Water hardness in Kokubunji
61 mg/L moderately hard
Tap water in Kokubunji is moderately hard: 61 mg/L (3.6 gpg · 3.4 °dH · 6.1 °fH).
Source: Tokyo Metropolitan Waterworks Bureau (東京都水道局), tap-water quality results FY2025 (2025) · data checked 2026-10-21
Range across supply zones: 56–65 mg/L.
Kokubunji is supplied by the Tokyo Metropolitan Waterworks Bureau, which draws about 80% of its water resources from the Tone and Arakawa river systems and about 20% from the Tama River system, with groundwater adding to the supply in the Tama area. Hardness is reported at 61.0 mg/L CaCO₃, ranging from 55.7 to 64.9 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 | 61 mg/L |
|---|---|
| Utility | Tokyo Metropolitan Waterworks Bureau (東京都水道局) |
How Kokubunji compares
- Tap water in Kokubunji is 22 % harder than the median of the 228 cities we track in Japan (50 mg/L).
- By hardness it ranks 76 of 228 in Japan (1 = hardest); 67 % of these cities have softer water.
- In Tokyo it is number 28 of 53 cities; the regional median is 62 mg/L.
- Nearest city with clearly softer water: Musashi-murayama, 6 km away (48 mg/L).
- Nearest city with clearly harder water: Musashino, 8 km away (82 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 Kokubunji
| 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 Kokubunji water hard or soft?
It is moderately hard: 61 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 Kokubunji 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 Kokubunji water?
3.6 gpg, equal to 3.4 °dH, 6.1 °fH or 61 mg/L calcium carbonate.
What dishwasher salt setting should I use in Kokubunji?
For 61 mg/L (3.4 °dH) check your manufacturer's table: level 2–3.
Do I need a water softener in Kokubunji?
Not essential; a scale inhibitor protects the heater cheaply.
Source
Source: Tokyo Metropolitan Waterworks Bureau (東京都水道局), tap-water quality results FY2025 (2025). Method: figure published by the water supplier. Tokyo Metropolitan Waterworks Bureau (東京都水道局), tap-water quality results FY2025 · Outdated figure? Tell us