How much silicone does your mold need?
Enter the size of the object you are casting and get the silicone volume, weight and cost for a block mold, plus the inner dimensions of the box to print around it. Free, no card, nothing to upload.
A shape-hugging shell wraps the model instead of burying it in a block, and the generator measures the real cavity so the figure is exact rather than estimated.
How this is worked out
A block mold is a box with your object suspended inside it, filled with silicone. So the silicone you need is simply the inside volume of the box, minus the volume of the object. No estimation is involved in that step at all. The only uncertain part is your object's volume, which is why the water-displacement option is worth the two minutes it takes: it is exact, and it works on shapes no formula describes.
If you pick a shape instead, the geometric ones are exact formulas rather than rules of thumb. A cylinder fills 78.5% of its bounding box, a sphere 52.4%, a cone 26.2%. The two remaining options, a relief tile and an irregular figurine, are genuine estimates and the page says so when you pick one, because a figurine can sit anywhere between a quarter and a half of its box depending on how much of it is limbs.
Margins are a cubic term, which is the expensive part
Adding "just a few more millimetres" of silicone all round does not add a few more millimetres of cost. Every extra millimetre grows the box in three directions at once. On a 50 mm cube master, going from a 10 mm margin to a 20 mm one costs you more than two and a half times the silicone. Try it in the calculator above, with the price filled in. If a mold is coming out unaffordable, the margins are almost always where it went wrong, not the object.
Do not run short mid-pour
This is the one mistake with no recovery. Silicone starts to skin, and a second batch poured onto a partly-cured first one will not bond to it; you get a weak seam that tears on the first demold. Mix more than you think you need. The waste allowance above is roughly 15 ml plus 5%, which is what actually stays behind in a cup, rather than the usual flat 10% that leaves small pours short.
Which kind of mold are you making?
- Block mold (what this page calculates): object suspended in a box, silicone poured around it, cut open to demold. Simple, forgiving, uses the most silicone.
- Two-part box mold: printed directly, no silicone at all, for wax, soap, plaster and concrete. Free in the two-part mold generator.
- Hugging shell: a printed formwork that follows the model's outline, so the silicone is a skin rather than a block. Uses a fraction of the material. Built by adaptive silicone, which measures the cavity and tells you the exact volume.
- Hollow vessel: cups, vases and anything with an inner cavity needs a core as well as a shell. See the vase mold generator.
Questions
How do I measure an object that floats?
Hold it fully under with a skewer or a fork while you read the level. The skewer displaces a millilitre or so, which is well inside the accuracy you need. Do not just push it half under: the reading is only right when the whole object is submerged.
Tin-cure or platinum-cure?
Start with tin-cure. It is cheaper, more forgiving about mixing ratios, and it tolerates contamination that stops platinum curing altogether. Move to platinum-cure when you want longer mold life and finer detail, or when you are casting food-grade. More in the silicone mold guide, and if a batch never sets, the silicone not curing guide covers why.
Can I print the mold box here too?
Yes, and that is the point of the box dimensions above. Any generator on the site will build you a printable box, or use the silicone mold generator, which builds the formwork around your actual model and works the silicone volume out from the real geometry rather than a bounding box.
What does each cast cost after that?
That is the other half of the sum, and the mold is usually the part people forget to include. The casting cost calculator takes the mold cost from this page, spreads it over the number of casts the mold survives, and tells you what one finished piece really costs.