Write down the volume in a standardized set of units. That is, precisely record how much the volume is, with units. Easy units for conversion include cubic inches, cubic feet, liters, gallons and kilograms.
Convert the volume of water into gallons. Multiply the volume by the conversion factor corresponding to your units of measurement. The conversion factors are cubic inches: 0.004; cubic feet: 7.481; liters: 0.264; gallons: 1; and kilograms: 0.264. For example, if you are dealing with a 28-liter volume of water, multiply this value by 0.264 to yield 7.392 gallons.
Convert gallons into pounds by multiplying by 8.345. For the example, multiplying 7.397 gallons by 8.345 yields 61.728 pounds -- the weight of your body of water.