If San Jose Water supplies your Cupertino home, its own report already holds the figures you need. Here is how to read them, and what to ask before a softener or a drinking water system goes in.
Water Softener San Jose is an independent referral service, not an installer or plumber. When you send the form with your Cupertino ZIP code, your details are passed to an independent local water treatment pro, who can contact you and prepare a quote.
The report that covers Cupertino
San Jose Water’s 2025 annual water quality report covers two systems: its main system (CA4310011) and the City of Cupertino system (CA4310018). Check your bill first: if it names San Jose Water, this is your report. If another company supplies you, ask your water provider for its annual water quality report instead.
The report gives hardness as calcium carbonate by source, and includes a map of which source feeds which area. Find your area on the map, then read the figure for that source.
| Source (San Jose Water, 2025) | Average hardness | On the USGS scale |
|---|---|---|
| Valley Water treated surface water | 99 ppm, about 5.8 grains per gallon | Moderately hard |
| Mountain surface water | 151 ppm, about 9 grains per gallon | Hard |
| Groundwater | 288 ppm, about 16.8 grains per gallon | Very hard |
What the figure means for a softener
The USGS grades hardness as 0 to 60 mg/L soft, 61 to 120 moderately hard, 121 to 180 hard and more than 180 very hard. A softener is set in grains per gallon, about 17 mg/L each. The difference between sources is large: a home on groundwater-fed supply, at about 16.8 grains on average, asks close to three times as much of a softener as one on Valley Water’s treated water at about 5.8.
So the quote should name the hardness it was sized for and say whether it came from the report or a test at your tap. Size also depends on how much water your household uses. NSF/ANSI 44 is the standard for residential softeners; ask whether the unit is certified to it.
A softener plus reverse osmosis at the sink
A softener and a drinking water system do different jobs. The softener deals with hardness through the house; a point-of-use reverse osmosis system treats water at one tap, such as the kitchen sink. EPA says RO systems can potentially remove contaminants such as lead, VOCs, PFAS and arsenic, and that a typical system sends five gallons or more of reject water down the drain per gallon treated, while WaterSense labeled units send 2.3 gallons or less. NSF/ANSI 58 is the standard for RO systems.
San Jose Water’s 2025 report says it monitors 88 groundwater wells and 9 surface sources for PFAS; the report itself has the results. If you want both systems, ask how the RO unit will be fed and whether both can be quoted on one visit. Our page on reverse osmosis in San Jose covers the RO install in detail.
Before the work starts
- Confirm which company supplies you, and find your source on its map.
- Ask what hardness the softener was sized for, and where the figure came from.
- Ask where the softener will go: as general practice, on the main line before the water heater, with a bypass valve.
- Ask where the drain lines for the softener and any RO unit will run.
- Check with the City of Cupertino about any permit.
- Get every quote in writing.
Salt-based softeners add sodium to the water in exchange for the hardness removed; on a sodium-restricted diet, ask your doctor. San Jose Water notes some softening systems can make water more corrosive to a water heater’s anode, so keep the water heater maintained. Hard water itself is not a health risk, San Jose Water says.
In Cupertino? Send the form with your ZIP code and an independent local pro can prepare a quote.
More on the work itself: water softener installation in San Jose and, if you already have a softener, water softener repair in San Jose.
Questions people ask
How hard is the water in Cupertino?
If San Jose Water supplies you, its 2025 report, which covers the City of Cupertino system, gives averages by source of about 5.8, 9 and 16.8 grains per gallon. Find your area on its source map.
Is all of Cupertino on San Jose Water?
Check your bill. If another company supplies you, ask your water provider for its annual water quality report.
Can I add reverse osmosis to a softened home?
Yes. Ask how the RO unit will be fed and whether both can be quoted on one visit.