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 by weight is the single most important factor for strong, bubble-free casts. Enter your mold dimensions above and the calculator returns 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 AC100 or 3:1 for AC730.
AC100 vs AC730: which to use?
AC100 is the most popular formulation — ideal for casting into silicone molds for trays, coasters, jewelry dishes, 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 (e.g., over expanded foam shapes or wood substrates).
Worked examples: three common mold sizes
Here’s the math for three typical project sizes, assuming standard base and liquid pricing.
Small coaster (4" x 4" x 0.4")
| AC100 (2.5:1) | AC730 (3:1) | |
|---|---|---|
| Volume | 105 ml | 105 ml |
| Total mix weight | 210 g | 210 g |
| Base powder | 150 g | 157 g |
| Liquid | 60 g | 53 g |
Trinket tray (6" x 4" x 0.6")
| AC100 (2.5:1) | AC730 (3:1) | |
|---|---|---|
| Volume | 236 ml | 236 ml |
| Total mix weight | 472 g | 472 g |
| Base powder | 337 g | 354 g |
| Liquid | 135 g | 118 g |
Decorative pot (5" x 5" x 4")
| AC100 (2.5:1) | AC730 (3:1) | |
|---|---|---|
| Volume | 1,639 ml | 1,639 ml |
| Total mix weight | 3,278 g | 3,278 g |
| Base powder | 2,341 g | 2,459 g |
| Liquid | 937 g | 819 g |
Note: pot is shown as solid volume. For a hollow pot, subtract the cavity volume from these figures.
Pigments, sealers, and consumables
The mix itself isn’t the whole material cost. Pigment and sealer add real money per cast and many makers forget to track them.
- Pigment. Keep additions under 5% of total weight to avoid weakening the cast. Dry pigments blend into the powder first; liquid pigments go into the liquid component first. Expect $0.10-$0.50 per cast for small items.
- Sealer. Jesmonite is porous when bare. A water-based or acrylic sealer adds about $0.20-$0.50 per piece and is essential for trays or anything that touches liquid.
- Mold release. If you use one (most silicone molds don’t require it), add about $0.05 per cast.
Pro tip: Add powder to liquid, not the other way around. Mix for at least two minutes until smooth and scrape the sides and bottom of your bucket to avoid dry pockets that cause weak spots in the finished cast. Working time is 10-15 minutes after mixing — pour as soon as it’s smooth.
Jesmonite calculator in other languages
Jesmonite has a strong international maker community and this calculator gets used in five+ languages. If you landed here looking for a calculadora jesmonite (Spanish/Portuguese), a calculatrice jesmonite, calculateur jesmonite, or calcul jesmonite (French), a jesmonite kalkulator (Polish), a jesmonite Rechner (German), or a tool for the jesmonite verhouding (Dutch — mix ratio), this is the right page. The mix ratios — 2.5:1 for AC100 and 3:1 for AC730 — are identical worldwide; the math doesn’t change with language.
Selling your jesmonite pieces
Once you’ve dialed in per-cast cost, layer in fees and labor before you list:
- Etsy Profit Calculator — reverse 2026 Etsy fees to find your listing price.
- Resin Volume Calculator — if you also work with epoxy.
- Craft Concrete Calculator — for concrete and plaster casting.
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.