Preparation and Submersion
Fill the barrel about two-thirds of the way with warm, clean water. You must be able to fully submerge yourself without any water splashing over the side.
Measure the distance from the top of the barrel to the surface of the water, in centimeters.
Climb carefully into the barrel, nude or wearing minimal attire. Take a deep breath and submerge yourself completely beneath the surface.
Have your assistant measure the new distance from the top of the barrel to the water surface. Your assistant should indicate to you when he or she is done so that you can come up for air as quickly as possible.
Calculating the Volume
Measure the diameter of the barrel and divide by two to get its radius, r, in centimeters.
Subtract the height of the water surface when nothing was in the water from its height when you were submerged and call this h.
Calculate the volume of the cylinder of water you displaced using the formula Pi * r^2 * h. This gives the volume of your body in cubic centimeters, or milliliters. If you prefer to use liters, divide your result by 1,000.