Water Hardness in Hawaii
Across 34 measured cities — with 10 more covered by labeled county-level estimates — Hawaii's median tap water hardness is 102.2 mg/L (6.0 gpg): moderately hard on the USGS scale. Below: every covered city, the state's hardest and softest water, and the geology that explains it.
Data updated July 6, 2026. Sources: utility-reported water quality data and USGS/EPA Water Quality Portal samples.
Hawaii Cities by Hardness Category
Measured cities only — labeled county estimates never enter the category counts or the median.
| Category | Range | Measured cities | Share |
|---|---|---|---|
| Soft | 0-60 mg/L | 11 | 32.4% |
| Moderately hard | 60-120 mg/L | 21 | 61.8% |
| Hard | 120-180 mg/L | 2 | 5.9% |
| Very hard | >180 mg/L | 0 | 0.0% |
Hardest and Softest Water in Hawaii
Measured values only; labeled county estimates and values under active dispute never rank.
Hardest water in Hawaii
- Wailuku — 150.0 mg/L (8.8 gpg), hard
- Schofield Barracks — 125.0 mg/L (7.3 gpg), hard
- Kaanapali — 117.0 mg/L (6.8 gpg), moderately hard
- Waikoloa — 117.0 mg/L (6.8 gpg), moderately hard
- Ahuimanu — 105.0 mg/L (6.1 gpg), moderately hard
Every Covered City in Hawaii
All 44 covered Hawaii cities, sorted by population served. "County estimate" rows are labeled context values from USGS/EPA Water Quality Portal county sampling — they never enter the median above. City names link to each city's full water quality page.
| City | Hardness (mg/L) | gpg | Category | Data |
|---|---|---|---|---|
| Urban Honolulu | 102.6 | 6.0 | Moderately hard | Utility-reported |
| Aiea | 102.2 | 6.0 | Moderately hard | Utility-reported |
| Ewa Beach | 102.2 | 6.0 | Moderately hard | Utility-reported |
| Halawa | 102.2 | 6.0 | Moderately hard | Utility-reported |
| Hickam Housing | 102.2 | 6.0 | Moderately hard | Utility-reported |
| Iroquois Point | 102.2 | 6.0 | Moderately hard | Utility-reported |
| Pearl City | 102.2 | 6.0 | Moderately hard | Utility-reported |
| Waimalu | 102.2 | 6.0 | Moderately hard | Utility-reported |
| Kailua | 102.8 | 6.0 | Moderately hard | Utility-reported |
| Ahuimanu | 105.0 | 6.1 | Moderately hard | Utility-reported |
| East Honolulu | 105.0 | 6.1 | Moderately hard | Utility-reported |
| Heeia | 105.0 | 6.1 | Moderately hard | Utility-reported |
| Kahaluu | 105.0 | 6.1 | Moderately hard | Utility-reported |
| Kaneohe | 105.0 | 6.1 | Moderately hard | Utility-reported |
| Maunawili | 105.0 | 6.1 | Moderately hard | Utility-reported |
| Waimanalo | 105.0 | 6.1 | Moderately hard | Utility-reported |
| Wailuku | 150.0 | 8.8 | Hard | Utility-reported |
| Jbpph | 74.8 | 4.4 | Moderately hard | Utility-reported |
| Waipahu | 55.4 | 3.2 | Soft | County estimate |
| Mililani | 55.4 | 3.2 | Soft | County estimate |
| Hilo | 90.9 | 5.3 | Moderately hard | Utility-reported |
| Makawao | 20.0 | 1.2 | Soft | County estimate |
| Holualoa | 56.5 | 3.3 | Soft | Utility-reported |
| Kahaluu-Keauhou | 56.5 | 3.3 | Soft | Utility-reported |
| Kaiminani | 56.5 | 3.3 | Soft | Utility-reported |
| Kaloko | 56.5 | 3.3 | Soft | Utility-reported |
| Waikoloa | 117.0 | 6.8 | Moderately hard | Utility-reported |
| Lahaina | 31.8 | 1.9 | Soft | Utility-reported |
| Schofield Barracks | 125.0 | 7.3 | Hard | Utility-reported |
| Waimea | 5.9 | 0.3 | Soft | Utility-reported |
| Kaanapali | 117.0 | 6.8 | Moderately hard | Utility-reported |
| Kula | 12.5 | 0.7 | Soft | Utility-reported |
| Pukalani | 12.5 | 0.7 | Soft | Utility-reported |
| Hawi | 47.9 | 2.8 | Soft | Utility-reported |
| Kapaau | 47.9 | 2.8 | Soft | Utility-reported |
| Kaunakakai | 20.0 | 1.2 | Soft | County estimate |
| Honolulu | 55.4 | 3.2 | Soft | County estimate |
| Lanai City | 20.0 | 1.2 | Soft | County estimate |
| Kunia | 55.4 | 3.2 | Soft | County estimate |
| Kualapuu | 20.0 | 1.2 | Soft | County estimate |
| Maunaloa | 20.0 | 1.2 | Soft | County estimate |
| Kahului | 20.0 | 1.2 | Soft | County estimate |
| Puunene | 60.1 | 3.5 | Moderately hard | Utility-reported |
| Kihei | 51.2 | 3.0 | Soft | Utility-reported |
Above 120 mg/L (7 gpg), scale is usually worth treating — see our water softener guide, or size a system with the hardness calculator & converter.
The Geology Behind Hawaii's Water Hardness
Hawaii sits in the Pacific maritime volcanic terrain region.
The Pacific Northwest, Alaska, and Hawaii share young volcanic bedrock - basalt and other igneous rock with little carbonate to dissolve - and most supplies come from heavy rainfall, snowmelt, or fast-recharging volcanic aquifers. Water simply does not stay in contact with soluble carbonate long enough to harden. That is why the USGS maps these states among the softest in the nation.
Hawaii Water Hardness FAQ
Does Hawaii have hard water?
On the whole, no — Hawaii's statewide median sits below the hard threshold, though some cities run harder. The median is 102.2 mg/L (6.0 gpg) — moderately hard on the USGS scale, where anything above 120 mg/L (7 gpg) counts as hard. 2 of Hawaii's 34 measured cities fall in the hard or very hard bands (6%), so check your own city rather than the state average.
How hard is tap water in Hawaii?
Across the 34 Hawaii cities with measured values in our index, the median hardness is 102.2 mg/L (6.0 gpg) — moderately hard on the USGS scale. Individual cities range from 5.9 to 150.0 mg/L, so check your own city rather than assuming the state number.
Which city in Hawaii has the hardest water?
Wailuku has the hardest measured tap water in our Hawaii index at 150.0 mg/L (8.8 gpg) — hard on the USGS scale. Rankings use measured utility values only; labeled county estimates and values under active dispute are excluded.
Do I need a water softener in Hawaii?
2 of Hawaii's 34 measured cities (6%) fall in the hard or very hard bands — above 120 mg/L (7 gpg), the level where most water treatment professionals suggest considering an ion-exchange softener. Hardness varies utility to utility, so look up your own city's value before sizing equipment.
How do I find my city's exact water hardness?
Enter your ZIP code in our free lookup at tapwaterdata.com/zip to see the value we hold for your Hawaii city from utility-reported water quality data, check your water utility's annual Consumer Confidence Report (CCR), or use a home test-strip kit.
Stay Informed About Your Water Quality
Get EPA reports, filter recommendations, and safety alerts for your area.
Join 10,000+ people protecting their families. Unsubscribe anytime.