Water hardness in Seattle
1.5 gpg soft
Tap water in Seattle is soft: 1.5 gpg (26 mg/L · 1.5 °dH · 2.6 °fH).
Source: SPU Quarterly Surface Water Analysis, samples 19-20 May 2026 (2026-05) · data checked 2026-10-22
2 supply zones — hardness varies by neighbourhood; the figure above is the median.
Hardness by district / supply zone
| gpg | mg/L | ||
|---|---|---|---|
| Cedar supply (distribution M-1A, S. Stacy St. & Utah Ave. S.) | 1.5 | 27 | soft |
| Tolt supply (distribution B-2, NW 122nd St. & 1st Ave. NW) | 1.5 | 26 | soft |
Seattle's water comes from two Cascade Range watersheds, the Cedar River and the South Fork Tolt River, both on largely protected forest land. Rain and snowmelt pick up very few minerals on the way, so hardness is only 26.4 mg/L CaCO₃. The two sources differ a little by neighborhood, but scale is rarely an issue and a softener adds little.
USGS classes: soft 0–60, moderately hard 61–120, hard 121–180, very hard >180 mg/L.
Water data
| Total hardness | 1.5 gpg |
|---|---|
| Utility | Seattle Public Utilities (SPU) |
How Seattle compares
- Tap water in Seattle is 83 % softer than the median of the 1372 cities we track in the United States (9.1 gpg).
- By hardness it ranks 1297 of 1372 in the United States (1 = hardest); 5 % of these cities have softer water.
- In Washington it is number 20 of 27 cities; the regional median is 2.8 gpg.
- Nearest city with clearly softer water: Redmond, 17 km away (1.2 gpg).
- Nearest city with clearly harder water: Renton, 16 km away (4 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 Seattle
| Brand | Setting for this water |
|---|---|
| Bosch | H00 |
| Miele | 1 °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 Seattle water hard or soft?
It is soft: 26 mg/L as CaCO₃ (1.5 gpg, 1.5 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Seattle 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 Seattle water?
1.5 gpg, equal to 1.5 °dH, 2.6 °fH or 26 mg/L calcium carbonate.
What dishwasher salt setting should I use in Seattle?
For 26 mg/L (1.5 °dH) check your manufacturer's table: level 0–1, usually no salt.
Do I need a water softener in Seattle?
No. With soft water a softener adds nothing.
Source
Source: SPU Quarterly Surface Water Analysis, samples 19-20 May 2026 (2026-05). Method: own measurement. SPU Quarterly Surface Water Analysis, samples 19-20 May 2026 · Outdated figure? Tell us