Alginate is sold as a powder. It needs to be mixed with water to make it work.
How Much Water?
There are two ways to calculate this.
One method is simple but not that accurate.
The other method is very accurate, but takes a little more work.
SIMPLE: The simple way is to measure by volume. Make sure your alginate is not too compressed in the bag and measure out 3 parts water for 2 parts of alginate powder.
ACCURATE: The more accurate method is to weigh out both the water and the powder. Measure out 4 parts water to 1 part powder (by weight). It is convenient that water weighs almost exactly 1 pound per pint. For every pound of water, you'll use 1/4 pound of alginate.
This second method is more accurate because alginate is a compressible (fluff-able) powder and can be in any one of a thousand density states at any one time. Compressed- and your alginate will be thick. Fluffed and it will be thin. Weighing out your alginate eliminates this issue.
Use only with Accu-Cast alginates.
Other alginate manufacturers recommend a 1:1 ratio by volume. This is very easy to measure accurately, but notice that they are not adding as much water to their powder as Accu-Cast is. This means that Accu-Cast alginates "go" farther than other brands. More value for your money.
How Much Alginate do I Need?