Converting from cubic meters to kilograms is essentially calculating an object's weight from its volume. The comparison between a cubic meter of water and a cubic meter of air shows that although the volume is the same, water weighs much more than air. This is because an object's weight depends not only on volume but on density. With an object’s density and volume, the object's weight can be calculated.
Write down the volume and density of the object you're interested in weighing. Include the units of your measurements. For example, if you want to calculate the weight for 3 cubic meters of water, you would write down the volume of 3 cubic meters and density of 1,000 kilograms per cubic meter.
Convert the units of volume to "cubic meters" and density to "kilograms per cubic meter" if they are not already in those units.
Double-check your units by ensuring the volume unit cancels out the bottom part of the density unit. For example, if your volume is in "cubic meters" and your density is in "kilograms per cubic meter," the "cubic meters" cancel out, leaving you with a measurement in "kilograms."
Multiply the volume by the density. For example, with the 3 cubic meters of water, multiply 3 by 1,000 to get a result of 3,000.
Write down the multiplication results with the units figured earlier. This should give you a weight in kilograms. For example, the weight for the 3 cubic meters of water is 3,000 kilograms.
Technically speaking, an object's density multiplied by its volume provides the object's mass, not its weight. In everyday use, however, mass and weight can be used interchangeably, as in this article's example.
The units of measurement for the volume must match the bottom of the units of measure for density. For example, if the volume units are in "cubic meter" and the density units are in "grams per cubic centimeter," the density needs to be converted to "kilograms per cubic meter" before multiplying the volume and density.
These calculations assume a standard temperature and pressure. If the pressure or temperature varies, it will affect the volume of the object, which then affects its weight.