Honolulu, HI water hardness
Honolulu Board of Water Supply publishes a hardness range of 25 to 300 mg/L for its system, on the page cited below. On the USGS scale that is hard water, though the utility's own range crosses more than one class — it spans soft and very hard water from end to end.
The published figure, exactly as it appears
This is the hardness line transcribed from the source, with nothing added:
Water sources across Oʻahu may have hardness values between 25 and 300 parts per million (ppm).
Honolulu Board of Water Supply
Units as published: parts per million (ppm). What kind of figure it is: a range with no average. Retrieved 2026-10-17.
Honolulu hardness in mg/L and grains per gallon
Water-quality reports use mg/L (identical to ppm). Softener instructions and plumbing specifications use grains per gallon. Both are below. Where a figure is our arithmetic rather than the utility's own number, it says so.
| Figure | mg/L (ppm) | Grains per gallon | Whose number |
|---|---|---|---|
| Low end of published range | 25 | 1.5 | mg/L published, gpg is our conversion |
| High end of published range | 300 | 17.5 | mg/L published, gpg is our conversion |
| Plotted on the scale above | 162.5 | 9.5 | midpoint of the published range (no average is published); the gpg here is our conversion at 17.1, so it can differ slightly from a utility’s own rounded gpg figure |
The conversion, both ways: gpg = mg/L ÷ 17.1 and mg/L = gpg × 17.1. Worked for this system: 162.5 ÷ 17.1 = 9.5 gpg. The converter does °dH and °f as well, and why the factor is 17.1 explains where the number comes from.
What this figure does not tell you
Service area and source water
| Utility | Honolulu Board of Water Supply |
|---|---|
| Service area as published | The Honolulu Board of Water Supply's sources across the island of Oʻahu. The page states that the hardness level at an address may vary depending on factors such as which pumps are in service, and directs address-specific questions to its Water Quality Department. |
Provenance of every number on this page
| Source document | Honolulu Board of Water Supply, Water Quality frequently asked questions |
|---|---|
| Source URL | https://www.boardofwatersupply.com/frequently-asked-questions/water-quality/ |
| Published by | Honolulu Board of Water Supply |
| Report year | Not stated on the source page |
| Year note | The page states no year for the hardness span and cites no report. |
| Retrieved | 2026-10-17 |
| Classification bands | USGS Water Science School, retrieved 2026-10-16 |
Notes the report forces on this page
- This is the span across the board's sources island-wide, not a figure for any one source, and the board publishes no average. The plotted point is the midpoint of that span and is not a number the board published.
- The published span crosses all four USGS classes, from soft at 25 ppm to very hard at 300 ppm. A single classification cannot honestly be read off it, and the class shown here describes the midpoint only.
- The page states that the level at an address may vary depending on which pumps are in service.
- The board publishes no grains-per-gallon figure; every grains figure on this page is our conversion at 17.1 mg/L per grain.
If you are sizing a softener for Honolulu water
At 9.5 gpg, the sizing arithmetic starts from your household's water use, not from the hardness alone. The calculator below is pre-filled with this system's figure and every other input is an editable assumption.
Whether a softener is worth installing at this hardness is a judgement, not arithmetic, and it is not one this site makes for you. Nothing here has been tested by the author.
Where Honolulu sits against other verified systems
- Portland, OR 9 mg/L 0.5 gpg Soft
- Boston, MA 16 mg/L 1 gpg Soft
- Manchester, NH 17.9 mg/L 1 gpg Soft
- Raleigh, NC 25 mg/L 1.47 gpg Soft
- New York, NY 32 mg/L 2 gpg Soft
- Charlotte, NC 32 mg/L 1.9 gpg Soft
- Memphis, TN 46 mg/L 2.7 gpg Soft
- Anchorage, AK 56 mg/L 3.3 gpg Soft
- Oakland, CA 68.4 mg/L 4 gpg Moderately hard
- Nashville, TN 77.3 mg/L 4.5 gpg Moderately hard
- Sacramento, CA 83 mg/L 4.9 gpg Moderately hard
- Fresno, CA 89.3 mg/L 5.2 gpg Moderately hard
- Saint Paul, MN 90 mg/L 5 gpg Moderately hard
- Austin, TX 93 mg/L 5.4 gpg Moderately hard
- Washington, DC 95 mg/L 5.6 gpg Moderately hard
- Birmingham, AL 98 mg/L 5.7 gpg Moderately hard
- Kansas City, MO 100 mg/L 5.8 gpg Moderately hard
- Denver, CO 105 mg/L 6.1 gpg Moderately hard
- Tulsa, OK 114.5 mg/L 6.7 gpg Moderately hard
- Louisville, KY 115 mg/L 6.7 gpg Moderately hard
- Columbus, OH 120 mg/L 7 gpg Moderately hard
- Cleveland, OH 120 mg/L 7 gpg Moderately hard
- Philadelphia, PA 125 mg/L 7.3 gpg Hard
- Ann Arbor, MI 126 mg/L 7.4 gpg Hard
- Cincinnati, OH 133 mg/L 7.8 gpg Hard
- Milwaukee, WI 137 mg/L 8.01 gpg Hard
- St. Louis, MO 138 mg/L 8.1 gpg Hard
- Fort Worth, TX 145 mg/L 8.5 gpg Hard
- Honolulu, HI 162.5 mg/L 9.5 gpg Hard
- Wichita, KS 170 mg/L 9.9 gpg Hard
- Omaha, NE 170 mg/L 10 gpg Hard
- Los Angeles, CA 192 mg/L 11.2 gpg Very hard
- Lincoln, NE 192 mg/L 11.2 gpg Very hard
- Tampa, FL 220 mg/L 12.9 gpg Very hard
- Phoenix, AZ 237 mg/L 13.9 gpg Very hard
- Mesa, AZ 239.4 mg/L 14 gpg Very hard
- San Diego, CA 276 mg/L 16 gpg Very hard
- Las Vegas, NV 280 mg/L 16 gpg Very hard
- Chandler, AZ 282.2 mg/L 16.5 gpg Very hard
- San Antonio, TX 299.3 mg/L 17.5 gpg Very hard
- Anaheim, CA 300 mg/L 18 gpg Very hard
- Indianapolis, IN 317 mg/L 19 gpg Very hard
- Madison, WI 324.9 mg/L 19 gpg Very hard
- Scottsdale, AZ 352.5 mg/L 20.6 gpg Very hard
mg/L as calcium carbonate. Each classification band is drawn the same width, so the axis is not linear in mg/L.
Systems with similar published hardness
- Wichita, KS City of Wichita Water Utilities, 170 mg/L (9.9 gpg)
- Omaha, NE Metropolitan Utilities District, 170 mg/L (9.9 gpg)
- Fort Worth, TX Fort Worth Water, 145 mg/L (8.5 gpg)
- St. Louis, MO City of St. Louis Water Division, 138 mg/L (8.1 gpg)
All 44 verified systems How to find hardness in your own utility's report If you are on a well instead