Many of my projects include 2 part moulding and casting.  Mould making requires pouring silicone, which isn’t hugely cheap, so it’s in our interests to mix up exactly the right amount and no more, to avoid wasting any.

One way of doing this is to purposefully only mix up small batches of silicone, and pour them one at a time until you have reached a satisfactory depth.  You can either do this one after the other or I guess you could wait for each layer to dry (each layer of silicone will stick to the next). But this wastes something even more precious than silicone – time!

When I started mould making I did everything by eye.  I have a pretty good eye for spacial stuff, but I would usually end up wasting half an inch of silicone in the bottom of my pouring cup.  Over 3 or so pours this amounts to a whole mould pour wasted. I then progressed to a rough formula based on counting the number of lego brick squares I’d used to build my mould boxes, and eventually have refined the formula to the point that it is near exact.  If you don’t use lego bricks, then it can be converted by measuring the size of your mould box in millimetres.

Counting Lego Bricks
This method works perfectly for me because I use lego for my mould boxes and it’s quicker and easier than measuring the size of the box.  Begin by counting bricks to work out the cubic area of your desired pour.  You want the height, width and desired depth, multiplied together.  For example the mould box below is 6 high x 12 wide, and I want to pour to a depth of 1.5 bricks, so  6 x 14 x 1.5 = a cubic area of 126 bricks.


  • Lego Brick Cubic Area = Height x Width x Depth of Bricks *

Next we must define the size of the parts to be cast.  Obviously, larger parts displace more cubic area meaning less silicone will be required.  I simply class each mould as having small, medium or large parts.  Examples follow but remember it’s not the area taken up by the part that makes it displace space but the cubic area.  A banner or cloak could be very large but if flat it won’t take up any space in the mould.

  • Small – pouches, heads, weapons, anything flat and small details
  • Medium – billowing cloaks, legs, arms and backpacks
  • Large – whole figures and bulky vehicle parts

The size of the parts defines a multiplier that we’ll apply to the cubic area to arrive at the total amount of silicone required.

  • Small Parts – 0.8
  • Medium Parts – 0.72
  • Large Parts – 0.64

For example, given I had cast 4 cloaks in the mould box above, 2 of which were billowing and had a lot of depth to them, I classed the mould as having medium parts and multiplied the cubic area of 126 by 0.72 to arrive at a total silicone weight of 91g.

In summary, to calculate the silicone required:

  • Required Silicone Mix (g) = Lego Brick Cubic Area x Part Size Multiplier

You then use whatever mix ratio your silicone mix should have (most common tends to be 10:1) to work out the amount of silicone and catalyst to use. If your silicone is a 10:1 mix you multiply the total required by 0.091 to get your catalyst, and the rest is silicone.  For this mould, that is 8g of Catalyst and 83g of silicone.

  • Required Catalyst (g) = Required Silicone Mix (g) x 0.091
  • Required Silicone (g) = Required Silicone Mix (g) – Required Catalyst (g)

And if all this seems like far too much hassle, here is a spreadsheet you can use.  Simply fill in the blue cells and it will tell you the right amount of silicone and catalyst, assuming your silicone mix ratio is 10:1.

Converting mm to Lego Bricks
If you don’t use lego bricks for your moulds, firstly I suggest you investigate the possibility, and secondly you can use the method above by knowing that 1 brick wide or high is 8 mm, while 1 brick deep is 10mm.

  • Width or height in lego bricks = width or height in mm/8
  • Depth in lego bricks = depth in mm/10

* Given 1 “brick unit” is deeper than it is high or wide, some innacuracy would be introduced into the formula for very deep or very shallow moulds.  I have never had to pour a mould less than 1 brick or more than 2 bricks deep so the problem seems remote, but it’s worth bearing in mind.