Water hardness in Shinjuku
70 mg/L moderately hard
Tap water in Shinjuku is moderately hard: 70 mg/L (4.1 gpg · 3.9 °dH · 7 °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: 62–79 mg/L.
Shinjuku Ward 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. The reported figure is 69.7 mg/L CaCO₃ (range 62.4 to 78.8 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 | 70 mg/L |
|---|---|
| Utility | Tokyo Metropolitan Waterworks Bureau (東京都水道局) |
How Shinjuku compares
- Tap water in Shinjuku is 39 % harder than the median of the 228 cities we track in Japan (50 mg/L).
- By hardness it ranks 28 of 228 in Japan (1 = hardest); 88 % of these cities have softer water.
- In Tokyo it is number 4 of 53 cities; the regional median is 62 mg/L.
- Nearest city with clearly softer water: Inagi, 20 km away (56 mg/L).
- Nearest city with clearly harder water: Hakone, 83 km away (110 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 Shinjuku
| Brand | Setting for this water |
|---|---|
| Bosch | H00 |
| Miele | 4 °dH |
| AEG / Electrolux | 2 |
| Beko | 1st |
| Hoover | S1 |
| LG | H1 |
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 Shinjuku water hard or soft?
It is moderately hard: 70 mg/L as CaCO₃ (4.1 gpg, 3.9 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Shinjuku 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 Shinjuku water?
4.1 gpg, equal to 3.9 °dH, 7 °fH or 70 mg/L calcium carbonate.
What dishwasher salt setting should I use in Shinjuku?
For 70 mg/L (3.9 °dH) check your manufacturer's table: level 2–3.
Do I need a water softener in Shinjuku?
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