Seattle
Seattle, United States has an estimated water hardness of 24 ppm (mg/L as calcium carbonate), which is classed as soft. Low scale risk. Use the lower end of detergent guidance.
Seattle water hardness in every unit
| Unit | Average |
|---|---|
| mg/L or ppm (CaCO₃) | 24 |
| mmol/L | 0.24 |
| German degrees (°dH) | 1.34 |
| French degrees (°fH) | 2.4 |
| Clark degrees (°e) | 1.68 |
| Grains per US gallon (gpg) | 1.4 |
Published range: 21–26 ppm.
Where Seattle's water comes from
- Supplier: Seattle Public Utilities (SPU)
- Source: The Cedar River and Tolt River supplies, each with its own treatment plant
SPU publishes quarterly analyses of both supplies. Values are very low, so most households have little need for hardness-related appliance settings. SPU quarterly surface water analysis (February 2026) (checked 2026-10-05).
How Seattle compares
With an average of 24 ppm, Seattle ranks 18th hardest of the 19 cities in our directory, 42 ppm below the directory median of 66 ppm. Among the cities in our directory in North America, Calgary is the hardest at 200 ppm and Vancouver is the softest at 5 ppm.
What the range means
Published figures for Seattle run from 21 to 26 ppm. The spread is relatively narrow (5 ppm), so the average is a reasonable guide, though your own tap can still differ from the citywide figure.
What soft water means at home
Soft water rarely leaves visible scale. Kettles, taps and shower screens stay clearer for longer, and detergents lather easily, so the main risk is overdosing. Start at the minimum amount on the pack and increase only if results are poor.
Appliance settings for 24 ppm water
- Dishwasher: salt no; lowest hardness band. Salt is usually unnecessary unless your manual says otherwise.
- Coffee machine: descale every 3–4 months; filter: usually optional.
- Laundry: use the soft-water dose. Start at the label minimum and adjust for soil level.
- Kettle: descale inspect every 3 months.
- Humidifier: tap water is usually suitable; clean weekly.
Frequently asked questions
How hard is the water in Seattle?
The estimate for Seattle, United States is about 24 mg/L as calcium carbonate (1.34 °dH, 2.4 °fH, 1.4 gpg), with a published range of 21–26 mg/L. That is classed as soft water. Single quarterly sample (24 Feb 2026) of the Cedar and Tolt supplies; varies by season.
Do I need dishwasher salt in Seattle?
Usually not. Salt is usually unnecessary unless your manual says otherwise. Always follow your dishwasher manual.
How often should I descale a coffee machine in Seattle?
At this hardness, descale every 3–4 months. Water filter: usually optional.
Do I need a water softener in Seattle?
Almost certainly not. Soft water does not usually justify a softener.
Is hard water in Seattle safe to drink?
Water hardness is mainly an aesthetic and scale issue. The World Health Organization does not set a health-based guideline value for it. Hardness affects taste, scale and detergent use rather than drinking safety, and your utility is responsible for meeting drinking-water standards. If you have a specific health or dietary concern, ask your doctor.
Who supplies tap water in Seattle?
Seattle Public Utilities (SPU). Source: The Cedar River and Tolt River supplies, each with its own treatment plant.
How to check your own water
City averages are a starting point. To get your actual figure, look for the latest water-quality report from Seattle Public Utilities quarterly surface water analysis or your local supplier, or dip a hardness test strip in a glass of cold tap water and read it against the chart. Strips usually report in °dH, gpg or ppm; enter the number into the Waterdial checker to convert it and see the matching appliance settings. A manual reading always beats a city estimate.
Source
Seattle Public Utilities quarterly surface water analysis, checked 2026-10-05. Guidance is general; your appliance manual takes priority.
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