Hard water in Cupertino, CA
Reported by San Jose Water · October 2025
- CA typical
- 121 mg/L
- EPA health violations since 2016
- 0
- Softener for 3 people
- 24k grain
- Data
- Utility report
Tap water in Cupertino has about 186 mg/L of hardness, which is 10.9 grains per gallon. On the USGS scale that counts as very hard. The number comes from San Jose Water, dated October 2025.
Cupertino sits 65 mg/L above the typical California figure of 121. Of the 10 nearest cities we track, 6 have softer water and 4 have harder. The range nearby runs from 29 mg/L in Palo Alto to 298 in Los Altos.
What 186 mg/L means at home
Scale forms fast here. Kettles and coffee makers need regular descaling, and showerheads clog unless you clean them.
Tank water heaters collect mineral sediment that insulates the water from the burner, so flushing the tank matters more than usual.
Dishes and glass shower doors spot easily. Soap scum and a dull film on hair or laundry are common complaints.
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
At 186 mg/L most of the trouble is scale, and only a few things deal with it.
Ion-exchange water softener
The standard whole-house fix. It swaps calcium and magnesium for sodium or potassium and needs salt plus a drain. Size it with the calculator below.
Salt-free conditioner
No salt and no drain line. It changes how minerals form scale instead of removing them, so water still tests hard.
Reverse osmosis or carbon filter
Under-sink reverse osmosis removes minerals from drinking water only. Plain carbon filters improve taste but do not soften.
3 people × 75 gallons a day × 10.9 gpg = 2,446 grains a day. A 24,000-grain unit regenerates about every 9 days.
EPA record for Cupertino's water system
No health-based violations since 2016
Serves about 16,336 people from surface water.
- Health-based
- 0
- Monitoring or reporting
- 0
- Open now
- 0
EPA lists City of Cupertino as the system serving Cupertino. The hardness figure above comes from a different system's report, which TapWaterData uses for Cupertino.
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 Cupertino water hard?
Yes. It measures about 186 mg/L (10.9 grains per gallon), which the USGS classes as very hard.
What is the water hardness in Cupertino in grains per gallon?
About 10.9 gpg. One grain per gallon equals 17.1 mg/L, so 186 mg/L divided by 17.1 gives 10.9.
What size water softener do I need in Cupertino?
For 3 people using 75 gallons each per day, 10.9 gpg works out to about 2,446 grains a day. A 24,000-grain softener covers that with a regeneration about every 9 days.
Has the Cupertino water system had EPA violations?
EPA records show no health-based violations for City of Cupertino since 2016.
Nearby cities
- Saratoga178 mg/LHard
- Los Altos298 mg/LVery hard
- Sunnyvale298 mg/LVery hard
- Santa Clara248 mg/LVery hard
- Campbell178 mg/LHard
- Mountain View134 mg/LHard
- Los Altos Hills274 mg/LVery hard
- Palo Alto29 mg/LSoft
- Los Gatos178 mg/LHard
- Stanford146 mg/LHard
Hardness data: TapWaterData US Water Hardness (utility report, October 7, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.