Silicone fills the space the master does not
A block mold is a box with a master in it and rubber everywhere else. So the silicone volume is a subtraction: the inside volume of the box up to the fill line, minus the volume the master displaces. Everything after that is unit conversion and a mix ratio.
What makes it worth calculating rather than guessing is that silicone is the most expensive material in most small studios, and the failure mode is asymmetric. Mix too much and you have wasted twenty dollars. Mix too little and you must stop mid-pour, mix a second batch, and pour it onto silicone that has already begun to skin — which gives you a visible cure line, a weak plane through the mold, and usually trapped air at the interface. Overestimating is cheap; underestimating ruins the mold.
That asymmetry is why the waste allowance is not optional. Silicone is viscous and clings: to the mixing cup, the spatula, the sides of the container it came in. Ten percent is a sensible floor for a small pour, and more for a mold with fine detail where you will be dribbling a thin stream from height to break bubbles.
Displacement, density and the ratio
Box volume is length × width × pour depth, all measured inside the box and up to the line you actually intend to fill, not to the top of the walls. Half an inch of unfilled wall is a large fraction of a small mold's volume.
Displaced volume is the master's volume, and the reliable way to get it is Archimedes rather than geometry. Seal the master if it is porous, submerge it fully in a measuring jug of water, and read the rise. That number is the volume, whatever the shape, and it costs a minute. Calculating it from dimensions only works for simple solids and always overestimates a shape with hollows or undercuts — which means you buy silicone you do not need.
Density converts volume to weight, and weight is what you actually work in, because studio scales are far more precise than measuring jugs for viscous liquids. One cubic inch is exactly 16.387064 cm³, so a cubic inch of a 1.15 g/cm³ silicone weighs 16.387064 × 1.15 = 18.845 g. Data sheets sometimes give specific gravity instead; it is the same number.
The mix ratio splits the total. For a product quoted as r parts base to 1 part catalyst, the catalyst is m ÷ (r + 1) and the base is the remainder. A 10:1 product takes one eleventh catalyst — not one tenth, which is the error that produces a mold that never fully cures. On a 1,244 g mix that is 113.1 g of catalyst against the 124.4 g a tenth would give: a 10% overdose, enough to change the working time noticeably.
One thing to check on the data sheet: whether the ratio is by weight or by volume. Many silicones are formulated so the two are equivalent, and many are not, because the catalyst has a different density from the base. This calculator splits by weight. If your product's ratio is by volume, convert it using the two components' densities before entering it.
Worked example: a 6 × 4 in box, 3 in deep, around a 12 in³ master
You are making a block mold of a small sculpture. Your box measures 6 × 4 in inside and you intend to pour to a depth of 3 in, which leaves half an inch of silicone over the top of the master. Water displacement puts the master at 12 in³. You are using a tin-cure silicone at 1.15 g/cm³, 10:1 by weight, at $25 a pound, and you want a 10% waste allowance.
- Box volume. 6 × 4 × 3 = 72 in³.
- Net silicone volume. 72 − 12 = 60 in³. The master occupies 12 ÷ 72 = 16.7% of the box, so the walls are generous.
- With waste. 60 × 1.10 = 66 in³.
- In millilitres. 66 × 16.387064 = 1,081.5 mL.
- Mass. 1,081.5 × 1.15 = 1,243.8 g, which is 1,243.8 ÷ 453.59237 = 2.742 lb.
- Catalyst. 1,243.8 ÷ 11 = 113.07 g.
- Base. 1,243.8 − 113.07 = 1,130.71 g.
- Cost. 2.742 × $25 = $68.55.
Zero the scale with the mixing cup on it, weigh in 1,130.7 g of base, then add catalyst until the scale reads 1,243.8 g. Weighing the catalyst into the base rather than separately avoids transferring it twice and losing some of it on a second cup — and on a 10:1 product, losing five grams of catalyst on a cup wall is a four percent under-catalysation.
Sizing the box, which is where the money goes
The calculation is fixed once the box is fixed, so the box is the real decision. Every extra half inch of clearance around a 6 × 4 × 3 in master adds meaningfully to the volume, and silicone is priced by weight.
