Water hardness in Diamond Bar
9.7 gpg hard
Tap water in Diamond Bar is hard: 9.7 gpg (166 mg/L · 9.3 °dH · 16.6 °fH).
Source: WVWD 2025 Consumer Confidence Report (2025 data) (2025) · data checked 2026-10-20
Range across supply zones: 97–280 mg/L.
2 supply zones — hardness varies by neighbourhood; the figure above is the median.
Hardness by district / supply zone
| gpg | mg/L | ||
|---|---|---|---|
| Miramar Plant (TVMWD, ~39% of supply) | 5.7 | 97 | moderately hard |
| Weymouth Plant (MWD, ~60% of supply) | 13.7 | 234 | very hard |
Walnut Valley Water District's 2025 report lists two treatment plants. Water from the Metropolitan Water District's Weymouth plant averages 234 mg/L (13.7 gpg), range 189 to 280 mg/L. Water from the Miramar plant, run by Three Valleys MWD, averages 97 mg/L (5.7 gpg). The page shows the median of 165.5 mg/L (9.7 gpg) across both, within a range of 97 to 280 mg/L. The district does not publish which plant serves which neighborhood, so your water may be much softer or harder than the median. Ask the district before sizing a softener.
USGS classes: soft 0–60, moderately hard 61–120, hard 121–180, very hard >180 mg/L.
Water data
| Total hardness | 9.7 gpg |
|---|---|
| Utility | Walnut Valley Water District (WVWD) |
How Diamond Bar compares
- Tap water in Diamond Bar is 7 % harder than the median of the 1372 cities we track in the United States (9.1 gpg).
- By hardness it ranks 644 of 1372 in the United States (1 = hardest); 53 % of these cities have softer water.
- In California it is number 161 of 336 cities; the regional median is 9.5 gpg.
- Nearest city with clearly softer water: Rancho Cucamonga, 25 km away (7.3 gpg).
- Nearest city with clearly harder water: Pomona, 6 km away (15.5 gpg).
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 Diamond Bar
| 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 Diamond Bar water hard or soft?
It is hard: 166 mg/L as CaCO₃ (9.7 gpg, 9.3 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Diamond Bar 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 Diamond Bar water?
9.7 gpg, equal to 9.3 °dH, 16.6 °fH or 166 mg/L calcium carbonate.
What dishwasher salt setting should I use in Diamond Bar?
For 166 mg/L (9.3 °dH) check your manufacturer's table: level 4–5.
Do I need a water softener in Diamond Bar?
Recommended if you have a water heater, boiler or tankless unit: it saves repairs and detergent.
Source
Source: WVWD 2025 Consumer Confidence Report (2025 data) (2025). Method: figure published by the water supplier. WVWD 2025 Consumer Confidence Report (2025 data) · Outdated figure? Tell us