San Mateo, CA water hardness

Tap water hardness
35mg/L
2.1gpg
Soft

Reported by California Water Service - San Mateo · October 2025

CA typical
121 mg/L
EPA health violations since 2016
0
Data
Utility report

San Mateo water measures 35 mg/L (2.1 gpg) of hardness, soft by USGS standards. The number comes from California Water Service - San Mateo, dated October 2025.

San Mateo on the hardness scale, with nearby cities
060120180240300360+CA typical 121Burlingame: 31 mg/LBurlingame 31Redwood City: 32 mg/LRedwood City 32North Fair Oaks: 32 mg/LNorth Fair Oaks 32Foster City: 33 mg/LFoster City 33San Carlos: 33 mg/LSan Carlos 33Hillsborough: 36 mg/LHillsborough 36Belmont: 37 mg/LBelmont 37Millbrae: 44 mg/LMillbrae 44San Bruno: 44 mg/LSan Bruno 44South San Francisco: 64 mg/LSouth San Francisco 64San Mateo 35mg/L as calcium carbonate060120180240300360+CA 121Burlingame: 31 mg/LBurlingame 31Foster City: 33 mg/LFoster City 33San Carlos: 33 mg/LSan Carlos 33Hillsborough: 36 mg/LHillsborou. 36Belmont: 37 mg/LBelmont 37Millbrae: 44 mg/LMillbrae 4435 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

At 35 mg/L, San Mateo is 86 mg/L below the typical California figure of 121. Of the 10 nearest cities we track, 5 have softer water and 5 have harder. The range nearby runs from 31 mg/L in Burlingame to 64 in South San Francisco.

What 35 mg/L means at home

Soap and shampoo lather with very little product. Kettles and showerheads stay mostly free of white crust.

Some people notice that soap feels slow to rinse off in soft water. That slippery feel is normal and harmless.

Hardness is not a health problem, and EPA sets no limit for it. It is a matter of scale and soap.

What to do about it

Nothing, as far as hardness goes. At 35 mg/L a softener would have almost nothing to remove.

Carbon filter

If the water tastes of chlorine, a pitcher or faucet carbon filter takes care of that. It does not change hardness.

EPA record for San Mateo's water system

California Water Service - San Mateo · PWSID CA4110008

No health-based violations since 2016

Serves about 103,536 people from surface water.

Health-based
0
Monitoring or reporting
0
Open now
0

TapWaterData ties San Mateo to this system. EPA's service-area records do not name San Mateo, so check the utility printed on your water bill.

Health-based means a contaminant above its legal limit or a missed treatment step. Monitoring and reporting violations are late or missing tests and notices. Source: EPA ECHO, 2026Q3 export, violations with periods starting 2016 or later. Full EPA record.

Questions

Is San Mateo water hard?

No. It measures about 35 mg/L (2.1 grains per gallon), which the USGS classes as soft.

What is the water hardness in San Mateo in grains per gallon?

About 2.1 gpg. One grain per gallon equals 17.1 mg/L, so 35 mg/L divided by 17.1 gives 2.1.

Has the San Mateo water system had EPA violations?

EPA records show no health-based violations for California Water Service - San Mateo since 2016.

Nearby cities

All 515 cities in California

Hardness data: TapWaterData US Water Hardness (utility report, October 14, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.