Wall thickness. Around half an inch of silicone on every side of the master is a common working minimum for a block mold. Thinner walls tear at the parting line and let a heavy casting distort the cavity; much thicker walls buy nothing but cost. The calculator warns when the master fills more than 70% of the box, because that is the point at which the remaining rubber is thin somewhere.
Pour depth over the top. Enough silicone above the highest point of the master to hold together when the mold is opened — again about half an inch on a small mold. Measure the pour depth to your fill line rather than to the top of the walls, or you will mix silicone for air.
Box shape. A box that hugs the master's outline uses far less silicone than a rectangular one around an irregular object. Building the box from foam board cut to follow the shape, or using a blanket or glove mold instead of a block mold, can halve the material on a complex master. A blanket mold does not fit this calculator's arithmetic at all — for that, estimate the surface area and multiply by the intended blanket thickness.
Cost per mold is worth watching against how many castings the mold will produce. A $70 mold that yields two hundred resin castings is 35 cents a casting; the same mold used three times is $23 a casting, and at that point a rigid mold or a different process may be cheaper.
Silicone weight per cubic inch by density
| Density (g/cm³) | Grams per in³ | Pounds per 100 in³ | Typical product |
|---|---|---|---|
| 1.00 | 16.387 | 3.613 | Reference: water |
| 1.07 | 17.534 | 3.866 | Many platinum-cure silicones |
| 1.10 | 18.026 | 3.974 | — |
| 1.15 | 18.845 | 4.155 | Many tin-cure silicones |
| 1.20 | 19.664 | 4.335 | — |
| 1.25 | 20.484 | 4.516 | — |
| 1.30 | 21.303 | 4.697 | Filled or high-durometer grades |
Read your own density off the data sheet rather than picking a row — the difference between 1.07 and 1.15 is 7.5% of the silicone bill on every mold you ever make.
Measure the master by displacement, not by geometry
Fill a measuring jug with enough water to submerge the master, note the level, seal the master if it is porous or unfired, push it fully under with a thin rod, and note the new level. The difference is the volume, exactly, whatever the shape. One US fluid ounce is 1.8047 in³ and one millilitre is 0.061024 in³, so any jug you own will do. Working the volume out from length × width × height instead treats the master as a solid rectangular block, which on a figurine or a shell can overestimate the volume by a factor of two — and every cubic inch you overestimate is a cubic inch of silicone you buy and then have to fit into a box that will not take it.
What this calculation does not cover
- Blanket, glove and brush-on molds. Those are surface area × thickness, not box minus object. Estimate the area you will cover and multiply by the blanket thickness.
- Two-part molds with a parting line. Each half is its own pour with its own box volume; run the calculator once per half, and remember the first half becomes part of the second half's displaced volume.
- Registration keys and sprues. Keys cut into the first half add silicone to the second; a pour spout and vents add a little more. Both are inside the waste allowance for a small mold.
- Cure shrinkage. Silicone shrinks slightly as it cures — the data sheet gives the figure. It changes the finished cavity's dimensions, not the quantity you have to mix.
- Thickening additives. Thixotropic additives change viscosity and slightly change the density; use the data sheet's figure for the mixed system if it gives one.
- Vacuum degassing. Silicone expands dramatically in a vacuum chamber before it collapses. Use a container at least three times the mix volume, which is a container question, not a quantity one.
Where the mold sits in the casting workflow
Mold making is the middle of three volume calculations. First the master, whose volume you measure by displacement. Then the mold, which this page sizes. Then the castings, and the resin they consume is the mold cavity's volume — which is exactly the master's displaced volume you already measured, multiplied by the number of castings. Size that side with the resin mold volume calculator, and if the piece is a flood coat over a surface rather than a casting, with the epoxy coverage calculator.
The displacement method here is the same one soap makers use to size a batch to a mold, and the soap mold volume calculator applies it with a different conversion constant on the end. Candle makers do the same thing with wax in the candle wax calculator. All three are the same three steps: volume, conversion, density.
The one material in this category that does not behave this way is clay, because it changes size between forming and firing — see the clay shrinkage calculator for how a shrinkage factor replaces a straight density conversion. Silicone's own cure shrinkage is small enough by comparison that it affects the mold's accuracy rather than the quantity you buy.