How hard is the water in Kauai County, HI?

2.6 grains per gallon

You do not need a softener here.

The median hardness measured in Kauai County, HI is 45 mg/L as calcium carbonate, which is 2.6 gpg. That is soft on the USGS scale. It comes from 64 samples at 3 sites between 1990 and 1996, drawn from every sampling site in the county, wells and surface water together.

How much it varies here

Half the sites came in between 1.5 and 3.1 gpg. The whole middle of that range gives the same answer, so a test of your own tap is unlikely to change it. Confidence in the county figure: medium (how it is measured).

What to do next

You can skip the softener. At 2.6 gpg the most a softener would fix is the odd water spot. If a salesperson in Kauai County says otherwise, ask to see their reading in grains per gallon and compare it with this one.

More on when a softener is not worth it.

From 0.3 to 3.7 gpg

The softest single sample on record in Kauai County was 5.4 mg/L and the hardest 62.8 mg/L, a 12-fold gap. The top quarter of sites read above 3.1 gpg and the bottom quarter below 1.5 gpg.

Wells 3.7 gpg, rivers and lakes 1.5 gpg

6 groundwater samples from 1 wells put the median at 62.8 mg/L. 58 surface samples from 2 sites put it at 25 mg/L. In Kauai County the wells come out 2.2 gpg harder. Groundwater sits in contact with rock for years and picks up calcium and magnesium as it goes, so wells usually read harder. The two sources are close, so the county figure should hold whichever one your tap is on.

Kauai County against the rest of Hawaii

Among the 5 Hawaii counties with data, Kauai County is number 4 of 5 counting from the hardest. Across the whole country it is harder than 20% of counties.

Softest in Hawaii: Kalawao County at 2.1 gpg. Hardest: Maui County at 24.7 gpg. Closest to Kauai County: Kalawao County (2.1), Honolulu County (3.6), Hawaii County (4).