Plaster Mold Mix Calculator

A plaster mold calculator estimates how much dry plaster and water to weigh for a mold: divide the mix volume by (consistency ÷ 100 + 1 ÷ specific gravity) to get the plaster weight, then multiply by consistency ÷ 100 for the water. Enter your mold size, optional model volume and allowance, and two data-sheet numbers.

How to use the plaster mold calculator

1. Choose the mold shape: rectangular (box) or round (cylinder).
2. Pick metric (centimeters and grams) or US units (inches and pounds).
3. Measure the inside of the mold or cottle: length and width, or diameter, plus the depth you plan to pour. Each dimension must be greater than 0 and no more than 1000 in the unit you chose.
4. Optional: enter the model volume in milliliters for the model or anything else inside the mold that the plaster will not fill. It is always in mL, even with US units (1 in³ ≈ 16.39 mL). It must be 0 or more and smaller than the mold volume. Leave it at 0 for an empty mold.
5. Optional: enter an extra allowance from 0 to 100% to cover spills and plaster left in the bucket. Leave it at 0 for no margin.
6. Enter the consistency from your plaster’s bag or technical data sheet. This is the weight of water per 100 parts of plaster (input bounds: greater than 0, up to 500).
7. Enter the true particle specific gravity of the plaster from the data sheet (input bounds: greater than 1, up to 10). Do not use the bulk density of the loose powder or the specific gravity of mixed slurry, because either will give wrong results.
8. Press Calculate mix and weigh out the estimated plaster and water shown. If an input is outside its bounds, a message next to the form tells you what to fix.

The result is an estimate. It assumes the volumes of plaster and water simply add, and it ignores trapped air and real variation in slurry volume. No waste margin is added unless you enter an allowance. Decimal commas such as 2,69 are accepted.

How the plaster and water amounts are calculated

The calculator works in four steps. First it finds the geometric volume of the mold. A box is length × width × height, and a cylinder is π × r² × h, where r is half the diameter. Inches are converted to centimeters with the exact definition 1 in = 2.54 cm, and one cubic centimeter equals one milliliter.

Second, it subtracts the model volume and applies your allowance:

Mix volume (mL) = (Geometric volume − Model volume) × (1 + Allowance ÷ 100)

Third, it estimates how much plaster fits in that mix volume. Each gram of plaster brings c ÷ 100 grams of water, where c is the consistency. The calculator assumes water weighs about 1 g per mL (specific gravity 1.0), so that water takes up roughly c ÷ 100 mL. The plaster particles take up 1 ÷ SG mL, which is why the true particle specific gravity is required:

Plaster (g) = Mix volume (mL) ÷ (c ÷ 100 + 1 ÷ SG)
Water (g) = c × Plaster ÷ 100

Finally, US results are converted with 1 lb = 453.59237 g and 1 oz = 28.349523125 g, both exact legal definitions. Under the 1 g/mL water approximation, the water weight in grams also gives its approximate volume in milliliters.

Worked example: USG No. 1 Pottery Plaster lists a normal consistency of 70 (70 lb water per 100 lb plaster) and a wet density of 1586 kg/m³. With c = 70, that wet density works out to about 0.933 g of plaster and 0.653 g of water per mL of slurry. In this model, that corresponds to an effective plaster SG of about 2.69. That SG figure is our derivation, not a value published by USG.

Getting accurate numbers for your plaster mold

Your inputs decide how accurate the estimate is, and the two plaster properties matter most. Consistency changes the strength and absorbency of a set mold. Pottery plasters are sold with a recommended value, and the calculator uses whatever number you enter. The input bounds in the form only reject impossible or mistyped values; they are not recommendations.

No specific gravity is prefilled, because we could not verify a published figure for the hemihydrate powder in the bag. The formula needs the true particle specific gravity. That is not the bulk density of loose powder, which includes air between the grains, and it is not the specific gravity of the wet slurry. For context, set gypsum (calcium sulfate dihydrate) has a density of about 2.32 g/cm³ and anhydrous calcium sulfate about 2.96 g/cm³. Neither is the powder you mix, so check your product’s data sheet. The USG-derived 2.69 is a rough reference for that plaster only.

• Measure inside dimensions, not the outside of the mold box.
• For a cottle around a model, enter the model’s volume in mL so it is subtracted. One way to find it is to measure how much water the model displaces, if the model can safely get wet.
• Use the allowance for spills and residue instead of padding the dimensions.
• Weigh both plaster and water, as USG advises for each mix. A digital scale in grams gives the finest control.
• Follow the water temperature range on your data sheet. USG lists 70 °F to 100 °F (21 °C to 38 °C) for No. 1 Pottery Plaster.

Methodology & sources

The geometry and unit conversions use standard, exact definitions. The mix volume is the geometric volume minus your model volume, scaled by your allowance. The mix formula assumes the volumes of plaster and water add together in the slurry and ignores trapped air and real slurry-volume variation, so all results are estimates. Water is treated as 1 g per mL, an assumed approximation based on its specific gravity of 1.0. The plaster properties are always your own inputs. Source data checked 2026-11-09.

• Wikipedia – Cylinder: volume V = π r² h
• Wikipedia – Inch: 1 in = 2.54 cm exactly
• Wikipedia – Pound (mass): 1 lb = 453.59237 g; 16 oz per lb
• Wikipedia – Ounce: 1 oz = 28.349523125 g
• Digitalfire – Specific gravity: water SG 1.0 = 1 g per cc (used as an approximation)
• USG No. 1 Pottery Plaster submittal sheet (IG1366): consistency, densities and mixing guidance used in the worked example
• Wikipedia – Calcium sulfate: dihydrate and anhydrous densities

Last updated: November 2026

FAQ

What is plaster consistency?

Consistency is the weight of water used per 100 parts of plaster by weight. For example, USG No. 1 Pottery Plaster lists a normal consistency of 70 lb of water per 100 lb of plaster.

Where do I find the specific gravity of my plaster?

Check the manufacturer’s technical data sheet for your plaster. The formula needs the true particle specific gravity of the plaster, not the bulk density of the loose powder or the specific gravity of the mixed slurry, and no value is prefilled because a verified figure for bag plaster was not available.

Why weigh water instead of measuring it by volume?

Consistency is defined by weight, and manufacturers such as USG advise weighing both plaster and water for each mix. The calculator assumes water at about 1 g per mL (specific gravity 1.0), an approximation that lets the water weight double as an approximate volume.

Can I subtract the model and add extra plaster for waste?

Yes. Enter the model’s volume in mL to subtract it from the mold space, and an allowance from 0 to 100% to scale up the mix: mix volume = (mold volume − model volume) × (1 + allowance ÷ 100). Both default to 0, and the model volume must be smaller than the mold volume.

How is the mold volume calculated?

A rectangular mold uses length × width × height, and a round mold uses π × radius² × height. Use the inside dimensions of the space the plaster will fill, up to the planned pour height.

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