Jesmonite Calculator | Mix Ratio, Pigment & Cost Tool
Wasting Jesmonite because you guessed the ratio? Not knowing how much material a coaster or tray mould needs is frustrating. Use our free tool below to calculate exact powder, liquid, total mix weight, safe pigment levels, and estimated material costs in seconds.
Jesmonite Material Calculator
Instantly calculate powder, liquid, and pigment for your mould casting and terrazzo pieces.
1. Mould Details & Volume
Tip: Weigh your mould filled with water. 1g of water = 1ml.
2. Product Settings
Default 2.5:1 is for Jesmonite AC100 / AC200.
Usually 2% max by weight.
Are you casting multiples?
Leave at 0 if unknown. Helps calculate how many mixes you must do.
Materials Checklist
- Base Powder (138g)
- Liquid (55g)
- Mixing Cup & Stirrer
- Digital Scales
- Silicone Mould
- Pigments/Chips
- Gloves & Paper Towels
- Jesmonite Sealer
How the Jesmonite Calculator Works
Estimating the right amount of casting compound can feel like a guessing game. Whether you are using Jesmonite AC100, AC200, or experimenting with terrazzo chips, leaving it to chance often results in wasted material or an underfilled mould.
This Jesmonite material calculator helps you determine the exact weight of base powder and acrylic liquid required. Simply enter your mould dimensions or water volume, select your mix ratio, and add a small waste allowance. The tool instantly outputs the total mix weight and provides a breakdown of powder, liquid, and optional pigment percentages.
The "Water Fill" Method: How to Measure Mould Volume
If you have a complex or irregular silicone mould (like a small sculpture or custom candle vessel), measuring its dimensions using length and width won't be accurate. The best technique for UK makers and craft sellers is the water fill test method.
- Place your empty casting mould on a digital weighing scale and press 'tare' to zero it out.
- Carefully fill the mould to the top with standard tap water.
- Weigh the water. Because 1 gram of water equals exactly 1 millilitre (ml) in volume, this gives you the exact ml volume of your mould.
- Select the "Custom Volume" shape on the calculator above, choose "ml", and enter your number!
Note: Jesmonite is denser than water. The calculator automatically multiplies your ml volume by the specific density of Jesmonite (roughly 1.75g per ml) to find the total mix weight in grams.
Understanding the Jesmonite Mix Ratio
Getting the powder to liquid ratio right is critical for curing time and overall strength. The standard Jesmonite AC100 mix ratio is 2.5 parts base powder to 1 part liquid by weight. This is a 2.5:1 ratio.
Never measure your materials by volume (e.g. using measuring cups visually). The mineral powder is heavy and compresses differently than the water based liquid. Always use reliable digital kitchen scales to weigh both parts accurately. Accurate weighing prevents sticky surfaces, air bubbles, and brittle home decor products.
The Casting Workflow
Quick Reference Table: Common Craft Project Estimates
| Project Type | Approx. Volume | Est. Total Mix (g) | Waste Level |
|---|---|---|---|
| Standard Coaster Mould | 100 ml | ~175g - 185g | 5% |
| Jewellery / Oval Tray Mould | 250 ml | ~440g - 480g | 10% |
| Small Plant Pot | 300 ml | ~525g - 600g | 15% |
| Large Serving Tray | 900 ml | ~1575g - 1730g | 10% |
Beginner Tips for Reducing Waste
- Overpour vs Underfill: It is always better to slightly overpour. Scraping the bottom of a mixing cup because you ran out leads to uneven finishes and rushed work. Use the waste allowance feature (typically 5-10%).
- Save the excess: Have a small "scrap mould" or gem mould nearby. Pour any leftover mixed Jesmonite into it to create terrazzo chips for future projects. This acts as a great waste saver tip!
- Avoid guessing by eye: Always weigh! Visual guessing causes weak mixtures that may not cure properly, forcing you to throw out the whole cast.
Frequently Asked Questions
Got questions about mixing, waste, or using the calculator? Find quick answers below.
To calculate how much Jesmonite you need, you first determine the volume of your mould in millilitres (using the water fill test or measuring dimensions). Multiply that volume by 1.75 (the average density of mixed Jesmonite AC100). Finally, add a 5% to 10% waste allowance to cover spillage and residue left in your mixing cup.
The standard mixing ratio for Jesmonite AC100 is 2.5 parts base powder to 1 part acrylic liquid, separated by weight. For example, if you need 350g in total, you will mix 250g of powder with 100g of liquid. Always check the manufacturer's label if you are using products like AC200 or AC730.
You must always measure Jesmonite materials by weight, using a precise digital weighing scale. Measuring by volume (like using a jug or cup) is inaccurate because the base powder has a completely different density and weight compared to the water based liquid.
It is recommended to add a 5% to 15% waste allowance to your total calculation. Simple shallow moulds like coasters usually only need 5%. Complex shapes with deep corners, like a plant pot or candle vessel, benefit from a 10% to 15% allowance to ensure you don't underfill the silicone mould.
Yes! This calculator defaults to a 2.5:1 ratio and a 1.75 specific density, which perfectly matches Jesmonite AC100 requirements for calculating total mix weight, base powder, and liquid amounts.
Yes, you can edit the ratio fields to fit other products. For example, if you are using Jesmonite AC730, you can change the ratio setting from 2.5 to 5.0 (since AC730 typically uses a 5:1 ratio). Please note that densities may vary slightly between different compounds.
Measure the diameter of the round mould across its widest point, and its total depth. Select 'Round/Cylinder' in the calculator, input those measurements, and the calculator will find the volume using the formula (Pi x radius squared x depth) and convert it to total mixed weight.
Jesmonite pigments should never exceed 2% by weight of your total mixed material. Adding too much colour paste disrupts the curing process, resulting in a weak or crumbly casting. The calculator provides a safe maximum pigment estimate based on this 2% rule.
Several factors affect a real pour. Differences in exact powder density, air bubbles trapped inside the mix, spillage, and variations in silicone mould thickness can all cause the final amount to look slightly over or under. Adding a 10% waste allowance helps absorb these variations.
Yes. By opening the 'Advanced Options' in the calculator, you can enter the price you paid per kilogram of base powder and per kilogram of liquid. The calculator then estimates your material cost per item, helping small business production planning and pricing.