Hawaii Water Hardness by County
Water in Hawaii has a median hardness of 37.8 mg/L as CaCO₃ — "soft" on the USGS scale — based on 3,112 measurements at 455 monitoring stations. Hawaii County has the state's hardest measured water (57.5 mg/L); Maui County the softest (16.1). Rank: #42 of the 50 states and D.C.
Hawaii water hardness map — what the numbers mean. Data verified 2026-07-22.
Of Hawaii's 5 measured counties, 5 soft. The hardest are Hawaii County (57.5 mg/L), Honolulu County (53.9), and Kalawao County (36). Maui County is the softest at 16.1 mg/L (soft).
No measured Hawaii county reaches the hard threshold, so effectively none of its residents in counties with gate-passing hardness data live in a hard-or-worse county.
Statewide, Hawaii's median works out to 2.2 grains per gallon and ranks #42 of the 50 states and D.C. Across 3,112 measurements at 455 stations — 272 of them since 2000 — the middle 80% of readings fall between 8.8 and 85.1 mg/L. The public record runs from 1968 to 2023.
See where Hawaii sits nationally on the national map, or what its number means on the hardness scale.
Download the Hawaii water hardness map (PNG) — Water hardness by county in Hawaii: 5 counties mapped from USGS/EPA data, statewide median 37.8 mg/L (soft); darker counties have harder water.
By county
Water hardness by county
The table lists all 5 measured Hawaii counties, hardest first. Their medians span 57.5 mg/L in Hawaii County to 16.1 in Maui County — a 3.6× range.
| County | Median | Band | Results | Sites | Newest |
|---|---|---|---|---|---|
| Hawaii County | 57.5 | Soft | 146 | 75 | 2013 |
| Honolulu County | 53.9 | Soft | 1,468 | 190 | 2015 |
| Kalawao County | 36 | Soft | 66 | 6 | 1995 |
| Kauai County | 32 | Soft | 522 | 77 | 1996 |
| Maui County | 16.1 | Soft | 910 | 107 | 2023 |
ZIP codes
ZIP codes in Hawaii
The ZIP codes below are grouped by the Hawaii county they fall in. Each ZIP takes its county's measured median — shown in the group header — so this is a ZIP-to-county guide, not a separate reading for every ZIP. To check the water system serving a specific address, use the lookup.
Hawaii County · 57.5 mg/L · Soft
96704†96710†9671996720†96725†96726†96727†96728†9673796738†96740†9674396749†9675096755†96760†9676496771†96772†96773†96774†96776†96777†96778†96780†96781†96783†96785†
Honolulu County · 53.9 mg/L · Soft
96701†96706†9670796712†96717†96730†96731†96734†9674496759†96762†96782†96786†96789†96791†967929679596797†9681396814968159681696817†968189681996820968219682296825968269684896850968539685796858†9685996860†96863
Kalawao County · 36 mg/L · Soft
96742†
Kauai County · 32 mg/L · Soft
96703†96705†96714†96716†96722†96741†9674696747†9675196752†96754†96756†9676596766†96769†96796†
Maui County · 16.1 mg/L · Soft
96708†96713†96729†9673296748†96753†96757†96761†967639676896770†96779†96790†96793†
† 65 ZIP codes are not served by a mapped community water system — most are private-well areas. See well water hardness.
Source
Groundwater vs surface water
Hawaii's groundwater measures harder than its surface water — 82 mg/L across 398 samples versus 33.6 across 2,694, about 2.4× harder. Wells draw from groundwater, so well owners in Hawaii tend to see the harder number — 82 mg/L, moderately hard. Groundwater readings there run 38.2 to 514 mg/L across the middle 80% of samples. The comparison draws on 271 groundwater and 180 surface-water monitoring sites; 42 readings above 2,000 mg/L were logged but held out of the medians as outliers.
Groundwater (wells)
82 mg/L as CaCO₃ · 398 results at 271 stations.
Surface water (rivers, lakes)
33.6 mg/L as CaCO₃ · 2,694 results at 180 stations.
Groundwater measures about 2.4× harder than surface water here.
Chemistry
Water chemistry in Hawaii
Beyond the hardness number, Hawaii's water carries a fuller chemical signature — here is how its measured parameters compare to national benchmarks.
TDS
65 mg/L · n=235
65 mg/L — below the national county median of 226.
pH
7.7 pH · n=65
pH 7.7 — within the EPA secondary range of 6.5–8.5, on the alkaline side.
Alkalinity
50 mg/L as CaCO3 · n=380
50 mg/L — modest buffering capacity.
Calcium : Magnesium
8.1 : 5.6 mg/L ≈ 1.45 : 1 · n=290
Calcium is about 47% of Hawaii's hardness (8.1 mg/L Ca to 5.6 mg/L Mg), a little more magnesium-rich than the national ~68% calcium share.
Parameter medians may include a small share of quality-control samples; the hardness lane is fully QC-filtered.
Aquifers
Major aquifers under Hawaii
One aquifer system supply much of Hawaii's measured groundwater.
- Hawaii volcanic-rock aquifers — median 84 mg/L across 403 samples at 270 sites: moderately hard.
Together these systems account for 403 measured samples at 270 sites. Aquifer medians pool samples across a whole system and can mask local variation — see the national aquifer map for how these compare across states.
Trend
Hardness over time
Hawaii's decade median moved from 53 mg/L in the 1960s to 23.7 in the 2010s. Over the series, the decade median peaked at 57.4 mg/L in the 2000s and bottomed at 23.7 in the 2010s. Decade medians reflect where and what agencies sampled in each era, not just changes in the water itself.
Scale
Where Hawaii sits on the scale
On the USGS scale, Hawaii's 37.8 mg/L is soft — 2.2 grains per gallon, below the 60 mg/L soft cutoff — among the softest water measured in the US.
Questions
Frequently asked questions
- Is Hawaii's water hard?
- Hawaii's median hardness is 37.8 mg/L as CaCO₃, "soft" on the USGS scale and below the national county median of 140 mg/L.
- Which county in Hawaii has the hardest water?
- Hawaii County, with a median of 57.5 mg/L as CaCO₃ across 146 measurements.
- Is well water harder than city water in Hawaii?
- Groundwater measures harder here — about 2.4× the surface water — so wells tend to run harder (groundwater 82 vs surface water 33.6 mg/L).
- How do I find my exact hardness in Hawaii?
- Search your ZIP on the lookup tool, read your utility's Consumer Confidence Report, or test at home with a kit.
- Is Hawaii's water harder than the national average?
- No. Hawaii's median of 37.8 mg/L is below the national county median of 140 mg/L; it ranks #42 of the 50 states and D.C.
Compare
States with similar hardness
Two states measure closest to Hawaii's 37.8 mg/L: Virginia at 42 mg/L and Connecticut at 42.9 mg/L. Hawaii measures harder than 18% of states. Browse the full ranking on the statistics page.
More: hardness scale · check your address · national statistics · the national map.
By Marko Visic, MPharm, BSc Physics. Source: USGS/EPA Water Quality Portal — 3,112 Hawaii measurements. Data last verified . Methodology.
Cite this page: “Hawaii Water Hardness by County.” Hard Water Map. Retrieved 2026-07-22, https://hardwatermap.com/water-hardness/hawaii/