Convert Parts Per

The parts-per notation is used primarily when one wants to specify very low concentrations of a substance. To illustrate how low the concentrations are: 1 parts-per million corresponds to 0.001 per mille.

The parts-per notation is not part of the SI system, but is still often used in chemistry to describe dilute solutions. For example, the parts-per notation is used when talking about contaminants in water.