Silicone Mold and Resin Calculator: How Much to Mix
Choose a material and mold shape, enter the size (or the master's volume), and get the total grams and the Part A / Part B split.
Your measurements
Pick your material and enter the mold size. Everything updates as you type.
Material
Mix 100:96 by weight, 1:1 by volume. Smooth-On: 45 min pot life, 6 h cure at 73°F / 23°C before demolding.
Mold
Enter the inside size of the mold box.
Sets the volume formula. For an irregular cavity, weigh water instead.
How deep the material will be, not the height of the box walls.
The space the original object takes up inside the box. Find it by dunking it in a measuring jug of water. Use 0 for an empty mold.
Extra for waste
Material that stays on the cup walls and stirrer. 8% is a common starting point; use more for thick mixes.
Common buffers
Results
Total Platinum-Cure Silicone (Smooth-On Mold Star 30) to mix
g
= 32 oz (2 lb)
Mixed weight with 8% extra for waste, at 1.12 g/cm³. Weigh each part on a scale.
- Part A
- 462.9g
- = 16.33 oz
- or 405 mL by volume
- Part B
- 444.3g
- = 15.67 oz
- or 405 mL by volume
- Cavity volume
- 750mL
- = 25.36 fl oz
- Before the buffer
- Batch volume
- 810mL
- = 27.4 fl oz
- With the buffer
100:96 by weight, 1:1 by volume. Pre-mix Part A and Part B separately, dispense 1:1 by volume, then mix while scraping the sides and bottom several times until the color has no streaks.
Mold box
Inside size of the box with your numbers.
How this was calculated
Volume, then weight, then the split, with your numbers.
How this was calculated
Volume, then weight, then the split, with your numbers.
Step 1: Volume to fill
V = L × W × H − V_master
= 15 × 10 × 6 − 150 cm³
= 750 cm³ (mL)The inside of the mold minus the 150 cm³ the master takes up.
Step 2: Weight with a 8% buffer
W = V × density × (1 + buffer)
= 750 cm³ × 1.12 g/cm³ × 1.08
= 907.2 g1.12 g/cm³ is the mixed density from the maker's data. The buffer covers what stays in the cup and on the stirrer.
Step 3: Split by the weight ratio
A = W × R_A ÷ (R_A + R_B)
= 907.2 g × 100 ÷ 196
= 462.9 g Part A + 444.3 g Part B100:96 by weight. By volume it is 1:1: 405 mL + 405 mL.
Sources
Platinum-Cure Silicone (Smooth-On Mold Star 30): density, mix ratio and timing
Checked against sourceChecked against the maker's documentation. Checked 23 Oct 2026.
- Smooth-On Mold Star 30 product page and data at a glance, opened 23 Oct 2026
In short
Grams to mix = volume to fill × mixed density + 5–10%. A 15 × 10 × 6 cm box with a 150 cm³ master needs about 907 g of Mold Star 30 Silicone: 462.9 g Part A + 444.3 g Part B.
Products with a data sheet use the maker's density and ratio.
Casting materials compared
Mixed density sets the weight you need; the ratio sets how to split it. Generic epoxy entries are typical values, so use your kit's label if it differs.
| Material | Density | Mix ratio | Pot life | Demold | Basis |
|---|---|---|---|---|---|
| Mold Star 30 Silicone | 1.12 g/cm³ | 100:96 by weight, 1:1 by volume | 45 min | 6 h | Maker's data sheet |
| Mold Max 30 Silicone | 1.18 g/cm³ | 100:10 by weight | 45 min | 24 h | Maker's data sheet |
| Jesmonite AC100 | 1.845 g/cm³ | 2.5:1 by weight | See notes | See notes | Maker's data sheet |
| Jesmonite AC730 | 1.95 g/cm³ | 5:1 by weight | See notes | See notes | Maker's data sheet |
| Deep-Pour Epoxy (generic 2:1 by volume) | 1.11 g/cm³ | 2:1 by volume | See notes | See notes | Typical (estimate) |
| Table-Top Epoxy (generic 1:1 by volume) | 1.08 g/cm³ | 1:1 by volume | See notes | See notes | Typical (estimate) |
| Smooth-Cast 300 Resin | 1.05 g/cm³ | 100:90 by weight, 1:1 by volume | 3 min | 10 min | Maker's data sheet |
| Dragon Skin FX-Pro | 1.062 g/cm³ | 1:1 by weight or volume | 12 min | 40 min | Maker's data sheet |
| USG No. 1 Pottery Plaster | 1.58 g/cm³ (est.) | 100:70 by weight | See notes | See notes | Maker's data sheet |
| USG Hydrocal White | 1.762 g/cm³ | 100:42.5 by weight | See notes | See notes | Maker's data sheet |
| Clear Flex 50 Urethane | 1.04 g/cm³ | 1:2 by weight | 25 min | 16 h | Maker's data sheet |
Mold volume formulas
Measure the inside of the mold box. Subtract the master's volume to get the volume to fill.
| Shape | Volume | Default size in the calculator |
|---|---|---|
| Rectangular Mold Box | length × width × height | 15 × 10 × 6 cm |
| Cylindrical Cup Mold | π × (diameter ÷ 2)² × height | Ø9 × 12 cm |
| Shallow Tray / Coaster Mold | length × width × depth | 20 × 15 × 2 cm |
| Hemisphere / Dome Mold | ⅔ × π × (diameter ÷ 2)³ | Ø10 cm |
| Custom Geometric Block | length × width × height | 12 × 12 × 8 cm |
Worked example: Mold Star 30 Silicone in a box mold
Volume, then weight, then the split between the two parts.
Step 1: Volume to fill
V = L × W × H − V_master
= 15 × 10 × 6 − 150 cm³
= 750 cm³ (mL)The inside of the mold minus the 150 cm³ the master takes up.
Step 2: Weight with a 8% buffer
W = V × density × (1 + buffer)
= 750 cm³ × 1.12 g/cm³ × 1.08
= 907.2 g1.12 g/cm³ is the mixed density from the maker's data. The buffer covers what stays in the cup and on the stirrer.
Step 3: Split by the weight ratio
A = W × R_A ÷ (R_A + R_B)
= 907.2 g × 100 ÷ 196
= 462.9 g Part A + 444.3 g Part B100:96 by weight. By volume it is 1:1: 405 mL + 405 mL.
Mold casting: common questions
Volume, displacement and weight vs volume ratios.
How do I work out how much silicone I need for a mold?
Find the volume to fill (the inside of the mold box minus the volume of the master), multiply by the rubber's mixed density in g/cm³, and add 5–10% for what stays in the cup. A 15 × 10 × 6 cm box with a 150 cm³ master needs about 907 g of Mold Star 30 Silicone.
How do I find the volume of an irregular master?
Use water displacement: put a container of water on a scale, zero it, and hold the master fully under the surface without touching the sides. The added grams equal the master's volume in cm³; enter that as the master volume. Alternatively, with the master fixed in the mold box, fill the box with water, weigh the water and use the Water Displacement mode. Seal porous masters first.
Should I mix by weight or by volume?
Use the basis the maker gives. Many silicones and urethanes list both; a ratio like 100:10 by weight is not the same as 100:10 by volume because the two parts have different densities. The calculator shows the split in the basis each product uses.