How hard is the water in Mount Prospect, IL?

Tap water hardness
112mg/L
6.5gpg
Moderately hard

Reported by Mount Prospect · May 2025

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

Mount Prospect water measures 112 mg/L (6.5 gpg) of hardness, moderately hard by USGS standards. The number comes from Mount Prospect, dated May 2025.

Mount Prospect on the hardness scale, with nearby cities
060120180240300360+IL typical 139Bensenville: 104 mg/LBensenville 104Arlington Heights: 115 mg/LArlington Heights 115Des Plaines: 126 mg/LDes Plaines 126Northbrook: 131 mg/LNorthbrook 131Wheeling: 132 mg/LWheeling 132Elk Grove Village: 141 mg/LElk Grove Village 141Buffalo Grove: 154 mg/LBuffalo Grove 154Rolling Meadows: 229 mg/LRolling Meadows 229Glenview: 229 mg/LGlenview 229Palatine: 350 mg/LPalatine 350Mount Prospect 112mg/L as calcium carbonate060120180240300360+IL 139Arlington Heights: 115 mg/LArlington. 115Des Plaines: 126 mg/LDes Plaines 126Wheeling: 132 mg/LWheeling 132Elk Grove Village: 141 mg/LElk Grove. 141Rolling Meadows: 229 mg/LRolling Me. 229Glenview: 229 mg/LGlenview 229112 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 112 mg/L, Mount Prospect is 27 mg/L below the typical Illinois figure of 139. Of the 10 nearest cities we track, 1 has softer water and 9 have harder. The range nearby runs from 104 mg/L in Bensenville to 350 in Palatine.

What 112 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 Mount Prospect

3 people × 75 gallons a day × 6.5 gpg = 1,474 grains a day. A 24,000-grain unit regenerates about every 16 days.

Size it for your household

EPA record for Mount Prospect's water system

Mount Prospect · PWSID IL0311980

No health-based violations since 2016

Serves about 41,000 people from surface water.

Health-based
0
Monitoring or reporting
14
Open now
1

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 Mount Prospect water hard?

Somewhat. It measures about 112 mg/L (6.5 grains per gallon), which the USGS classes as moderately hard.

What is the water hardness in Mount Prospect in grains per gallon?

About 6.5 gpg. One grain per gallon equals 17.1 mg/L, so 112 mg/L divided by 17.1 gives 6.5.

What size water softener do I need in Mount Prospect?

For 3 people using 75 gallons each per day, 6.5 gpg works out to about 1,474 grains a day. A 24,000-grain softener covers that with a regeneration about every 16 days.

Has the Mount Prospect water system had EPA violations?

EPA records show no health-based violations for Mount Prospect since 2016. There were also 14 monitoring or reporting violations, which are about late or missing tests and notices.

Nearby cities

All 192 cities in Illinois

Hardness data: TapWaterData US Water Hardness (utility report, May 19, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.