Water analysis involves the detection of very small amounts of substances. If the results were expressed as a percentage the numbers would be very small. To make the results easier to read, the results of a water analysis are usually expressed in parts per million (ppm). One part per million equals one part in a million parts (i.e. 1 lb in 1,000,000 lbs of water).

When results are very small they are expressed as parts per billion (ppb) in these cases. One part per billion is equal to one-thousandth of one part per million (0.001 ppm).

The metric system reports results in milligrams per liter (mg/L) or micrograms per liter (µg/L). One microgram per liter is equal to one-thousandth of one milligram per liter (0.001 mg/l). Some results may express results as µg /cc, which is almost the same as mg/l (1 mL = 1.000028 cm^{3}). Milligrams per liter (mg/L) is very close to parts per million (ppm). This formula for the relationship is:

ppm x solution density = mg/L

If the solution density is equal to 1, then ppm = mg/L. This is normally the case in water samples found in industrial water systems. Milligrams per liter (mg/L) and parts per million (ppm) begin to move away from each other when the water density is greater or less than 1. Examples of this would be when the water contains a high amount of solids.

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