How do you calculate the density of creams and body lotions?

Denisty is mass over volume, usually expressed as grams (abbreviated g) per mililiter (abbreviated ml).

1 milimeter equals 1 cubic centimeter, so those volumes are interchangeable.

The density of water is 1 gram per ml (g/ml), as a reference point. The term specific gravity refers to the density of something relative to water.

So for your question, take a known volume of your lotion and weigh it. We use fluid ounces normally here in the United States. 1 fluid ounce = 30 ml. Of note, 8 fluid ounces equal one cup. Next weigh that known volume of lotion. We typically use pounds (lbs) in United States. 1 lb = 454 g. Divide your weight in grams by your volume in milliliters and you have your density.

If you are weighing your lotion in some container, weigh the container first. Then by simple subtraction you have your lotion weight.

Hope that makes sense.