(original source unknown. I also added a few notes, examples, and conversions)
Recommended Plaster: #1 Pottery Plaster
How do I calculate how much water or plaster I need?
Calculating volume for solid shapes
For rectangular shapes: Volume = Height x Width x Depth
For circular shapes: Volume = Pi x Radius2 x Height (Note: Pi is apx 3.14. and that's supposed to be radius squared, but I cannot get the text formatting to cooperate.)
Divide the volume by 80 to find the number of quarts of water you will need to make enough plaster.
How much plaster do I add to water for a proper mixture?
The recommended consistency is 7lbs water to 10lbs plaster. So a 7:10 ratio, by weight 2.85lb plaster to 1 qt (2lbs) water (Note: If you want stronger plaster, you can multiply the weight by 3. Softer plaster, multiply the weight by 2.75)
Your mold is 5" x 5" x 4" so your volume is 100 cubic inches.
100 divided by 80 =1.25. You need 1.25 quarts of water
Multiply 1.25 by 2.85 = 3.56. You need 3.56 lbs of plaster.
1/2 quart = 1 lb = 16 oz
1 quart = 2 lbs = 32 oz (above example 1.25 quarts of water= 40 oz. Multiply 1.25 by 32oz )
1 lb = 453.6 g
What method do I use to add plaster to the water?
Always add plaster to water not the reverse!
Sift plaster into water with a spoon or by hand. Continue doing so until a small mountain forms and remains above the water level. Once that mountain forms, let the plaster "soak" for 1-3 minutes. The greater the amount, the longer the soak - anything in a one gallon bucket needs only a minute or so.
Once the soak cycle is complete you can mix the plaster for a couple minutes. In order to get the most consistent mix, it is best to mix the plaster with a hand blender for small batches, or a drill with a mixing attachment for large batches. Mixing by hand is possible, but the smaller air bubbles tend not to release in the mix and might appear in the surface of your mold.
If mixed by a mixer, be sure to mix a bit after by hand to feel the consistency. The plaster is ready to pour in the mold once it changes from watery to creamy. Work quickly before the plaster starts to set.
Keri Radasch also has similar notes on her site and a link to Ian Anderson's guidelines as well.