In upstream oil processing, the units milligram per liter (mg/l) and parts per million (ppm) are commonly used in produced water treatment plant. It is common to have a statutory requirement for maximum oil in water concentration (either mg/l or ppm) that can be discharged into open waters. A practice amongst many engineers is to assume these two units are equal or interchangeable such that x *mg/l* is equal to x *ppm*. This assumption is fundamentally wrong and can only hold if the density of oil is equal to the density of water.

This brief article will show the conversion method from *mg/l* to *ppm* and vice versa.

**Example 1**

Convert 40mg/l oil in water to ppm

Oil density – 790 kg/m^{3}

In Oil processing terminology, 40mg/l translates into 40mg of oil in 1 liter of oily water (oil + water)

The above calculation procedure from mg/l to ppm can be written as a simple equation of

**Example 2**

Convert 40 ppm oil in water to mg/l

Oil density – 850 kg/m^{3}

In Oil processing terminology, 40ppm translates into 40 parts of oil in 1 million parts of oily water (oil + water).

This conversion can be easily carried out by using mg/l as the subject of the formulae determined in example 1 above.