How hard is the water in Waimalu, HI?

Tap water hardness
102mg/L
6.0gpg
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

The water in Waimalu carries roughly 102 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 May 2025.

Waimalu on the hardness scale, with nearby cities
060120180240300360+HI typical 55Waipahu: 55 mg/LWaipahu 55Honolulu: 55 mg/LHonolulu 55Aiea: 102 mg/LAiea 102Halawa: 102 mg/LHalawa 102Pearl City: 102 mg/LPearl City 102Hickam Housing: 102 mg/LHickam Housing 102Iroquois Point: 102 mg/LIroquois Point 102Ewa Beach: 102 mg/LEwa Beach 102Ahuimanu: 105 mg/LAhuimanu 105Kahaluu: 105 mg/LKahaluu 105Waimalu 102mg/L as calcium carbonate060120180240300360+HI 55Waipahu: 55 mg/LWaipahu 55Aiea: 102 mg/LAiea 102Halawa: 102 mg/LHalawa 102Pearl City: 102 mg/LPearl City 102Hickam Housing: 102 mg/LHickam Hou. 102Iroquois Point: 102 mg/LIroquois P. 102102 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 102 mg/L, Waimalu is 47 mg/L above the typical Hawaii figure of 55. Of the 10 nearest cities we track, 2 have softer water and 2 have harder. The range nearby runs from 55 mg/L in Waipahu to 105 in Ahuimanu.

What 102 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 Waimalu

3 people × 75 gallons a day × 6.0 gpg = 1,346 grains a day. A 24,000-grain unit regenerates about every 17 days.

Size it for your household

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

Somewhat. It measures about 102 mg/L (6.0 grains per gallon), which the USGS classes as moderately hard.

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

About 6.0 gpg. One grain per gallon equals 17.1 mg/L, so 102 mg/L divided by 17.1 gives 6.0.

What size water softener do I need in Waimalu?

For 3 people using 75 gallons each per day, 6.0 gpg works out to about 1,346 grains a day. A 24,000-grain softener covers that with a regeneration about every 17 days.

Has the Waimalu 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.