (parts water per 100 parts plaster by weight):

Water (g) = Volume (cm³) × 0.671 (= 11 g water per cubic inch ÷ 16.387 cm³ per cubic inch)

Plaster (g) = Water × (100 ÷ Consistency)

Estimating the water from the mould volume first, then deriving the plaster from the consistency, keeps the mix faithful to the datasheet — harder plasters (lower consistency) automatically come out denser and stronger. Consistencies used: Plaster of Paris 67, Herculite No. 2 42, Crystacal R 35, dental stone 30.

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Last updated: June 2026

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