Resin Mold Calculator
Mold casting is all about volume. Whether you're filling a silicone cube mold, a cylinder for a tumbler, or a sphere mold for paperweights, you need the internal volume in fluid ounces. This guide gives you the formulas, worked examples, and pre-calculated tables for the most common mold shapes and sizes.
Use the free calculator to get a precise resin volume estimate for your mold project.
Calculate Resin Amount →The Core Formula: Cubic Inches to Fluid Ounces
All mold volume calculations follow the same process: find the internal volume in cubic inches, then convert to fluid ounces.
This conversion factor (1.805) is based on the density of typical epoxy resin, which is approximately 9 fl oz per cubic inch of displaced space when fully mixed. It applies to both epoxy and most polyurethane casting resins.
Volume Formulas by Mold Shape
Box / Rectangular Mold
| Mold Size (in) | Volume (cu in) | Resin Needed (fl oz) | Resin Needed (mL) |
|---|---|---|---|
| 2 × 2 × 2 (small cube) | 8 cu in | 4.4 fl oz | 130 mL |
| 3 × 3 × 1 (coaster) | 9 cu in | 5.0 fl oz | 148 mL |
| 4 × 4 × 1.5 | 24 cu in | 13.3 fl oz | 393 mL |
| 6 × 4 × 2 (medium block) | 48 cu in | 26.6 fl oz | 787 mL |
| 8 × 5 × 2 | 80 cu in | 44.3 fl oz | 1,311 mL |
| 12 × 6 × 2 (charcuterie slab) | 144 cu in | 79.8 fl oz | 2,360 mL |
Cylinder Mold
| Diameter × Height (in) | Volume (cu in) | Resin Needed (fl oz) | Resin Needed (mL) |
|---|---|---|---|
| 2 dia × 2 high | 6.3 cu in | 3.5 fl oz | 103 mL |
| 3 dia × 3 high | 21.2 cu in | 11.7 fl oz | 347 mL |
| 4 dia × 4 high | 50.3 cu in | 27.9 fl oz | 825 mL |
| 5 dia × 5 high | 98.2 cu in | 54.4 fl oz | 1,609 mL |
| 6 dia × 6 high | 169.6 cu in | 94.0 fl oz | 2,780 mL |
Sphere Mold
| Sphere Diameter (in) | Volume (cu in) | Resin Needed (fl oz) | Resin Needed (mL) |
|---|---|---|---|
| 1 in | 0.52 cu in | 0.3 fl oz | 9 mL |
| 2 in | 4.19 cu in | 2.3 fl oz | 68 mL |
| 3 in | 14.1 cu in | 7.8 fl oz | 231 mL |
| 4 in | 33.5 cu in | 18.6 fl oz | 550 mL |
| 5 in | 65.4 cu in | 36.2 fl oz | 1,071 mL |
| 6 in | 113.1 cu in | 62.7 fl oz | 1,854 mL |
Hemisphere (Dome) Mold
| Dome Diameter (in) | Volume (cu in) | Resin Needed (fl oz) |
|---|---|---|
| 2 in | 2.09 cu in | 1.2 fl oz |
| 3 in | 7.07 cu in | 3.9 fl oz |
| 4 in | 16.8 cu in | 9.3 fl oz |
| 6 in | 56.5 cu in | 31.3 fl oz |
Cone Mold
| Base Diameter × Height (in) | Volume (cu in) | Resin Needed (fl oz) |
|---|---|---|
| 3 dia × 3 high | 7.1 cu in | 3.9 fl oz |
| 4 dia × 4 high | 16.8 cu in | 9.3 fl oz |
| 5 dia × 6 high | 39.3 cu in | 21.8 fl oz |
Tips for Mold Casting
- Mix extra: Always prepare 5–10% more than your calculated volume to account for mixing losses, residue left in the cup, and waste.
- Use a scale: For small molds (under 4 oz), weighing the resin components by gram is more accurate than measuring by volume.
- De-gas deep pours: Casting resin in deep molds can trap air bubbles. Use a vacuum chamber or heat gun after pouring.
- Max pour depth: Most casting resins have a maximum pour depth of 1–2 inches per layer. Deep sphere or cylinder molds may need multiple pours.
- Release agent: Apply mold release spray or petroleum jelly to non-silicone molds to prevent sticking. Silicone molds usually don't need a release agent.
Frequently Asked Questions
How do I calculate how much resin I need for a mold?
Calculate the mold's internal volume in cubic inches using its shape formula, then convert to fluid ounces by dividing by 1.805. Add 5–10% extra for mixing losses. For a box mold: Volume = length × width × height ÷ 1.805 = fl oz needed.
How much resin does a sphere mold need?
Use the formula V = (4/3) × π × r³, then divide by 1.805 to get fl oz. A 3-inch diameter sphere needs about 7.8 fl oz. A 4-inch sphere needs about 18.6 fl oz. Most sphere molds are sold in halves — you fill both halves separately and join them.
Should I overfill a resin mold?
Slightly overfilling is normal — plan for 5–10% extra. Resin can shrink slightly as it cures, and surface tension can cause the top to dome or dip. For open-top molds, a small overflow allows you to level the surface. For closed molds, fill exactly to avoid pressure buildup.