Concentration in 'parts' jargon is useful, especially when discussing very dilute solutions.

These units are a mass ratio. Parts per million (ppm) is grams of solute/1,000,000 grams of solution. It is a similar unit to % by mass.

In calculations, it is useful to look at a sample size that is the same as the parts units. For instance, use 1,000,000 for ppm calculations.

In very dilute solutions where these units are frequently used, the amount of solvent is within experimental error of the amount of solution. Consequently in these dilute solutions, molarity determinations are frequently converted to mass ratios by assuming that the mass of 1 liter of water is 1 kg. The results are reported in ppm or ppb units in situations where only 1 or 2 significant figures are reported. Water purity is a frequently reported this way. Regulatory agencies frequently publish upper limits of contaminates in units of ppm.