Water Hardness in Hawaii (HI)
Hawaii has an average water hardness of 40 ppm (2.3 grains per gallon), which is classified as Soft . Hawaii enjoys naturally soft water that requires no softening treatment. Residents benefit from easy soap lathering, spot-free dishes, and long-lasting plumbing and appliances. Water hardness varies between cities — from 18 ppm in Hilo to 85 ppm in Kailua-Kona.
Water Hardness by City in Hawaii
The table below shows water hardness data for 8 cities in Hawaii, sorted from hardest to softest. Click any city name for detailed information including water source, treatment recommendations, and estimated hard water costs.
| City | Hardness (PPM) | Grains/Gallon | Level | Water Source | pH | TDS | Utility |
|---|---|---|---|---|---|---|---|
| Kailua-Kona | 85 ppm | 5 gpg | Slightly Hard | Ground | 7.5 | 130 | Hawaii County Department of Water Supply |
| Honolulu | 70 ppm | 4.1 gpg | Slightly Hard | Ground | 7.5 | 110 | Honolulu Board of Water Supply |
| Pearl City | 65 ppm | 3.8 gpg | Slightly Hard | Ground | 7.4 | 100 | Honolulu Board of Water Supply |
| Kailua | 55 ppm | 3.2 gpg | Soft | Ground | 7.3 | 85 | Honolulu Board of Water Supply |
| Kaneohe | 48 ppm | 2.8 gpg | Soft | Ground | 7.2 | 75 | Honolulu Board of Water Supply |
| Kahului | 30 ppm | 1.8 gpg | Soft | Mixed | 7.1 | 50 | Maui County Department of Water Supply |
| Lihue | 25 ppm | 1.5 gpg | Soft | Surface | 7 | 40 | Kauai Department of Water |
| Hilo | 18 ppm | 1.1 gpg | Soft | Surface | 6.8 | 30 | Hawaii County Department of Water Supply |
Understanding Water Hardness in Hawaii
Water Sources in Hawaii
Many cities in Hawaii rely on groundwater from wells and aquifers as their primary water source. Groundwater tends to be harder than surface water because it spends more time in contact with limestone, dolomite, and other mineral-rich rock formations deep underground. This prolonged contact allows more calcium and magnesium to dissolve into the water, resulting in higher hardness levels.
Hardness Variation Across Hawaii
Water hardness in Hawaii ranges from 18 ppm in Hilo to 85 ppm in Kailua-Kona — a difference of 67 ppm. This variation means that treatment recommendations differ significantly between cities. Even the hardest water in Hawaii is moderate enough that a water softener is optional rather than essential, though homeowners who value spotless dishes and softer-feeling water may still find one worthwhile. Meanwhile, Hilo enjoys naturally soft water that needs no treatment at all — an advantage that saves residents money on both equipment and ongoing maintenance.
How Hawaii Compares to Other States
Below is a comparison of Hawaii with five states that have similar average water hardness levels. Understanding how your state compares can help you gauge whether your water is typical for the region or unusually hard or soft.
| State | Avg Hardness | Level | Cities |
|---|---|---|---|
| Hawaii (HI) | 40 ppm | Soft | 8 |
| Connecticut (CT) | 35 ppm | Soft | 8 |
| Oregon (OR) | 30 ppm | Soft | 8 |
| Rhode Island (RI) | 30 ppm | Soft | 8 |
| South Carolina (SC) | 50 ppm | Soft | 8 |
| Arkansas (AR) | 55 ppm | Soft | 8 |
Find Your City's Water Hardness
Use our water hardness lookup tool to find your specific city in Hawaii, or browse all 50 states to compare water hardness across the country. If your water is hard, visit our treatment solutions guide to find the right softener or conditioner for your home.