There is no need to estimate this. A mould tells you exactly how much mix it needs, and the method takes about a minute.
The water method
It works for any mould, including ones you made yourself.
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Fill the mould with water
Right up to the level you intend to cast to.
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Tip that water into a container on the scales
Note the weight in grams. That figure is close enough to the total mix weight you need.
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Add about ten per cent
Some material stays behind in the container and on the spatula. Running out halfway through a pour is worse than having a little left over.
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Dry the mould completely
Water left in the mould will affect the surface where it sits. Dry it properly before you cast.
Typical objects
Approximate total mix for common pieces, as a sense of scale rather than a specification:
| Object | Approximate mix | Sensible bag |
|---|---|---|
| Candle holder | 250g | 1kg |
| Small tray | 420g | 1kg |
| Large tray | 980g | 1kg |
| Sculptural vessel | 1,750g | 5kg |
| A workshop of 12 pieces | 5,800g | 10kg |
These are the figures behind the project calculator on the FormCast+ page. Your mould will have its own answer — measure it.
Choosing a bag size
A 1kg bag is a first project and a second attempt. It is the right size for finding out whether you enjoy this.
A 5kg bag is the size for making rather than trying — a run of small pieces, or one larger form with material left for a repeat.
10kg and 20kg are for repeat production, classes and studios, where running out mid-session costs more than the material does.
Questions people ask
How do I work out the quantity for a mould I made myself?
The same way as any other: fill it with water, weigh that water in grams, add about ten per cent, then dry the mould thoroughly before casting.
What can I do with leftover mix?
Pour it into any spare mould, or into a flat tray to make a slab. Broken up once set, that slab becomes terrazzo chips. Do not pour leftover mix down a drain.
How many pieces will a 1kg bag make?
It depends entirely on the object. As a rough sense of scale, a candle holder is around 250g of total mix and a small tray around 420g — and that total includes the water, not just the powder.
