Water hardness in Ome
48 mg/L soft
Tap water in Ome is soft: 48 mg/L (2.8 gpg · 2.7 °dH · 4.8 °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: 26–57 mg/L.
Ome 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. The utility's 2025 data give 47.7 mg/L CaCO₃ within a range of 25.9 to 56.7 mg/L. 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 | 48 mg/L |
|---|---|
| Utility | Tokyo Metropolitan Waterworks Bureau (東京都水道局) |
How Ome compares
- Tap water in Ome is 5 % softer than the median of the 228 cities we track in Japan (50 mg/L).
- By hardness it ranks 122 of 228 in Japan (1 = hardest); 46 % of these cities have softer water.
- In Tokyo it is number 49 of 53 cities; the regional median is 62 mg/L.
- Nearest city with clearly softer water: Okutama, 17 km away (34 mg/L).
- Nearest city with clearly harder water: Mizuho, 10 km away (63 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 Ome
| 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 Ome water hard or soft?
It is soft: 48 mg/L as CaCO₃ (2.8 gpg, 2.7 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Ome 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 Ome water?
2.8 gpg, equal to 2.7 °dH, 4.8 °fH or 48 mg/L calcium carbonate.
What dishwasher salt setting should I use in Ome?
For 48 mg/L (2.7 °dH) check your manufacturer's table: level 0–1, usually no salt.
Do I need a water softener in Ome?
No. With soft water a softener adds nothing.
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