How hard is the water in Ontario, CA?

Tap water hardness
104mg/L
6.1gpg
Moderately hardReports range 85–104

Reported by Cucamonga Valley Water District · September 2025

CA typical
121 mg/L
EPA health violations since 2016
0
Softener for 3 people
24k grain
Data
Utility report

The water in Ontario carries roughly 104 mg/L of dissolved calcium and magnesium, or 6.1 grains per gallon. That is moderately hard water. The number comes from Cucamonga Valley Water District, dated September 2025. Reports for this system disagree, with values from 85 to 104 mg/L.

Ontario on the hardness scale, with nearby cities
060120180240300360+CA typical 121Rancho Cucamonga: 68 mg/LRancho Cucamonga 68Chino Hills: 81 mg/LChino Hills 81Fontana: 92 mg/LFontana 92Norco: 99 mg/LNorco 99Eastvale: 114 mg/LEastvale 114Jurupa Valley: 122 mg/LJurupa Valley 122Upland: 126 mg/LUpland 126Pomona: 138 mg/LPomona 138Claremont: 157 mg/LClaremont 157Montclair: 166 mg/LMontclair 166Ontario 104mg/L as calcium carbonate060120180240300360+CA 121Rancho Cucamonga: 68 mg/LRancho Cuc. 68Norco: 99 mg/LNorco 99Eastvale: 114 mg/LEastvale 114Jurupa Valley: 122 mg/LJurupa Val. 122Upland: 126 mg/LUpland 126Montclair: 166 mg/LMontclair 166104 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 104 mg/L, Ontario is 17 mg/L below the typical California figure of 121. Of the 10 nearest cities we track, 4 have softer water and 6 have harder. The range nearby runs from 68 mg/L in Rancho Cucamonga to 166 in Montclair.

What 104 mg/L means at home

You may see faint white spots on glasses and shower doors after the water dries.

Scale builds up slowly. A kettle or coffee maker needs descaling now and then, maybe every few months.

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

Optional. Plenty of households at this level live with it and descale the kettle now and then.

Water softener

Worth a look if spots on glass bother you, or if your tankless water heater manual asks for softer water.

Carbon filter

Improves taste and removes chlorine. It leaves calcium and magnesium in place.

Softener size for Ontario

3 people × 75 gallons a day × 6.1 gpg = 1,375 grains a day. A 24,000-grain unit regenerates about every 17 days.

Size it for your household

EPA record for Ontario's water system

Ontario Municipal Utilities Company · PWSID CA3610034

No health-based violations since 2016

Serves about 185,010 people from surface water.

Health-based
0
Monitoring or reporting
0
Open now
0

EPA lists Ontario Municipal Utilities Company as the system serving Ontario. The hardness figure above comes from a different system's report, which TapWaterData uses for Ontario.

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 Ontario water hard?

Somewhat. It measures about 104 mg/L (6.1 grains per gallon), which the USGS classes as moderately hard.

What is the water hardness in Ontario in grains per gallon?

About 6.1 gpg. One grain per gallon equals 17.1 mg/L, so 104 mg/L divided by 17.1 gives 6.1.

What size water softener do I need in Ontario?

For 3 people using 75 gallons each per day, 6.1 gpg works out to about 1,375 grains a day. A 24,000-grain softener covers that with a regeneration about every 17 days.

Has the Ontario water system had EPA violations?

EPA records show no health-based violations for Ontario Municipal Utilities Company since 2016.

Nearby cities

All 515 cities in California

Hardness data: TapWaterData US Water Hardness (utility report, September 23, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.