Plaster to Water Ratio Calculator

Based on the volume of liquid plaster needed to make a mold, this calculator determines the amount of plaster and water, both by weight, to mix together to yield the required volume. It uses a water to plaster ratio of 73. (More below form).

The mixing ratio is 1 part water to 1.375 parts plaster, which is approximately 73 parts water to 100 parts plaster. The 73/100 ratio is often a good choice for mold making, providing an effective porosity. The ratio of water to plaster is referred to as consistency. Calculated water and plaster amounts are shown by weight, which facilitates weighing each on a scale. This calculator was designed to be used with plasters such as USG® Pottery Plaster Number 1. We make no guarantee that the 73 consistency is right for your particular application. You should test one or more batches to make sure that you are satisified with the consistency for the molds you are making.

Does your studio use 73/100 consistency? If your studio uses a chart, check and see if for 80 cubic inches, 2 pounds 12 ounces of plaster and 2 pounds of water are specified. If so, your studio uses a 73 consistency. If that chart is like most, it's in increments of 20 cubic inches; With this calculator, you can calculate how much water and plaster to mix based on the exact cubic inches needed (e.g. 30, 95, 205 cubic inches).

Note: It's generally preferable to have a little extra plaster than too little (ie. round up by 10 or 20 cubic inches), even if ultimately it's not needed . Mixing a second batch to compensate for a shortage typically results in adhesion and strength issues at the join area due to each batch going through its chemical reaction at different times.

Calculating Required Plaster Volume

Prior to using this calculator, you need to know how much volume you need for the plaster mold you are making. PhotoPottery's Plaster Volume Calculator calculates the volume of liquid plaster needed for making a one-part mold or a section in a multi-part mold.