Honolulu, HI water hardness

Tap water hardness
55mg/L
3.2gpg
SoftCounty estimate

County estimate from 114 water samples in Honolulu County · July 2026

HI typical
55 mg/L
Data
County estimate

Water samples from around Honolulu put hardness at about 55 mg/L (3.2 gpg), which would be soft on the USGS scale. Treat this as a rough guide: it is the median of 114 samples taken across Honolulu County, mostly wells and streams, not a test of Honolulu's treated tap water.

Honolulu on the hardness scale, with nearby cities
060120180240300360+HI typical 55Halawa: 102 mg/LHalawa 102Aiea: 102 mg/LAiea 102Hickam Housing: 102 mg/LHickam Housing 102Waimalu: 102 mg/LWaimalu 102Iroquois Point: 102 mg/LIroquois Point 102Heeia: 105 mg/LHeeia 105Kaneohe: 105 mg/LKaneohe 105Waimanalo: 105 mg/LWaimanalo 105Ahuimanu: 105 mg/LAhuimanu 105East Honolulu: 105 mg/LEast Honolulu 105Honolulu 55mg/L as calcium carbonate060120180240300360+HI 55Halawa: 102 mg/LHalawa 102Aiea: 102 mg/LAiea 102Hickam Housing: 102 mg/LHickam Hou. 102Waimalu: 102 mg/LWaimalu 102Heeia: 105 mg/LHeeia 105Kaneohe: 105 mg/LKaneohe 10555 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

Honolulu sits close to the typical Hawaii figure of 55 mg/L. None of the 10 nearest cities we track has softer water. The range nearby runs from 102 mg/L in Halawa to 105 in Heeia.

What 55 mg/L means at home

Soap and shampoo lather with very little product. Kettles and showerheads stay mostly free of white crust.

Some people notice that soap feels slow to rinse off in soft water. That slippery feel is normal and harmless.

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

Nothing, as far as hardness goes. At 55 mg/L a softener would have almost nothing to remove.

Carbon filter

If the water tastes of chlorine, a pitcher or faucet carbon filter takes care of that. It does not change hardness.

EPA record for Honolulu's water system

We could not match Honolulu to a community water system in EPA's records. Your water bill or the yearly Consumer Confidence Report names the system and its ID.

Questions

Is Honolulu water hard?

No. It measures about 55 mg/L (3.2 grains per gallon), which the USGS classes as soft.

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

About 3.2 gpg. One grain per gallon equals 17.1 mg/L, so 55 mg/L divided by 17.1 gives 3.2.

Nearby cities

All 23 cities in Hawaii

Hardness data: TapWaterData US Water Hardness (county estimate, July 4, 2026), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.