Understanding “ppm” (Parts per Million)

“ppm” stands for “parts per million” and can also be expressed as “milligrams per liter” (mg/L). This measurement indicates the mass of a chemical or contaminant in relation to the volume of water. When you see ppm or mg/L on a lab report, they refer to the same concept.

To grasp the meaning of ppm, imagine adding four drops of ink to a 55-gallon barrel of water and thoroughly mixing it. This would result in an ink concentration of 1 ppm. Here are a few analogies that can help you visualize the scale of ppm:

• One inch in 16 miles.
• One second in 11.5 days.
• One minute in two years.

For an even smaller concentration measurement, we have “parts per billion” (ppb). One ppb represents one part in 1 billion. Due to its lower concentration, consider these analogies:

• One sheet in a roll of toilet paper stretching from New York to London.
• One second in nearly 32 years.
• One pinch of salt in 10 tons of potato chips.