Plaster Mix Calculator
Plaster and water for a mould at any consistency — where a higher number means a wetter, softer mix, not a thicker one.
Volume is worked out from the two components' own volumes, which is close enough for sizing a batch but not an exact account of what happens chemically as gypsum sets. Mix a little extra — running short partway through a pour is not recoverable. Nothing is uploaded.
A higher consistency is a wetter mix
Plaster is quoted as a consistency: parts of water by weight per
100 parts of plaster. Consistency 70 means 700 grams of water to a kilo of
plaster. The number counts the water, not the plaster — so a higher consistency is a
wetter mix, which sets softer, weaker and more absorbent.
That reads backwards to almost everyone who meets it, because "high consistency" sounds
thick. It is the opposite: 50 is a dense, hard, slow-drawing mould and 100 is a soft, thirsty
one that will not survive many pulls.
| Consistency | Water per 1000 g plaster | Mixed volume | Share that is water |
|---|---|---|---|
| 50 | 500 g | 931 cm³ | 53.7% |
| 60 | 600 g | 1031 cm³ | 58.2% |
| 70 | 700 g | 1131 cm³ | 61.9% |
| 80 | 800 g | 1231 cm³ | 65.0% |
| 100 | 1000 g | 1431 cm³ | 69.9% |
Most of a mould is the space the water left
At a working consistency of 70, water is
61.9% of the mixed volume — most of
it. It does not stay: it leaves during drying, and what remains is pore space.
That porosity is the entire working principle of a slip mould. It is what pulls water out of
the slip and builds a wall against the surface, and a mould that has not dried between casts
has no capacity left to do it. So the consistency number is really choosing a trade: a denser
mix is a stronger mould that draws more slowly and lasts longer, a wetter one draws fast and
wears out. Neither is correct in the abstract — it depends on whether you are casting a run
or making a mould that has to survive one.
Sizing a batch
A 2,000 cm³ mould at consistency 70 takes
1768 g of plaster and
1238 g of water. Doubling the mould doubles both,
which is the one part of this that behaves the way you would expect.
A wetter mix needs less plaster for the same volume, since more of the volume is
water — which is the saving people are occasionally tempted by, and it is paid for in
strength and mould life rather than being free. Weigh both components; measuring plaster by
volume is unreliable because it settles and packs differently every time.
How to use
- Enter the volume of the mould.
- Set the consistency you want.
- Weigh out the plaster and the water.
- Compare consistencies before you commit.
Frequently asked questions
What does consistency mean in a plaster mix?
Parts of WATER by weight per 100 parts of plaster. Consistency 70 is 700 grams of water to a kilo of plaster. The number counts the water, not the plaster.
So is a higher consistency thicker?
The opposite, and it catches almost everyone. A higher number is more water — a wetter mix that sets softer, weaker and more absorbent. Consistency 50 is a dense, hard, slow-drawing mould; 100 is a soft, thirsty one that will not survive many pulls.
How much of a mould is actually plaster?
Less than half by volume. At a working consistency of 70, water is about 62% of the mix. It leaves during drying, and the space it leaves behind is the porosity.
Why does the porosity matter?
It is the entire working principle of a slip mould — it is what pulls water out of the slip and builds a wall against the surface. A mould that has not dried between casts has no capacity left to do it, which is why casting runs slow down rather than stopping outright.
Which consistency should I use?
It is a trade rather than a right answer. Denser mixes give stronger moulds that draw more slowly and last longer; wetter mixes draw fast and wear out. Which you want depends on whether you are casting a production run or making a mould that has to survive one job.
Can I save plaster by using a wetter mix?
Yes, and it is not free. More of the volume is water, so the same mould takes less plaster — paid for in strength and mould life. Worth knowing that is the trade rather than discovering it three casts in.
Should I measure by volume instead?
No — weigh both. Plaster settles and packs differently every time it is scooped, so volume measurement is unreliable in exactly the material where the ratio matters most. Mix a little extra too: running short partway through a pour is not recoverable.
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.