Slip Casting

Plaster Mixing Calculator

Calculate exact plaster and water weights needed to fill a mold volume using the WCR method.

Reviewed by ClayCalc Editorial · Updated May 2026

Target Volume to Fill

Plaster Grade

Custom Water:Plaster Ratio (optional)

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Results

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Enter your measurements above and click Calculate.

The Water:Plaster Ratio (WCR) Method

The WCR (Water:Plaster Ratio) expresses the weight of water relative to plaster weight. For Pottery Plaster #1, the standard WCR is 0.70 — 70 parts water per 100 parts plaster by weight. Lower WCR produces harder, denser plaster (Hydrostone uses 0.32).

Formula

Given a target volume, this calculator solves for the plaster weight needed using the combined density formula:

V_total = P_weight / plaster_density + P_weight × WCR / water_density → P_weight = V_total / (1/plaster_density + WCR)

Pottery Plaster #1 has a set density of approximately 1.52 g/cm³. Water density is 1.0 g/cm³.

Grade Comparison

Grade	WCR	Hardness	Best For
Pottery Plaster #1	0.70	Medium	Jiggering, bat molds
Casting Plaster	0.72	Softer	Slip casting molds
Hydrocal	0.45	Hard	Master models
Hydrostone	0.32	Very Hard	High-durability molds

Method & review note

About this calculation

Uses weight, volume and ratio relationships. Product data sheets and measured viscosity/thixotropy take priority over generic presets.

Reference: USG — No. 1 Pottery Plaster. See the editorial methodology for testing, precision and corrections.

Input handling: Values are sent to the ClayCalc server for this request and are not written to the application database.

Read the Pottery Plaster Mixing

lightbulb Quick Tips

check_circle Always sift plaster into water — never the reverse. Sprinkle it evenly across the entire surface.
check_circle Slake (rest) 2 minutes after all plaster is added before mixing. This prevents air bubbles in the mix.
check_circle Pour plaster in a continuous stream, tilted to let air escape from the mold before it fills.
warning Never pour plaster down the drain — it will set and block your plumbing permanently.

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Frequently Asked Questions

What is water:plaster ratio (WCR)? expand_more

WCR is the ratio of water weight to plaster weight. Pottery Plaster #1 has a WCR of 0.70, meaning 70g of water per 100g of plaster. A lower WCR (like Hydrostone at 0.32) produces harder, denser plaster.

Why do you add plaster TO water, not water to plaster? expand_more

Adding water to dry plaster causes lumping and inconsistent hydration. Sifting plaster slowly into water allows even wetting and produces a smoother, more consistent mix. Always add plaster to water.

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

Slake undisturbed for 2 minutes after all plaster is added. Mix gently for 2–3 minutes. The plaster is ready when it leaves a slight trail on the surface (called "trailing"). Pour before heat becomes noticeable.

Can I use builders plaster (plaster of Paris) for pottery molds? expand_more

Craft or builders plaster is softer and less durable than USG Pottery Plaster #1 or equivalent. Molds made from it wear out faster. For production slip casting, always use a dedicated pottery-grade plaster.