Water hardness in Toshima
68 mg/L moderately hard
Tap water in Toshima is moderately hard: 68 mg/L (3.9 gpg · 3.8 °dH · 6.8 °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: 63–78 mg/L.
Toshima is supplied by the Tokyo Metropolitan Waterworks Bureau, which reports a hardness of 67.6 mg/L in its FY2025 results. The range is 62.5 to 78.1 mg/L, so it falls in the moderate category. The figure is the utility's reported value, not a measurement at individual homes.
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 | 68 mg/L |
|---|---|
| Utility | Tokyo Metropolitan Waterworks Bureau (東京都水道局) |
How Toshima compares
- Tap water in Toshima is 35 % harder than the median of the 228 cities we track in Japan (50 mg/L).
- By hardness it ranks 39 of 228 in Japan (1 = hardest); 83 % of these cities have softer water.
- In Tokyo it is number 11 of 53 cities; the regional median is 62 mg/L.
- Nearest city with clearly softer water: Higashiyamato, 26 km away (48 mg/L).
- Nearest city with clearly harder water: Ichihara, 41 km away (86 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 Toshima
| 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 Toshima water hard or soft?
It is moderately hard: 68 mg/L as CaCO₃ (3.9 gpg, 3.8 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Toshima 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 Toshima water?
3.9 gpg, equal to 3.8 °dH, 6.8 °fH or 68 mg/L calcium carbonate.
What dishwasher salt setting should I use in Toshima?
For 68 mg/L (3.8 °dH) check your manufacturer's table: level 2–3.
Do I need a water softener in Toshima?
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