Silicone Mold Volume Calculator
Silicone for a block mould, box less part — and why the margin you leave matters four times more than what the part displaces.
If you cannot guess how much of its box the part fills, measure it: drop the part in a measuring jug of water and read the rise. That is its volume exactly, and it takes less time than arguing about it. Mix a little extra for what stays in the cup. Nothing is uploaded.
The margin is the whole calculation
A block mould is a box with a part-shaped hole in it, and the clearance you leave appears in all three dimensions at once. Halving it from 0.5 in to 0.25 in does not halve the silicone — it leaves 46% of it, a saving of 54%.
| Margin | Box | Silicone | By weight | Against ½ in |
|---|---|---|---|---|
| 0.125 in | 6.33 cu in | 71 ml | 78 g | 0.27× |
| 0.25 in | 9.38 cu in | 121 ml | 133 g | 0.46× |
| 0.375 in | 13.23 cu in | 184 ml | 203 g | 0.70× |
| 0.5 in | 18.00 cu in | 262 ml | 288 g | 1.00× |
| 0.75 in | 30.63 cu in | 469 ml | 516 g | 1.79× |
| 1 in | 48.00 cu in | 754 ml | 829 g | 2.88× |
Note the last two columns disagree: silicone is sold by weight and mixed by volume, and at a specific gravity of 1.1 you need more grams than millilitres. A litre of mould is about 1100 g of product, so ordering by the volume figure leaves you short.
And the part — the thing everyone worries about — is the smaller lever
The usual worry is how much the part displaces. But it can never displace more than its own bounding box, so the whole lever is capped: going from a typical part to a completely solid block saves only 13%, against 54% for halving the margin. The margin lever is 4.3 times the part lever.
| How solid the part is | Fills | Part volume | Silicone |
|---|---|---|---|
| A thin flat part | 10% | 0.40 cu in | 17.60 cu in (1.10×) |
| A sparse or hollow part | 25% | 1.00 cu in | 17.00 cu in (1.06×) |
| A typical part | 50% | 2.00 cu in | 16.00 cu in (1.00×) |
| A sphere — the rounded ceiling | 52% | 2.09 cu in | 15.91 cu in (0.99×) |
| A chunky part | 75% | 3.00 cu in | 15.00 cu in (0.94×) |
| A solid block | 100% | 4.00 cu in | 14.00 cu in (0.88×) |
A rounded part cannot even reach the ceiling: a sphere fills exactly pi over six of its box, which is 52.4%, and nothing rounder exists. A thin flat part displaces so little that the silicone is 98% of the whole box — so a flat part costs more to mould than a chunky one of the same footprint, which is the opposite of what the worry assumes.
How to use
- Enter the length, width and height of the part.
- Set the margin you want on every side.
- Estimate how much of its box the part fills, or measure it in water.
- Read the silicone by volume and by weight.
Frequently asked questions
How much silicone does a block mould take?
The volume of the mould box less the volume of the part. The box is the part plus twice the margin in each of the three dimensions, so a 2 by 2 by 1 inch part with half an inch all round sits in an 18 cubic inch box.
What margin should I leave?
That is the decision this tool exists to make visible, because it is the expensive one. A margin appears in all three dimensions at once, so halving it from half an inch to a quarter does not halve the silicone — it leaves 46% of it.
Does the part displacing silicone matter?
Less than people expect. The part cannot displace more than its own bounding box, so the whole lever is capped: going from a typical part to a completely solid block saves about 12%, against 54% for halving the margin.
How do I measure how solid my part is?
Drop it in a measuring jug of water and read the rise. That is its volume exactly, in whatever units the jug is marked in, and it takes less time than estimating. Seal anything porous first, or it will drink the water and read low.
Is a flat part cheaper to mould?
No — more expensive, which surprises people. A thin flat part displaces so little that the silicone is about 98% of the whole box, while a chunky part of the same footprint takes a real bite out of it. Displacement works in your favour.
How much does a sphere displace?
Exactly pi over six of its bounding box, which is 52.4%, and nothing rounder exists. That is the practical ceiling for a rounded part: however solid it looks, half the box is still going to be silicone.
Why does the weight not match the volume?
Because silicone is sold by weight and mixed by volume, and it is denser than water. At a specific gravity of 1.1 a litre of mould is about 1100 grams of product — so ordering by the volume figure alone leaves you short.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.