Silicone mold calculator

A poured block mold is a box of liquid silicone with your model sitting in it. The silicone you need is the box volume minus the model — and because silicone kits are usually sold and often mixed by weight, the answer is in grams or ounces.

Box minus model

Glue the model to the base of a mold box, pour silicone until it covers the model by a comfortable margin, and let it cure. The silicone fills everything in the box that isn’t the model, so the amount is simply the box volume minus the model volume — the same method Smooth-On lays out in its “how much mold rubber do I need” guide. Silicone is sold by weight, so the calculator multiplies by the rubber’s specific gravity to get grams or ounces.

box = (model L + 2 × wall) × (model W + 2 × wall) × (model H + wall)
silicone volume = box − model
weight = volume × specific gravity × (1 + waste)
SG = 27.68 ÷ (cu in per lb), if your data sheet lists specific volume

Example: a 3 × 2 × 2 in block-shaped master with 1/2 in walls needs a 4 × 3 × 2.5 in box — 30 cu in. Take away the 12 cu in model and you have 18 cu in (about 295 ml) of silicone. At SG 1.2 that’s 354 g, or about 390 g with 10% extra.

Two worked examples show how much the model’s shape matters: how much silicone for a 4 inch coaster mold is a small, flat job that fits one trial kit, while how much silicone for a 12×8 tray mold takes several pounds because the rubber fills the tray’s hollow as well as surrounding it.

Model volume: measure it if you can

A block-shaped master fills its bounding box; a round one fills about 79% of it (π ÷ 4); a ball about 52% (π ÷ 6). Anything else — a figurine, a shell, a carved piece, a dish — is best measured by displacement: sink it in a measuring cup of water and read the rise. Getting this right matters, because an overestimated model volume is the way to run short of silicone.

Specific gravity and mix ratio

Silicones range in density — roughly 1.1 to 1.3 for many mold rubbers, though some are lighter or heavier. The data sheet lists it as specific gravity, or as specific volume in cubic inches per pound (divide 27.68 by that figure to get SG). Smooth-On’s Mold Star 15 SLOW, for example, lists SG 1.18 and 23.5 cu in/lb, and mixes 1A:1B by volume or by weight. Other platinum-cure silicones are 10A:1B by weight, and many tin-cure rubbers are around 100:5 by weight. Use the ratio on your kit.

Tips

• Seal porous models (wood, plaster, 3D prints) before molding, and use a release agent if the data sheet calls for one.
• Some materials inhibit platinum-cure silicone — Smooth-On lists latex, sulfur clays, certain woods and newly cast epoxy or polyester among them. A coat of clear acrylic lacquer on the model, fully dried, is their suggested barrier. Test on a scrap first.
• Pour into the lowest corner in a thin stream and let the silicone rise over the model; it pushes air out ahead of it.
• Smooth-On’s Mold Star tutorial pours about 1/2 in (1.25 cm) over the model’s high point — a good starting wall for small molds. Thinner walls flex more and tear sooner.
• Small jewelry molds are cheap to make but tricky to fill accurately; once yours is cured, see how much resin for 12 pendant molds for measuring tiny cavities with water.