Guides · A plain-language glossary
Water Terms Explained: PPM, MCL & GPD
PPM, MCL, NTU, GPD — certification labels and test reports are full of acronyms. Here is what they mean without the lab jargon.
PPM / TDS
PPM (parts per million) measures concentration, often of total dissolved solids (TDS). A low TDS reading is not the same as "clean" — some dangerous substances are present even at low ppm.
A TDS meter tells you how much dissolved material, not which material.
MCL
Maximum Contaminant Level — the enforceable limit set for public water systems. It is a legal ceiling for regulated tap water, not a "safe to have zero" target.
NSF/ANSI standards
42 covers aesthetic issues (taste, odor, chlorine). 53 covers health-related claims (e.g., lead, cysts). 58 covers reverse-osmosis systems. 401 covers emerging contaminants like some PFAS.
NTU
Nephelometric Turbidity Units — a measure of cloudiness from suspended particles. High turbidity can shield pathogens, which is why turbidity matters before disinfection.
GPD / GPM
Gallons per day / per minute — capacity and flow. A system rated in GPD is about daily throughput; GPM is about how fast water flows at the tap.
FAQ
Quick answers
+Is low TDS automatically healthy?
No. Low TDS mainly means fewer dissolved solids. Some regulated contaminants are relevant at low ppm, so a proper test matters more than a raw TDS number.
+Why do filters list multiple NSF standards?
Each standard covers a specific claim. More standards listed means the maker has certified reduction claims for more contaminant classes — check which ones matter to you.