Water hardness in Otsu
37 mg/L soft
Tap water in Otsu is soft: 37 mg/L (2.2 gpg · 2.1 °dH · 3.7 °fH).
Source: Otsu City Public Enterprise Bureau, seasonal water quality standard tests FY2025 (2025) · data checked 2026-10-21
Range across supply zones: 31–42 mg/L.
Otsu's tap water is supplied by Otsu City Public Enterprise Bureau; the intake mix is not confirmed here, but piped water in Shiga Prefecture comes mainly from rivers and reservoirs fed by short, steep catchments. The reported figure is 37.0 mg/L CaCO₃ (range 31.0 to 42.0 mg/L, 2025 data). Scale builds slowly at this level; a light white film on kettles and taps is possible, and a softener is rarely needed.
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 | 37 mg/L |
|---|---|
| Utility | Otsu City Public Enterprise Bureau |
How Otsu compares
- Tap water in Otsu is 26 % softer than the median of the 228 cities we track in Japan (50 mg/L).
- By hardness it ranks 184 of 228 in Japan (1 = hardest); 19 % of these cities have softer water.
- Nearest city with clearly softer water: Tondabayashi, 61 km away (17 mg/L).
- Nearest city with clearly harder water: Shimamoto, 22 km away (83 mg/L).
Ranking of the hardest and softest water in Japan →
What it means at home
- Little or no limescale in kettles, water heaters or washing machines.
- Soap lathers easily; water does not dry skin or hair.
- Dishwasher: salt setting at minimum or off.
- No softener needed; a jug filter only changes taste.
How much salt will you use for softening?
No. Water this soft does not need softening.
Descale a coffee machine or kettle about every 6 months (typical manufacturer guidance for this hardness).
Estimates for planning, not a quote. Check your appliance manual for exact settings.
Dishwasher setting in Otsu
| Brand | Setting for this water |
|---|---|
| Bosch | H00 |
| Miele | 2 °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 Otsu water hard or soft?
It is soft: 37 mg/L as CaCO₃ (2.2 gpg, 2.1 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Otsu 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 Otsu water?
2.2 gpg, equal to 2.1 °dH, 3.7 °fH or 37 mg/L calcium carbonate.
What dishwasher salt setting should I use in Otsu?
For 37 mg/L (2.1 °dH) check your manufacturer's table: level 0–1, usually no salt.
Do I need a water softener in Otsu?
No. With soft water a softener adds nothing.
Source
Source: Otsu City Public Enterprise Bureau, seasonal water quality standard tests FY2025 (2025). Method: figure published by the water supplier. Otsu City Public Enterprise Bureau, seasonal water quality standard tests FY2025 · Outdated figure? Tell us