How hard is the water in Cotati, CA?

Tap water hardness
106mg/L
6.2gpg
Moderately hard

Reported by Rohnert Park, City of · September 2025

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

Cotati water measures 106 mg/L (6.2 gpg) of hardness, moderately hard by USGS standards. The number comes from Rohnert Park, City of, dated September 2025.

Cotati on the hardness scale, with nearby cities
060120180240300360+CA typical 121Boyes Hot Springs: 62 mg/LBoyes Hot Springs 62Novato: 63 mg/LNovato 63Lucas Valley-Marinwood: 63 mg/LLucas Valley-Marinwood 63Sonoma: 84 mg/LSonoma 84Healdsburg: 93 mg/LHealdsburg 93Petaluma: 105 mg/LPetaluma 105Rohnert Park: 106 mg/LRohnert Park 106Windsor: 120 mg/LWindsor 120Santa Rosa: 121 mg/LSanta Rosa 121Sebastopol: 142 mg/LSebastopol 142Cotati 106mg/L as calcium carbonate060120180240300360+CA 121Boyes Hot Springs: 62 mg/LBoyes Hot. 62Sonoma: 84 mg/LSonoma 84Petaluma: 105 mg/LPetaluma 105Rohnert Park: 106 mg/LRohnert Pa. 106Santa Rosa: 121 mg/LSanta Rosa 121Sebastopol: 142 mg/LSebastopol 142106 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 106 mg/L, Cotati is 15 mg/L below the typical California figure of 121. Of the 10 nearest cities we track, 6 have softer water and 3 have harder. The range nearby runs from 62 mg/L in Boyes Hot Springs to 142 in Sebastopol.

What 106 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 Cotati

3 people × 75 gallons a day × 6.2 gpg = 1,395 grains a day. A 24,000-grain unit regenerates about every 17 days.

Size it for your household

EPA record for Cotati's water system

Cotati, City of · PWSID CA4910016

No health-based violations since 2016

Serves about 7,290 people from groundwater.

Health-based
0
Monitoring or reporting
0
Open now
0

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

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 Cotati water hard?

Somewhat. It measures about 106 mg/L (6.2 grains per gallon), which the USGS classes as moderately hard.

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

About 6.2 gpg. One grain per gallon equals 17.1 mg/L, so 106 mg/L divided by 17.1 gives 6.2.

What size water softener do I need in Cotati?

For 3 people using 75 gallons each per day, 6.2 gpg works out to about 1,395 grains a day. A 24,000-grain softener covers that with a regeneration about every 17 days.

Has the Cotati water system had EPA violations?

EPA records show no health-based violations for Cotati, City of since 2016.

Nearby cities

All 515 cities in California

Hardness data: TapWaterData US Water Hardness (utility report, September 16, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.