Water hardness in Oizumi
62 mg/L moderately hard
Tap water in Oizumi is moderately hard: 62 mg/L (3.6 gpg · 3.4 °dH · 6.2 °fH).
Source: Gunma Tobu Waterworks Authority (群馬東部水道企業団), tap-water quality results FY2025 (2025) · data checked 2026-10-21
Range across supply zones: 52–72 mg/L.
Oizumi is served by the Gunma Tobu Waterworks Authority, which receives surface water from the Tone River upstream of the Tone Barrage and from the Watarase River above the Takatsudo Dam; the area has been shifting from groundwater to surface water. Hardness is reported at 61.5 mg/L CaCO₃, ranging from 52.0 to 72.0 mg/L (2025 data). Expect gradual white deposits on kettles, taps and showerheads; descaling now and then with citric acid is usually enough.
Japanese waterworks report hardness (硬度) in mg/L CaCO₃; the standard is 300 mg/L and the target 10–100 mg/L.
Water data
| Total hardness | 62 mg/L |
|---|---|
| Utility | Gunma Tobu Waterworks Authority (群馬東部水道企業団) |
How Oizumi compares
- Tap water in Oizumi is 23 % harder than the median of the 228 cities we track in Japan (50 mg/L).
- By hardness it ranks 74 of 228 in Japan (1 = hardest); 68 % of these cities have softer water.
- In Gunma it is number 4 of 11 cities; the regional median is 51 mg/L.
- Nearest city with clearly softer water: Itakura, 19 km away (41 mg/L).
- Nearest city with clearly harder water: Chiyoda, 6 km away (79 mg/L).
Ranking of the hardest and softest water in Japan →
What it means at home
- Visible kettle scale every few weeks; vinegar or citric acid is enough.
- Water heater and washing machine: some long-term scale; a scale inhibitor slows it.
- Dishwasher: medium salt setting.
- Softener optional; pays off mainly with a gas boiler or tankless heater.
How much salt will you use for softening?
Optional. A scale inhibitor or a small softener protects the heater; soap savings are modest.
Descale a coffee machine or kettle about every 3 months (typical manufacturer guidance for this hardness).
Estimates for planning, not a quote. Check your appliance manual for exact settings.
Dishwasher setting in Oizumi
| Brand | Setting for this water |
|---|---|
| Bosch | H00 |
| Miele | 3 °dH |
| AEG / Electrolux | 1 |
| Beko | 1st |
| Hoover | S0 |
| LG | H0 |
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 Oizumi water hard or soft?
It is moderately hard: 62 mg/L as CaCO₃ (3.6 gpg, 3.4 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Oizumi safe to drink?
Hardness is not a health issue; it is about limescale, taste and appliances. Mains water must meet national drinking-water standards. Japanese waterworks report hardness (硬度) in mg/L CaCO₃; the standard is 300 mg/L and the target 10–100 mg/L.
How many grains per gallon is Oizumi water?
3.6 gpg, equal to 3.4 °dH, 6.2 °fH or 62 mg/L calcium carbonate.
What dishwasher salt setting should I use in Oizumi?
For 62 mg/L (3.4 °dH) check your manufacturer's table: level 2–3.
Do I need a water softener in Oizumi?
Not essential; a scale inhibitor protects the heater cheaply.
Source
Source: Gunma Tobu Waterworks Authority (群馬東部水道企業団), tap-water quality results FY2025 (2025). Method: figure published by the water supplier. Gunma Tobu Waterworks Authority (群馬東部水道企業団), tap-water quality results FY2025 · Outdated figure? Tell us