What is the correct plaster-to-water ratio?

Standard Plaster of Paris uses 1.5:1 (plaster:water by weight). Harder plasters like Crystacal use 2.2:1. Always add plaster to water (never water to plaster) and sprinkle evenly to avoid lumps.

Should I measure by weight or volume?

Always measure by weight using digital scales. Volume measurement is unreliable because plaster powder has varying density depending on how compressed it is. Use grams for plaster and millilitres (= grams) for water.

How long does plaster take to set?

Plaster of Paris sets in 10-15 minutes, Herculite in 15-20 minutes, Crystacal in 20-30 minutes. Temperature affects set time — warm water accelerates setting. Adding a pinch of salt or reducing water speeds up the set.

What plaster is strongest for casting?

Crystacal R is the strongest common plaster for art casting, with high compression strength. Herculite No. 2 is a good mid-range option. Standard Plaster of Paris is weakest but cheapest. For very high strength, use Jesmonite or resin instead.

How do I mix plaster without bubbles?

Sprinkle plaster evenly across the water surface until small islands form. Let it slake (absorb) for 2-3 minutes. Then mix gently from the bottom using a flat stick. Do not whisk — this traps air. Tap the container to release bubbles.

ArtCalcs

Plaster Mix Calculator — Plaster to Water Ratio

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

Volume Needed

Volume of cavity to fill

Plaster Type

How We Calculate This

How Plaster Mix Calculation Works

Total weight = Volume x Set plaster density

Water = Total / (1 + Ratio)

Plaster = Total - Water

From the desired volume, the calculator works backwards using the set plaster density and the mixing ratio to determine exact plaster and water weights.

Frequently Asked Questions

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Last updated: February 2026

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