Water hardness in Kumamoto
70 mg/L moderately hard
Tap water in Kumamoto is moderately hard: 70 mg/L (4.1 gpg · 3.9 °dH · 7 °fH).
Source: Kumamoto City Waterworks and Sewerage Bureau (熊本市上下水道局), tap-water quality results FY2025 (2025) · data checked 2026-10-21
Range across supply zones: 41–109 mg/L.
Kumamoto supplies its entire water from groundwater, the only city of over 500,000 people in Japan to do so; the aquifer is recharged in part by flooding rice fields in the middle Shirakawa basin below Aso's volcanic deposits. The reported figure is 70.2 mg/L CaCO₃ (range 41.0 to 109.0 mg/L, 2025 data); the spread is wide, so the value at your tap depends on where in the supply area you live. Deposits form on kettles, heaters and showerheads fairly quickly at this level, so regular descaling matters.
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 | Kumamoto City Waterworks and Sewerage Bureau (熊本市上下水道局) |
How Kumamoto compares
- Tap water in Kumamoto is 40 % harder than the median of the 228 cities we track in Japan (50 mg/L).
- By hardness it ranks 27 of 228 in Japan (1 = hardest); 89 % of these cities have softer water.
- Nearest city with clearly softer water: Kurume, 59 km away (45 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 Kumamoto
| 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 Kumamoto 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 Kumamoto 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 Kumamoto 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 Kumamoto?
For 70 mg/L (3.9 °dH) check your manufacturer's table: level 2–3.
Do I need a water softener in Kumamoto?
Not essential; a scale inhibitor protects the heater cheaply.
Source
Source: Kumamoto City Waterworks and Sewerage Bureau (熊本市上下水道局), tap-water quality results FY2025 (2025). Method: figure published by the water supplier. Kumamoto City Waterworks and Sewerage Bureau (熊本市上下水道局), tap-water quality results FY2025 · Outdated figure? Tell us