How hard is the water in Ahuimanu, HI?

Tap water hardness
105mg/L
6.1gpg
Moderately hard

Reported by HNL-Windward-Pearl Harbor · May 2025

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

Ahuimanu water measures 105 mg/L (6.1 gpg) of hardness, moderately hard by USGS standards. The number comes from HNL-Windward-Pearl Harbor, dated May 2025.

Ahuimanu on the hardness scale, with nearby cities
060120180240300360+HI typical 55Honolulu: 55 mg/LHonolulu 55Aiea: 102 mg/LAiea 102Halawa: 102 mg/LHalawa 102Waimalu: 102 mg/LWaimalu 102Pearl City: 102 mg/LPearl City 102Kailua: 103 mg/LKailua 103Kahaluu: 105 mg/LKahaluu 105Heeia: 105 mg/LHeeia 105Kaneohe: 105 mg/LKaneohe 105Waimanalo: 105 mg/LWaimanalo 105Ahuimanu 105mg/L as calcium carbonate060120180240300360+HI 55Aiea: 102 mg/LAiea 102Halawa: 102 mg/LHalawa 102Waimalu: 102 mg/LWaimalu 102Kahaluu: 105 mg/LKahaluu 105Heeia: 105 mg/LHeeia 105Kaneohe: 105 mg/LKaneohe 105105 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

Ahuimanu sits 50 mg/L above the typical Hawaii figure of 55. None of the 10 nearest cities we track has harder water. The range nearby runs from 55 mg/L in Honolulu to 105 in Kahaluu.

What 105 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 Ahuimanu

3 people × 75 gallons a day × 6.1 gpg = 1,381 grains a day. A 24,000-grain unit regenerates about every 17 days.

Size it for your household

EPA record for Ahuimanu'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 Ahuimanu to this system. EPA's service-area records do not name Ahuimanu, 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 Ahuimanu water hard?

Somewhat. It measures about 105 mg/L (6.1 grains per gallon), which the USGS classes as moderately hard.

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

About 6.1 gpg. One grain per gallon equals 17.1 mg/L, so 105 mg/L divided by 17.1 gives 6.1.

What size water softener do I need in Ahuimanu?

For 3 people using 75 gallons each per day, 6.1 gpg works out to about 1,381 grains a day. A 24,000-grain softener covers that with a regeneration about every 17 days.

Has the Ahuimanu 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, May 21, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.