"Parts per million" sounds like a small quantity and it is. One part per million (ppm), for example, is the equivalent of one inch in a distance of 16 miles, one second in a little more than 11 days or one car in bumper-to-bumper traffic stretching all the way from Cleveland to San Francisco. Milligrams per liter (mg/L) is another unit scientists use to express small quantities. Converting from one to the other is perfectly simple.
Write the quantity in milligrams per liter that you want to convert to parts per million using the correct units. For example, you write "7 mg/L."
Multiply the number you have written by 1. For example, 7 x 1 = 7.
Write your answer with the correct units as parts per million. For example, you write "7 mg/L = 7 ppm."