--- title: "Plaster Mix Calculator: Plaster to Water Ratio | Craft Mirth" description: "How much plaster and water to fill a mould, weighed out from USG's published mixing ratios, with set times and the safety rules for plaster casting." url: https://craftmirth.me/plaster-mix-calculator/ --- # Plaster mix calculator Measure your mould, pick your plaster and get the plaster and water to weigh out, from the mixing ratio the maker publishes for it. ## Never cast a hand in plaster - Never make a cast that encloses a hand, foot or any other part of the body. Plaster gets hot as it sets and can cause severe burns that may need surgery. - Plaster dust can irritate the eyes, skin, nose and throat: mix in a well-ventilated place and wear eye protection. - Keep plaster and wet mix away from children. - If plaster gets in your eyes, rinse them with water for 15 minutes; wash it off skin with plenty of water. From the cautions on USG's plaster data sheets and DAP's safety data sheet for its hobby and craft plaster of Paris; see Sources below. ## Mixing and setting Weigh both the plaster and the water: the ratio decides how strong and absorbent the cast will be. USG asks for drinking-quality water between 21 and 38 °C (70 and 100 °F). Sift or scatter the plaster onto the water slowly and evenly, let it soak until it is wet through, then mix. USG gives a set time of 14 to 24 minutes for both plasters here: machine-mixed for Pottery Plaster, which takes longer mixed by hand, and hand-mixed for Casting Plaster – 20 Minute. Dry finished casts to a constant weight; USG says pottery plaster moulds are safe from damage up to 49 °C (120 °F). ## How the amounts are worked out USG publishes a wet density of 1,567 kg/m³ for its Pottery Plaster mixed at 65 parts water. From that, 100 g of plaster takes up about 40.3 ml of the slurry and each gram of water 1 ml, so every consistency gives its own split of plaster and water. ## Questions **What does a consistency of 65 mean?** 65 parts of water to 100 parts of plaster, by weight: 650 g of water for every kilogram of plaster. **Why did my plaster set too fast?** Warm water and long mixing both shorten the set time. Use cooler water within USG's range and mix only until smooth. **Can I measure the plaster in cups?** USG asks for both to be weighed for every mix, because the ratio of water to plaster decides how strong and absorbent the cast will be. ## Sources - USG, [USG Pottery Plaster submittal sheet](https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/usg-pottery-plaster-submittal-en-IG1365.pdf) (IG1365): normal consistency 65; machine-mix Vicat set 14–24 minutes; wet density 1,567 kg/m³; water at 21–38 °C; weighing, soaking, mixing and drying; safe from calcination up to 49 °C; caution against casting any part of the body. - USG, [USG Casting Plaster – 20 Minute submittal sheet](https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/usg-casting-plaster-submittal-en-IG2004.pdf) (IG2004): consistency 63 to 73; hand-mix Vicat set 14–24 minutes; the same caution. - DAP Products Inc., [Plaster of Paris (hobby and craft) safety data sheet](https://www.physics.purdue.edu/primelab/safety/MSDS/SDS/Bondex%20Plaster%20Of%20Paris%2053005%20%20-%20DAP.pdf) (revised 8 May 2015): do not cast any part of the body; failure may cause severe burns that may require surgical removal of affected tissue. - National Institute of Standards and Technology, [Guide to the SI, Appendix B.9](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9): conversion factors for inches, US fluid ounces and ounces.