How hard is the water in Naperville, IL?

Tap water hardness
120mg/L
7.0gpg
Moderately hard

Reported by Naperville · July 2025

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

The water in Naperville carries roughly 120 mg/L of dissolved calcium and magnesium, or 7.0 grains per gallon. That is moderately hard water. The number comes from Naperville, dated July 2025.

Naperville on the hardness scale, with nearby cities
060120180240300360+IL typical 139Wheaton: 124 mg/LWheaton 124Downers Grove: 131 mg/LDowners Grove 131Woodridge: 134 mg/LWoodridge 134Boulder Hill: 235 mg/LBoulder Hill 235Lisle: 287 mg/LLisle 287Bolingbrook: 287 mg/LBolingbrook 287Aurora: 287 mg/LAurora 287Darien: 287 mg/LDarien 287Romeoville: 303 mg/LRomeoville 303Warrenville: 474 mg/LWarrenville 474Naperville 120mg/L as calcium carbonate060120180240300360+IL 139Downers Grove: 131 mg/LDowners Gr. 131Woodridge: 134 mg/LWoodridge 134Lisle: 287 mg/LLisle 287Bolingbrook: 287 mg/LBolingbrook 287Aurora: 287 mg/LAurora 287Warrenville: 474 mg/LWarrenville 474120 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 120 mg/L, Naperville is 19 mg/L below the typical Illinois figure of 139. None of the 10 nearest cities we track has softer water. The range nearby runs from 124 mg/L in Wheaton to 474 in Warrenville.

What 120 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 Naperville

3 people × 75 gallons a day × 7.0 gpg = 1,580 grains a day. A 24,000-grain unit regenerates about every 15 days.

Size it for your household

EPA record for Naperville's water system

Naperville · PWSID IL0434670

No health-based violations since 2016

Serves about 156,406 people from surface water.

Health-based
0
Monitoring or reporting
16
Open now
0

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 Naperville water hard?

Somewhat. It measures about 120 mg/L (7.0 grains per gallon), which the USGS classes as moderately hard.

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

About 7.0 gpg. One grain per gallon equals 17.1 mg/L, so 120 mg/L divided by 17.1 gives 7.0.

What size water softener do I need in Naperville?

For 3 people using 75 gallons each per day, 7.0 gpg works out to about 1,580 grains a day. A 24,000-grain softener covers that with a regeneration about every 15 days.

Has the Naperville water system had EPA violations?

EPA records show no health-based violations for Naperville since 2016. There were also 16 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, July 14, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.