How hard is the water in Hilo, HI?

Tap water hardness
91mg/L
5.3gpg
Moderately hard

Reported by Hilo · April 2025

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

The water in Hilo carries roughly 91 mg/L of dissolved calcium and magnesium, or 5.3 grains per gallon. That is moderately hard water. The number comes from Hilo, dated April 2025.

Hilo on the hardness scale, with nearby cities
060120180240300360+HI typical 55Kula: 12 mg/LKula 12Pukalani: 12 mg/LPukalani 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 105Wailuku: 150 mg/LWailuku 150Hilo 91mg/L as calcium carbonate060120180240300360+HI 55Kula: 12 mg/LKula 12Pukalani: 12 mg/LPukalani 12Kahului: 20 mg/LKahului 20Lahaina: 32 mg/LLahaina 32Kihei: 51 mg/LKihei 51Wailuku: 150 mg/LWailuku 15091 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

Hilo sits 36 mg/L above the typical Hawaii figure of 55. Of the 10 nearest cities we track, 6 have softer water and 4 have harder. The range nearby runs from 12 mg/L in Kula to 150 in Wailuku.

What 91 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 Hilo

3 people × 75 gallons a day × 5.3 gpg = 1,197 grains a day. A 24,000-grain unit regenerates about every 20 days.

Size it for your household

EPA record for Hilo's water system

Hilo · PWSID HI0000101

No health-based violations since 2016

Serves about 39,477 people from groundwater.

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 Hilo water hard?

Somewhat. It measures about 91 mg/L (5.3 grains per gallon), which the USGS classes as moderately hard.

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

About 5.3 gpg. One grain per gallon equals 17.1 mg/L, so 91 mg/L divided by 17.1 gives 5.3.

What size water softener do I need in Hilo?

For 3 people using 75 gallons each per day, 5.3 gpg works out to about 1,197 grains a day. A 24,000-grain softener covers that with a regeneration about every 20 days.

Has the Hilo water system had EPA violations?

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

Nearby cities

All 23 cities in Hawaii

Hardness data: TapWaterData US Water Hardness (utility report, April 22, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.