How hard is the water in Sonoma, CA?

Tap water hardness
84mg/L
4.9gpg
Moderately hard

Reported by Valley of the Moon Water District · September 2025

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

Sonoma water measures 84 mg/L (4.9 gpg) of hardness, moderately hard by USGS standards. The number comes from Valley of the Moon Water District, dated September 2025.

Sonoma on the hardness scale, with nearby cities
060120180240300360+CA typical 121Napa: 54 mg/LNapa 54Boyes Hot Springs: 62 mg/LBoyes Hot Springs 62Novato: 63 mg/LNovato 63Lucas Valley-Marinwood: 63 mg/LLucas Valley-Marinwood 63American Canyon: 65 mg/LAmerican Canyon 65Vallejo: 82 mg/LVallejo 82Petaluma: 105 mg/LPetaluma 105Rohnert Park: 106 mg/LRohnert Park 106Cotati: 106 mg/LCotati 106Santa Rosa: 121 mg/LSanta Rosa 121Sonoma 84mg/L as calcium carbonate060120180240300360+CA 121Napa: 54 mg/LNapa 54Boyes Hot Springs: 62 mg/LBoyes Hot. 62American Canyon: 65 mg/LAmerican C. 65Petaluma: 105 mg/LPetaluma 105Rohnert Park: 106 mg/LRohnert Pa. 106Cotati: 106 mg/LCotati 10684 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 84 mg/L, Sonoma is 37 mg/L below 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 54 mg/L in Napa to 121 in Santa Rosa.

What 84 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 Sonoma

3 people × 75 gallons a day × 4.9 gpg = 1,100 grains a day. A 24,000-grain unit regenerates about every 21 days.

Size it for your household

EPA record for Sonoma's water system

Sonoma, City of · PWSID CA4910012

No health-based violations since 2016

Serves about 10,619 people from groundwater.

Health-based
0
Monitoring or reporting
5
Open now
1

EPA lists Sonoma, City of as the system serving Sonoma. The hardness figure above comes from a different system's report, which TapWaterData uses for Sonoma.

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 Sonoma water hard?

Somewhat. It measures about 84 mg/L (4.9 grains per gallon), which the USGS classes as moderately hard.

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

About 4.9 gpg. One grain per gallon equals 17.1 mg/L, so 84 mg/L divided by 17.1 gives 4.9.

What size water softener do I need in Sonoma?

For 3 people using 75 gallons each per day, 4.9 gpg works out to about 1,100 grains a day. A 24,000-grain softener covers that with a regeneration about every 21 days.

Has the Sonoma water system had EPA violations?

EPA records show no health-based violations for Sonoma, City of since 2016. There were also 5 monitoring or reporting violations, which are about late or missing tests and notices.

Nearby cities

All 515 cities in California

Hardness data: TapWaterData US Water Hardness (utility report, September 30, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.