How hard is the water in Kailua, HI?

Tap water hardness
103mg/L
6.0gpg
Moderately hard

Reported by HNL-Windward-Pearl Harbor · October 2025

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

The water in Kailua carries roughly 103 mg/L of dissolved calcium and magnesium, or 6.0 grains per gallon. That is moderately hard water. The number comes from HNL-Windward-Pearl Harbor, dated October 2025.

Kailua on the hardness scale, with nearby cities
060120180240300360+HI typical 55Honolulu: 55 mg/LHonolulu 55Halawa: 102 mg/LHalawa 102Aiea: 102 mg/LAiea 102Waimalu: 102 mg/LWaimalu 102Kaneohe: 105 mg/LKaneohe 105Waimanalo: 105 mg/LWaimanalo 105Heeia: 105 mg/LHeeia 105Ahuimanu: 105 mg/LAhuimanu 105East Honolulu: 105 mg/LEast Honolulu 105Kahaluu: 105 mg/LKahaluu 105Kailua 103mg/L as calcium carbonate060120180240300360+HI 55Kaneohe: 105 mg/LKaneohe 105Waimanalo: 105 mg/LWaimanalo 105Heeia: 105 mg/LHeeia 105Ahuimanu: 105 mg/LAhuimanu 105East Honolulu: 105 mg/LEast Honol. 105Kahaluu: 105 mg/LKahaluu 105103 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

Kailua sits 48 mg/L above the typical Hawaii figure of 55. Of the 10 nearest cities we track, 4 have softer water and 6 have harder. The range nearby runs from 55 mg/L in Honolulu to 105 in Kaneohe.

What 103 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 Kailua

3 people × 75 gallons a day × 6.0 gpg = 1,352 grains a day. A 24,000-grain unit regenerates about every 17 days.

Size it for your household

EPA record for Kailua's water system

HNL-Windward-Pearl Harbor · PWSID HI0000331

No health-based violations since 2016

Serves about 631,389 people from groundwater.

Health-based
0
Monitoring or reporting
0
Open now
0

TapWaterData ties Kailua to this system. EPA's service-area records do not name Kailua, so check the utility printed on your water bill.

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 Kailua water hard?

Somewhat. It measures about 103 mg/L (6.0 grains per gallon), which the USGS classes as moderately hard.

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

About 6.0 gpg. One grain per gallon equals 17.1 mg/L, so 103 mg/L divided by 17.1 gives 6.0.

What size water softener do I need in Kailua?

For 3 people using 75 gallons each per day, 6.0 gpg works out to about 1,352 grains a day. A 24,000-grain softener covers that with a regeneration about every 17 days.

Has the Kailua water system had EPA violations?

EPA records show no health-based violations for HNL-Windward-Pearl Harbor since 2016.

Nearby cities

All 23 cities in Hawaii

Hardness data: TapWaterData US Water Hardness (utility report, October 14, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.