Without actually measuring your volume, such as through water displacement, you can find a close approximation based on your density and weight. The formula for density is mass divided by volume, so to calculate volume, divide the mass by the density. The average human has a density of 1 gram per cubic centimeter. Therefore, if you know your weight in grams, you can calculate your volume based on the average human density.
Determine your weight. For example, assume you weight 180 pounds.
Multiply your weight by 453.6 grams to convert your pounds to grams. In the example, 180 pounds times 453.6 equals 81,648 grams. This is your mass.
Divide your mass in grams by a density of 1 gram per cubic centimeter. In the example, 81,648 grams divided by 1 gram per cubic centimeter equals a volume of 81,648 cubic centimeters. Because there are 28,316.8 cubic centimeters in a cubic foot, your volume would be about 2.9 cubic feet.