Parts Per Million
A ratio expressing concentration as one unit of solute per million units of solution. For dilute aqueous samples, 1 ppm ≈ 1 mg/L.
Parts per million is the working unit for most consumer-facing water-quality numbers. It's a ratio, not a mass — which is why it stays meaningful regardless of sample size.
The conversions that matter
For drinking-water dilutions:
- 1 ppm = 1 mg/L (one milligram of solute per liter of water)
- 1 ppb = 1 µg/L (parts per billion — used for trace contaminants)
- 1 ppt = 1 ng/L (parts per trillion — used for PFAS and similar)
The shift between units matters: PFAS limits are typically ppt, lead limits are ppb, TDS readings are ppm. Three orders of magnitude separate each tier.
When ppm is the wrong unit
For very dilute contaminants — PFAS, pharmaceutical residues, some pesticides — quoting in ppm produces tiny decimals that mask the actual scale. Reports use ppt for these. Always check the unit when comparing two numbers; "10" in ppt is a thousand times less than "10" in ppb.