Slip Casting

Plaster Mold Volume Calculator

Calculate how much pottery plaster and water to mix for a block, cylinder, or oval casting mold based on cavity dimensions and wall thickness.

Updated May 2026

Cavity Shape

Cavity Length (cm)

Cavity Width (cm)

Cavity Diameter (cm)

Cavity Height / Depth (cm)

Mold Wall Thickness (cm)

Standard: 4–6cm. Thicker = more absorbent.

Plaster : Water Ratio (by weight)

Pottery Plaster No.1 standard: 1.20 (120g plaster per 100mL water).

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Results

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Enter your measurements above and click Calculate.

Calculating Plaster for a Mold

The amount of plaster needed is determined by the volume of plaster that makes up the mold walls — the total mold volume minus the empty cavity. The plaster-to-water ratio determines the final density and absorbency.

Mixing Formula

Plaster volume (cm³) = Total mold volume − Cavity volume
Slurry mass (g) = Plaster volume × slurry density (~1.55 g/cm³)
Water (g)    = Slurry mass / (1 + plaster:water ratio)
Plaster (g)  = Water (g) × plaster:water ratio

Standard Mixing Ratios

Ratio (P:W)	Result	Use
1.0 : 1	Soft, very absorbent	Experimental / large models
1.2 : 1	Standard pottery plaster	Slip casting, press molds ✓
1.4 : 1	Hard, less absorbent	Models and bats
1.6 : 1	Very hard, dense	Master models, waste molds

Correct Mixing Procedure

Measure water first into a clean bucket.
Sift plaster into the water slowly — never water into plaster.
Let the plaster slake for 60 seconds undisturbed.
Mix by hand for 2–3 minutes until smooth. Do not introduce air.
Pour when the mix leaves a faint trace on the surface (consistency of thin cream).
Tap the mold form to release air bubbles.

Why the 10% Safety Margin?

Even with precise measurements, real molds are never perfect geometric shapes. Plaster also adheres slightly to the mixing bucket. The 10% margin ensures you can complete the pour without running short — overpoured excess is far less damaging than an incomplete fill.

lightbulb Quick Tips

check_circle Never add water to already-mixed plaster to thin it — this destroys the crystal structure and produces a weak, crumbly mold. Start fresh if the mix gets too thick.
check_circle Seal wooden or cardboard forms with petroleum jelly or spray release before pouring — plaster bonds permanently to porous surfaces.
check_circle A new plaster mold must dry completely before use — typically 24–48h at room temperature. Using a wet mold slows casting dramatically and risks the mold crumbling.
check_circle Never put plaster waste down a drain — it sets hard and blocks pipes permanently. Let excess plaster set in the bucket, then bin it as solid waste.

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Frequently Asked Questions

What is the correct plaster-to-water ratio for pottery molds? expand_more

Pottery Plaster No. 1 (US Gypsum) is mixed at 1.2 parts plaster to 1 part water by weight (i.e., 120g plaster per 100mL water). This gives a strong, absorbent mold with good casting life. Stiffer ratios (less water) make harder, less absorbent molds.

How thick should the walls of a plaster mold be? expand_more

For one-part press molds, 4–6cm of plaster is standard. Thicker walls absorb more water and last longer but are heavier. For multi-part slip casting molds, 5–7cm is typical. Very thin walls (under 3cm) crack easily.

Why does the calculator add a 10% safety margin? expand_more

Mold cavities are rarely perfectly geometric, and you will always lose a small amount of plaster in the mixing bucket and to premature hardening. A 10% excess ensures you have enough to fill the form completely without running short mid-pour.

How do I know when the plaster is ready to pour? expand_more

Add plaster to water (not water to plaster). Let it slake for 1 minute, then mix by hand for 2–3 minutes until smooth and lump-free. Pour when it reaches the consistency of heavy cream (it will just leave a slight trail on the surface). Do not wait until it thickens — overly thick plaster creates weak, laminated molds.