Which are the some units to express density?

There is no unit of density but it is a measure of mass (weight) per unit of volume (space). If a brick weights 3kg and it has a volume of one thousandths of a cubic metre (.001 m3), it will have a density of 3Mg/m3 or 3 Mega grams per cubic metre. The units of density are Volume measured in cubic metres and Mass measured in grams or kg (kilograms)

In the old imperial system this would be pounds per cubic foot.