Finding cubic metres is the same thing as finding the volume of an item, measured in metres. Take a few measurements, do a simple calculation, and you have your answer. Just grab your metric ruler -- using one in inches and feet will work but requires different conversions.
Measure the height of your object.
Measure the length.
Measure the width.
Convert measurements to metres if they are not already. If measurement was taken in millimetres, divide by 1,000. If by centimetres, divide by 100.
Multiply the three measurements. The answer is given in cubic metres.
If all you have is a standard ruler, convert feet to metres by multiplying the measurement in feet by 0.3048.
If you are looking to put something in a cube, measure the inside height, length and width for an accurate answer.