Silicone Mold Calculator
How much silicone mould rubber a poured block mould takes, by weight, from the mould box, the model and the product’s specific gravity, with part A and part B.
Result
Enter specific gravity of the mixed rubber to see the result.
https://calcgallery.com/crafts/silicone-mold-calculator/
Results are arithmetic on the figures you enter and are estimates. Products, materials and animals vary, so follow the maker's instructions and label, make a test piece where it matters, and ask a vet about feeding. Nothing here is safety or veterinary advice. Read the full disclaimer
How to use the Silicone Mold Calculator
- Measure the inside of the mould box and the height the rubber will be poured to.
- Enter the model's volume, measured or found by water displacement.
- Enter the specific gravity and the mix ratio from the rubber's technical data sheet, and whether the ratio is by weight or by volume.
- Read the rubber to mix and how much part A and part B to weigh or measure out.
How it is calculated
box = length × width × rubber height (or π × (diameter ÷ 2)² × height)rubber = box − model volumeweight = rubber × (1 + extra) × specific gravity × 1 g/cm³part A = weight × A ÷ (A + B) (by weight) = rubber × (1 + extra) × A ÷ (A + B) (by volume)This is the method Smooth-On sets out for a poured mould: the volume of the box less the volume of the model, converted to weight through the rubber’s own figure. Their example, a 3 inch cube in a 4 inch box, is 64 less 27, or 37 cubic inches. No product figure is stored; each rubber’s specific gravity and ratio come from its own sheet.
Leave room around the model
The rubber height usually runs a little above the model so the mould has a solid top, and the box sits a little away from it on every side so the walls are not too thin. The page does not choose those gaps; measure the box as you have built it. The model’s share of the box volume is what the rubber does not fill.
What is left out
Brushed-on and glove moulds, which are sized by area and thickness, are not covered. Nor are mother moulds, sprues, release agents, pot life or cure times, which are all on the product sheet. For a soap mould, the soap mould batch calculator works out how much soap fills it.
With your numbers
- Rubber = 64 in³ − 27 in³ = 37 in³
- With 0% extra = 37 in³
- Weight = 37 in³ × specific gravity 1.12 = 1.5 lb
Quick reference
No extra is added by default. Some rubber always stays in the mixing cups and on the stirrer.
| Example | Rubber to mix |
|---|---|
| 4 in cube box, 27 in³ model, SG 1.12 | 1.5 lb |
| 6 × 4 × 3 in, 20 in³ model, 100:10 | 2.22 lb |
| 4 in box, 1:1 by volume, SG 1.12 | 1.5 lb |
| 3 in round × 2 in, 5 in³ model | 0.36 lb |
| 12 × 8 × 6 cm, 150 cm³ model | 477.12 g |
| 10 cm round × 8 cm, 200 cm³ model | 462.58 g |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
1 source
NIST Chemistry WebBook, water at 4 °C and 0.101325 MPa: 999.9749 kg/m³ (IAPWS-95), https://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185, rounded to the conventional 1000 kg/m³ of NIST SP 811 (2008) Appendix B, centimeter of water, conventional = 98.0665 Pa.
Used for
- The conventional density of water, 1000 kg/m³. Pure water at 4 °C, its densest, is 999.97 kg/m³.
Common questions about the Silicone Mold Calculator
- How much silicone do I need for a mould?
- Work out the volume of the box to the pour height, take away the volume of the model, and multiply by the rubber's specific gravity to get its weight. A 4 inch cube box around a 3 inch cube model holds 37 cubic inches of rubber; at a specific gravity of 1.12 that is about 1.5 pounds.
- Where do I find the specific gravity?
- On the product's technical data sheet, usually written in g/cc, which for this purpose is the same number. Smooth-On's sheets give it with the specific volume in cubic inches per pound: Mold Star 30, for example, lists 1.12 g/cc and 24.7 cubic inches per pound. To turn a specific volume into a specific gravity, divide 27.68 by it.
- What does a mix ratio of 100A to 10B mean?
- 100 parts of A to 10 parts of B by weight, so B is one eleventh of the mix. Enter 100 and 10. Some rubbers are mixed 1 to 1 by volume instead; choose By volume and the page gives each part as a volume to measure. Using a volume ratio as a weight ratio, or the reverse, gives the wrong proportions, because the two parts rarely weigh the same.
- How do I measure the model's volume?
- For a simple shape, from its dimensions. For anything else, fill the box with water to the pour height once with the model standing in it and once without; the difference in water is the model's volume.
- Is this the same as the epoxy resin calculator?
- No. The epoxy resin calculator sizes a resin pour by volume and counts the kits. This page sizes the rubber around a model, by weight, because silicone is often sold by weight and its specific gravity differs from product to product.