How to Calculate Jesmonite for Casting
Jesmonite is a water-based composite used for casting trays, coasters, pots, and decorative homeware. Getting the powder-to-liquid ratio right is critical for strong, bubble-free casts. Enter your mold dimensions above and this calculator will tell you exactly how much base and liquid you need, plus what it will cost.
The Math Behind the Mix
The calculator converts your mold dimensions from inches to milliliters (1 cubic inch = 16.387 ml), then multiplies by the jesmonite density of roughly 2.0 g/ml to get total mix weight. The base powder and liquid amounts are split according to your chosen ratio: 2.5:1 for standard AC100 or 3:1 for thicker AC730 formulations.
AC100 vs. AC730: Which to Use?
AC100 is the most popular formulation, ideal for casting into silicone molds for trays, coasters, and decorative objects. AC730 uses a 3:1 ratio and produces a thicker, more paste-like consistency suited for laminating over surfaces and creating shell casts. Choose the ratio that matches your project type.
Pro Tip: Always add the powder to the liquid (not the other way around) and mix for at least two minutes until smooth. Scrape the sides and bottom of your bucket to avoid dry pockets that cause weak spots in the finished cast.
Working with epoxy resin instead? Use our Resin Volume Calculator for two-part resin projects.
Frequently Asked Questions
What is the correct mix ratio for Jesmonite AC100?▼
The standard mix ratio for Jesmonite AC100 is 2.5 parts base powder to 1 part liquid by weight. Always weigh your components on a digital scale rather than measuring by volume, as the powder and liquid have different densities. Accurate ratios prevent soft spots, cracking, and surface defects.
How long does Jesmonite take to cure?▼
Jesmonite AC100 has a working time of about 10 to 15 minutes after mixing. It can be demolded in 30 to 60 minutes depending on thickness and room temperature. Full cure takes approximately 24 hours, though thicker casts may need longer. Avoid rushing demolding, as early handling can cause warping.
Can I add pigment to Jesmonite and does it affect the mix ratio?▼
Yes, you can add dry or liquid pigments to Jesmonite. Dry pigments should be blended into the powder before adding liquid, while liquid pigments are mixed into the liquid component first. Keep pigment additions under 5 percent of the total weight to avoid weakening the cast. The calculator accounts for pigment cost separately so you can track material expenses accurately.