Skip to content
HobbyHQ
Search

Mould making

Silicone volume estimator

Box volume less what the master displaces, plus the waste that stays in the pot, converted to the weight silicone is actually sold in.

The question this answers: “How much silicone do I actually mix for this block mould — and how much do I order, given it is sold by weight?”

To the top of the silicone, not the top of the walls — usually a centimetre or so over the highest point of the master.

Drop it in a measuring jug of water and read the rise. Guessing this is where the estimate goes wrong.

From your silicone's datasheet. Around 1.1 for many tin-cure rubbers, but check rather than assume.

What stays in the pot and on the spatula. Running short mid-pour ruins the mould.

Result

Box volume
576 ml
Silicone needed
426 ml
Box volume less what the master displaces.
With 10% waste
469 ml
To buy, by weight
515 g
Silicone is sold by weight and mixed by weight, so this is the number to order against. Specific gravity comes from your product datasheet.
Show the working
  1. Box volume12 × 8 × 6 cm576 ml
  2. Less the master576 − 150 ml displaced426 ml
  3. Plus waste426 × 1.1469 ml
  4. As weight469 ml × 1.1515 g

Box volume: 576 ml. Silicone needed: 426 ml. With 10% waste: 469 ml. To buy, by weight: 515 g

How it works

A cubic centimetre is a millilitre, so the box volume is length times width times pour depth. The master’s displaced volume comes off, a waste allowance goes on, and the result is multiplied by the specific gravity from your datasheet to give the weight to order against.

What it assumes

  • A rectangular box and a level pour. A cylindrical or irregular container needs its own volume worked out first.
  • That the displacement was measured rather than estimated. Guessing the volume of an irregular master is where this calculation goes wrong, and the jug of water takes a minute.
  • That the master is fully submerged at the stated pour depth. A pour that only just covers the highest point leaves a thin roof, which is a weak mould rather than an economy.

Where it stops being reliable

  • It is a block mould calculation. Brush-on, glove and two-part moulds have quite different volumes and are not modelled here.
  • It says nothing about whether a one-part mould will work for your master. Undercuts, parting lines and draft are geometry questions the page discusses and this does not.
  • It gives no mixing ratio, no cure time and no advice on which silicone to buy. Those are properties of a specific product and belong to its datasheet.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-19. More on Mould making.