The calculation in four steps
- Measure the box’s internal length, width and intended fill height. For a round box, measure the internal diameter and fill height.
- Calculate the container volume: length × width × fill height, or π × radius² × fill height for a cylinder.
- Subtract the volume of all objects inside the box that occupy space below the fill line.
- If you choose an extra allowance for handling losses, add it to the free-space estimate. Then use the exact silicone product’s mixing instructions to split the components.
Using centimetres gives cubic centimetres, and 1 cm³ = 1 mL. Keep all dimensions in the same unit before multiplying.
Worked example: a rectangular mould box
A box with internal dimensions 15 × 10 cm, filled to 8 cm, encloses 1,200 mL. An object occupying 250 mL leaves 950 mL for mixed silicone.
If you explicitly choose a 5% allowance, that adds 47.5 mL, giving 997.5 mL of mixed silicone. The 5% is an arithmetic example, not a product recommendation.
For a cylindrical container with a 10 cm internal diameter and 8 cm fill height, the container volume is π × 5² × 8 ≈ 628.3 mL before subtracting the object.
How do I find the object’s volume?
For a simple solid, calculate its actual geometric volume. For example, a solid 5 × 5 × 10 cm block occupies 250 mL. An irregular sculpture does not occupy its entire bounding box, so subtracting its maximum length × width × height can substantially underestimate the silicone needed.
For a water-compatible, non-porous solid, you can measure liquid displacement: a level rising from 500 mL to 750 mL means 250 mL is displaced. A porous, delicate or water-sensitive master needs a different measurement method suited to the object. An inaccurate object-volume estimate remains an estimate in the calculator.
For several objects, add their displaced volumes. Include other items that occupy the cavity, such as a solid support below the fill line. If you have already measured the free space directly, that measurement already excludes the object; do not subtract it again.
Volume tells you quantity; the label tells you how to mix it
The free-space result is a mixed volume. It does not establish a weight ratio. To convert it to total weight, use a documented mixed density for the exact material.
For example, Smooth-On’s Mold Star 30 data specifies 1:1 by volume, 100:96 by weight, and a mixed specific gravity of 1.12 g/cm³. These are different instructions for the same product. Use the Mold Star 30 calculator to select the measurement basis explicitly.
Choose your mould-box clearance and intended fill height according to the mould design and material instructions. This volume estimate does not prescribe wall thickness, cure conditions or the size of a batch you may mix at once.
Calculate your own mould quantity
Enter internal dimensions and the object’s measured volume. Leave any extra allowance at zero unless you deliberately choose one.
Common measuring mistakes
- Measuring outside dimensions instead of the internal cavity.
- Using the full box height when the planned fill line is lower.
- Subtracting an irregular object’s entire bounding box.
- Confusing mixed silicone volume with the amount of one component.
- Using a volume ratio as a weight ratio without checking the product.
Already know how much material you have measured? The known-ingredient mixer can scale a recipe using the ratio and basis you provide.