Water hardness in Mercer Island
1.6 gpg soft
Tap water in Mercer Island is soft: 1.6 gpg (28 mg/L · 1.6 °dH · 2.8 °fH).
Source: Seattle Public Utilities Quarterly Surface Water Analysis 2026-05-20 (2026) · data checked 2026-11-02
Range across supply zones: 27–30 mg/L.
Mercer Island is supplied with Seattle Public Utilities water from the Cedar River supply. SPU's May 2026 analysis gives a computed hardness of 28.1 mg/L as CaCO3, with a narrow range of 26.5 to 29.6 mg/L. This is soft water, so scale is minimal.
USGS classes: soft 0–60, moderately hard 61–120, hard 121–180, very hard >180 mg/L.
Water data
| Total hardness | 1.6 gpg |
|---|---|
| calcium_mg_L | 9.63 |
| magnesium_mg_L | 1.33 |
| Utility | City of Mercer Island (SPU Cedar River supply) |
How Mercer Island compares
- Tap water in Mercer Island is 82 % softer than the median of the 1372 cities we track in the United States (9.1 gpg).
- By hardness it ranks 1286 of 1372 in the United States (1 = hardest); 6 % of these cities have softer water.
- In Washington it is number 18 of 27 cities; the regional median is 2.8 gpg.
Ranking of the hardest and softest water in the United States →
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 Mercer Island
| 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 Mercer Island water hard or soft?
It is soft: 28 mg/L as CaCO₃ (1.6 gpg, 1.6 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Mercer Island safe to drink?
Hardness is not a health issue; it is about limescale, taste and appliances. Mains water must meet national drinking-water standards. USGS classes: soft 0–60, moderately hard 61–120, hard 121–180, very hard >180 mg/L.
How many grains per gallon is Mercer Island water?
1.6 gpg, equal to 1.6 °dH, 2.8 °fH or 28 mg/L calcium carbonate.
What dishwasher salt setting should I use in Mercer Island?
For 28 mg/L (1.6 °dH) check your manufacturer's table: level 0–1, usually no salt.
Do I need a water softener in Mercer Island?
No. With soft water a softener adds nothing.
Source
Source: Seattle Public Utilities Quarterly Surface Water Analysis 2026-05-20 (2026). Method: computed from supplier's calcium and magnesium. Seattle Public Utilities Quarterly Surface Water Analysis 2026-05-20 · Outdated figure? Tell us