Water hardness in Honolulu
9.5 gpg hard
Tap water in Honolulu is hard: 9.5 gpg (163 mg/L · 9.1 °dH · 16.3 °fH).
Source: Honolulu Board of Water Supply, Water Quality FAQs (hardness), updated 08/11/2026 (2026) · data checked 2026-10-23
Range across supply zones: 25–300 mg/L.
Honolulu's water comes mostly from groundwater in Oʻahu's volcanic basalt aquifers, pumped from wells across the island. Hardness varies by well, from 25 to 300 mg/L, with a median of 162.5 mg/L CaCO₃ for the city. Check your area's value, since some neighborhoods get much harder water than others.
USGS classes: soft 0–60, moderately hard 61–120, hard 121–180, very hard >180 mg/L.
Water data
| Total hardness | 9.5 gpg |
|---|---|
| Utility | Honolulu Board of Water Supply (BWS) |
How Honolulu compares
- Tap water in Honolulu is 5 % harder than the median of the 1372 cities we track in the United States (9.1 gpg).
- By hardness it ranks 652 of 1372 in the United States (1 = hardest); 53 % of these cities have softer water.
Ranking of the hardest and softest water in the United States →
What it means at home
- Fast scale on taps, shower screens and kettle; white spots on glassware.
- Water heater: efficiency loss and element failures within years if unprotected.
- Dry skin and dull hair after showering are common.
- Softener recommended; jug or reverse osmosis for drinking.
How much salt will you use for softening?
Recommended, especially with a heater or boiler: scale cuts efficiency and shortens its life.
Descale a coffee machine or kettle about every 2 months (typical manufacturer guidance for this hardness).
Estimates for planning, not a quote. Check your appliance manual for exact settings.
Dishwasher setting in Honolulu
| Brand | Setting for this water |
|---|---|
| Bosch | H02 |
| Miele | 9 °dH |
| AEG / Electrolux | 2 |
| Beko | 3rd |
| Hoover | S1 |
| LG | H2 |
From each brand's manual for the model family named in the source; your model may differ, so check its manual. All brands and how to change the setting →
FAQ
Is Honolulu water hard or soft?
It is hard: 163 mg/L as CaCO₃ (9.5 gpg, 9.1 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Honolulu safe to drink?
Hardness is not a health issue; it is about limescale, taste and appliances. Mains water must meet national drinking-water standards. USGS classes: soft 0–60, moderately hard 61–120, hard 121–180, very hard >180 mg/L.
How many grains per gallon is Honolulu water?
9.5 gpg, equal to 9.1 °dH, 16.3 °fH or 163 mg/L calcium carbonate.
What dishwasher salt setting should I use in Honolulu?
For 163 mg/L (9.1 °dH) check your manufacturer's table: level 4–5.
Do I need a water softener in Honolulu?
Recommended if you have a water heater, boiler or tankless unit: it saves repairs and detergent.
Source
Source: Honolulu Board of Water Supply, Water Quality FAQs (hardness), updated 08/11/2026 (2026). Method: computed from supplier's calcium and magnesium. Honolulu Board of Water Supply, Water Quality FAQs (hardness), updated 08/11/2026 · Outdated figure? Tell us