Seattle, WA water hardness

Name: Seattle, WA water hardness as published by Seattle Public Utilities
Creator: Seattle Public Utilities
License: https://www.usa.gov/government-works

Seattle Public Utilities does not publish one hardness figure for Seattle. It publishes 2 separate figures, one per source, in its Seattle Public Utilities Quarterly Surface Water Analysis, May 2026 (2nd Quarter 2026). Across them the published numbers run from 0.5 to 1.73 grains per gallon (8.6 to 29.6 mg/L at 17.1 mg/L per grain). On the USGS scale those figures are all soft water, so the classification does not depend on which source serves an address. Every published figure is in the table below.

Seattle Public Utilities published a figure for each of 2 sources rather than one for the system; the bar spans the lowest and highest of those published figures, 8.6 to 29.6 mg/L, converted from its 0.5 to 1.73 grains per gallon at 17.1 mg/L per grain. The plotted point is the midpoint of the lowest and highest figures published across this system's 2 areas; the utility publishes no system-wide figure, so this midpoint is not one of its numbers, converted at 17.1 mg/L per gpg. Classification bands from the USGS Water Science School.

Every figure Seattle Public Utilities publishes, by source

These are the only hardness numbers this utility publishes. There is no system-wide figure to quote, so the figure plotted above is a midpoint we calculated from the lowest and highest of them, and it is labelled as such everywhere it appears.

Source as published	mg/L (ppm)	Grains per gallon	USGS class
Cedar River supply (CLT-5 plant outlet and M-1A distribution) Distribution sample 1.55, plant outlet 1.73 grains/gallon.	26.5–29.6	1.55–1.73	Soft
Tolt River supply (TPT-3 plant outlet and B-2 distribution) Plant outlet 0.50, distribution sample 1.53 grains/gallon.	8.6–26.2	0.5–1.53	Soft

Figures in the grains column are the utility's own where it published them. Where it published only one unit, the other column is our conversion at 17.1 mg/L per grain, and the row above says which. Nothing in this table is averaged across sources.

The published figures, exactly as they appear

This is the hardness line transcribed from the source, with nothing added:

Cedar — CLT-5 Cedar Treatment Plant Outlet, M-1A Cedar Distribution: Hardness, Grains 1.73 1.55 Grains/gallon; Calcium Hardness 26.4 24.3 mg CaCO3/L. Tolt — TPT-3 Tolt Treatment Plant Outlet, B-2 Tolt Distribution: Hardness, Grains 0.50 1.53 Grains/gallon; Calcium Hardness 24.0 26.2 mg CaCO3/L
Seattle Public Utilities, Seattle Public Utilities Quarterly Surface Water Analysis, May 2026 (2nd Quarter 2026)

Units as published: grains/gallon (a separate “Calcium Hardness” row is given in mg CaCO3/L). What kind of figure it is: figures for each source, with no system-wide figure. Retrieved 2026-10-26.

Seattle hardness in mg/L and grains per gallon

Water-quality reports use mg/L (identical to ppm). Softener instructions and plumbing specifications use grains per gallon. Both are below. Where a figure is our arithmetic rather than the utility's own number, it says so.

Figure	mg/L (ppm)	Grains per gallon	Whose number
Low end of published span across areas	8.6	0.5	gpg published, mg/L is our conversion
High end of published span across areas	29.6	1.73	gpg published, mg/L is our conversion
Plotted on the scale above	19.1	1.1	midpoint of the lowest and highest figures published across this system's 2 areas; the utility publishes no system-wide figure, so this midpoint is not one of its numbers, converted at 17.1 mg/L per gpg; the gpg here is our conversion at 17.1, so it can differ slightly from a utility’s own rounded gpg figure

The conversion, both ways: gpg = mg/L ÷ 17.1 and mg/L = gpg × 17.1. Worked for this system: 19.1 ÷ 17.1 = 1.1 gpg. The converter does °dH and °f as well, and why the factor is 17.1 explains where the number comes from.

