Hard water in Wailuku, HI

Tap water hardness
150mg/L
8.8gpg
Hard

Reported by Wailuku · April 2025

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

Tap water in Wailuku has about 150 mg/L of hardness, which is 8.8 grains per gallon. On the USGS scale that counts as hard. The number comes from Wailuku, dated April 2025.

Wailuku on the hardness scale, with nearby cities
060120180240300360+HI typical 55Pukalani: 12 mg/LPukalani 12Kula: 12 mg/LKula 12Kahului: 20 mg/LKahului 20Lahaina: 32 mg/LLahaina 32Kihei: 51 mg/LKihei 51Honolulu: 55 mg/LHonolulu 55Kailua: 103 mg/LKailua 103East Honolulu: 105 mg/LEast Honolulu 105Waimanalo: 105 mg/LWaimanalo 105Kaneohe: 105 mg/LKaneohe 105Wailuku 150mg/L as calcium carbonate060120180240300360+HI 55Pukalani: 12 mg/LPukalani 12Kula: 12 mg/LKula 12Kahului: 20 mg/LKahului 20Lahaina: 32 mg/LLahaina 32Kihei: 51 mg/LKihei 51East Honolulu: 105 mg/LEast Honol. 105150 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 150 mg/L, Wailuku is 95 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 12 mg/L in Pukalani to 105 in East Honolulu.

What 150 mg/L means at home

Expect white crust on faucets and showerheads, plus spots on dishes that come out of the dishwasher.

Kettles collect scale quickly. Water heaters build up sediment at the bottom of the tank, which makes them work harder over time.

Soap and detergent need a little more to lather, and soap scum shows up in the tub.

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

At 150 mg/L most of the trouble is scale, and only a few things deal with it.

Ion-exchange water softener

The standard whole-house fix. It swaps calcium and magnesium for sodium or potassium and needs salt plus a drain. Size it with the calculator below.

Salt-free conditioner

No salt and no drain line. It changes how minerals form scale instead of removing them, so water still tests hard.

Reverse osmosis or carbon filter

Under-sink reverse osmosis removes minerals from drinking water only. Plain carbon filters improve taste but do not soften.

Softener size for Wailuku

3 people × 75 gallons a day × 8.8 gpg = 1,973 grains a day. A 24,000-grain unit regenerates about every 12 days.

Size it for your household

EPA record for Wailuku's water system

Wailuku · PWSID HI0000212

No health-based violations since 2016

Serves about 74,511 people from surface water.

Health-based
0
Monitoring or reporting
0
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 Wailuku water hard?

Yes. It measures about 150 mg/L (8.8 grains per gallon), which the USGS classes as hard.

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

About 8.8 gpg. One grain per gallon equals 17.1 mg/L, so 150 mg/L divided by 17.1 gives 8.8.

What size water softener do I need in Wailuku?

For 3 people using 75 gallons each per day, 8.8 gpg works out to about 1,973 grains a day. A 24,000-grain softener covers that with a regeneration about every 12 days.

Has the Wailuku water system had EPA violations?

EPA records show no health-based violations for Wailuku since 2016.

Nearby cities

All 23 cities in Hawaii

Hardness data: TapWaterData US Water Hardness (utility report, April 11, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.