The aerometer also called hydrometer is an instrument for measuring the density and weight for a fluid whether gas or liquid. Its body is made of hollow tube with a widened bottom to which a weight is attached. This weight, usually lead or mercury, keeps the instrument upright when it is placed in the substances to be measured. The hollow tube narrows upward into a graduated road or stem.

The density of the liquid is determined by reading where the scale penetrates the surface of the liquid. This instrument is based on the principle of Archimedes, which states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid, this is also known as the principle of buoyancy. Since the weight of aerometer is fixed, it is possible to measure the density and weight of a fluid based on how deep the device sinks in the elements.