What this figure does not tell you

Service area and source water

Utility	Seattle Public Utilities
Service area as published	Seattle, which SPU serves from two supplies. Per SPU's analyses page, more than 80 percent of Seattle residences receive Cedar River water; Broadview, Bitter Lake, Crown Hill, Northgate, Haller Lake, Pinehurst and parts of Maple Leaf, Wedgewood, Greenwood, View Ridge and Roosevelt regularly receive Tolt water.
Source water	The Cedar River and the South Fork Tolt River.

Provenance of every number on this page

Source document	Seattle Public Utilities Quarterly Surface Water Analysis, May 2026 (2nd Quarter 2026)
Source URL	https://www.seattle.gov/documents/Departments/SPU/Services/Water/Analyses/SPU_Quarterly-Surface-Water-Analysis_05-20-2026.pdf
Published by	Seattle Public Utilities
Report year	2026
Year note	One sampling event: samples collected on May 19 and May 20, 2026. The document is labelled “Non-regulatory data. Informational only”.
Retrieved	2026-10-26
Classification bands	USGS Water Science School, retrieved 2026-10-16

Notes the report forces on this page

These figures are from one sampling event (May 19-20, 2026) at two points per supply, which SPU labels non-regulatory and informational only. They are not an annual average and not a range over the year.
SPU's 2025 Drinking Water Quality Report carries no hardness line; the quarterly surface water analyses are where SPU publishes one.
SPU's two units do not agree for the Tolt plant outlet. It prints “Hardness, Grains” of 0.50 grains/gallon (about 8.6 mg/L) beside a “Calcium Hardness” of 24.0 mg CaCO3/L, and calcium hardness alone cannot exceed total hardness; 24.0 mg/L is about 1.4 grains/gallon. Both are reproduced as published and neither is adjusted. The other three sampling points are closer: Cedar outlet 1.73 grains/gallon beside 26.4 mg/L calcium hardness, Cedar distribution 1.55 beside 24.3, Tolt distribution 1.53 beside 26.2.
Every published figure here is soft on the USGS scale, so the discrepancy does not change the classification.
The plotted point is the midpoint of the lowest and highest grains-per-gallon figures across the two supplies, converted at 17.1 mg/L per grain. It is not a figure SPU published.
Other report years checked on 2026-10-26: 2025 (no hardness line).

If you are sizing a softener for Seattle water

At 1.1 gpg, the sizing arithmetic starts from your household's water use, not from the hardness alone. The calculator below is pre-filled with this system's figure and every other input is an editable assumption.

Whether a softener is worth installing at this hardness is a judgement, not arithmetic, and it is not one this site makes for you. Nothing here has been tested by the author.

Where Seattle sits against other verified systems

Soft Soft 0–60

Moderately hard Moderate 61–120

Hard Hard 121–180

Very hard Very hard 181+

Portland, OR 9 mg/L 0.5 gpg Soft
Boston, MA 16 mg/L 1 gpg Soft
Manchester, NH 17.9 mg/L 1 gpg Soft
Seattle, WA 19.1 mg/L 1.1 gpg Soft
Raleigh, NC 25 mg/L 1.47 gpg Soft
New York, NY 32 mg/L 2 gpg Soft
Charlotte, NC 32 mg/L 1.9 gpg Soft
Memphis, TN 46 mg/L 2.7 gpg Soft
Norfolk, VA 48 mg/L 2.8 gpg Soft
Anchorage, AK 56 mg/L 3.3 gpg Soft
Virginia Beach, VA 56 mg/L 3.3 gpg Soft
Oakland, CA 68.4 mg/L 4 gpg Moderately hard
Nashville, TN 77.3 mg/L 4.5 gpg Moderately hard
Sacramento, CA 83 mg/L 4.9 gpg Moderately hard
Fresno, CA 89.3 mg/L 5.2 gpg Moderately hard
Saint Paul, MN 90 mg/L 5 gpg Moderately hard
Austin, TX 93 mg/L 5.4 gpg Moderately hard
Minneapolis, MN 94 mg/L 5.5 gpg Moderately hard
Washington, DC 95 mg/L 5.6 gpg Moderately hard
Baltimore, MD 97.5 mg/L 5.7 gpg Moderately hard
Birmingham, AL 98 mg/L 5.7 gpg Moderately hard
Kansas City, MO 100 mg/L 5.8 gpg Moderately hard
Denver, CO 105 mg/L 6.1 gpg Moderately hard
Detroit, MI 107 mg/L 6.3 gpg Moderately hard
Aurora, CO 113 mg/L 6.6 gpg Moderately hard
Tulsa, OK 114.5 mg/L 6.7 gpg Moderately hard
Louisville, KY 115 mg/L 6.7 gpg Moderately hard
Columbus, OH 120 mg/L 7 gpg Moderately hard
Cleveland, OH 120 mg/L 7 gpg Moderately hard
Philadelphia, PA 125 mg/L 7.3 gpg Hard
Ann Arbor, MI 126 mg/L 7.4 gpg Hard
Cincinnati, OH 133 mg/L 7.8 gpg Hard
Chicago, IL 134.5 mg/L 7.9 gpg Hard
Houston, TX 135.5 mg/L 7.9 gpg Hard
Milwaukee, WI 137 mg/L 8.01 gpg Hard
St. Louis, MO 138 mg/L 8.1 gpg Hard
Grand Rapids, MI 142 mg/L 8.3 gpg Hard
Fort Worth, TX 145 mg/L 8.5 gpg Hard
Spokane, WA 159 mg/L 9.3 gpg Hard
Honolulu, HI 162.5 mg/L 9.5 gpg Hard
Wichita, KS 170 mg/L 9.9 gpg Hard
Omaha, NE 170 mg/L 10 gpg Hard
Los Angeles, CA 192 mg/L 11.2 gpg Very hard
Lincoln, NE 192 mg/L 11.2 gpg Very hard
Riverside, CA 197 mg/L 12 gpg Very hard
Tampa, FL 220 mg/L 12.9 gpg Very hard
Phoenix, AZ 237 mg/L 13.9 gpg Very hard
Mesa, AZ 239.4 mg/L 14 gpg Very hard
Round Rock, TX 258 mg/L 15.1 gpg Very hard
Pflugerville, TX 267 mg/L 15.6 gpg Very hard
San Diego, CA 276 mg/L 16 gpg Very hard
Las Vegas, NV 280 mg/L 16 gpg Very hard
Chandler, AZ 282.2 mg/L 16.5 gpg Very hard
San Antonio, TX 299.3 mg/L 17.5 gpg Very hard
Anaheim, CA 300 mg/L 18 gpg Very hard
Henderson, NV 304 mg/L 18 gpg Very hard
Indianapolis, IN 317 mg/L 19 gpg Very hard
Madison, WI 324.9 mg/L 19 gpg Very hard
Scottsdale, AZ 352.5 mg/L 20.6 gpg Very hard
Salt Lake City, UT 353.5 mg/L 20.7 gpg Very hard

mg/L as calcium carbonate. Each classification band is drawn the same width, so the axis is not linear in mg/L.

Seattle, WA is the highlighted row. Every other system on this chart has its own page with its own published figure and report date. Grains per gallon are the utility's own figure where it published one, and our conversion at 17.1 mg/L per grain where it did not.

Systems with similar published hardness

Manchester, NH Manchester Water Works, 17.9 mg/L (1 gpg)
Boston, MA Massachusetts Water Resources Authority (MWRA), 16 mg/L (0.9 gpg)
Raleigh, NC Raleigh Water, 25 mg/L (1.5 gpg)
Portland, OR Portland Water Bureau, 9 mg/L (0.5 gpg)

All 60 verified systems How to find hardness in your own utility's report If you are on a well instead