Washington Water Hardness by County
Water in Washington has a median hardness of 32.4 mg/L as CaCO₃ — "soft" on the USGS scale — based on 78,339 measurements at 7,030 monitoring stations. Island County has the state's hardest measured water (190 mg/L); Cowlitz County the softest (17). Rank: #43 of the 50 states and D.C.
Washington water hardness map — what the numbers mean. Data verified 2026-07-22.
Of Washington's 37 measured counties, 1 very hard, 4 hard, 12 moderately hard, and 20 soft. The hardest are Island County (190 mg/L), San Juan County (166.5), and Grant County (160). Cowlitz County is the softest at 17 mg/L (soft).
About 4% of Washington's roughly 7.7 million residents in counties with gate-passing hardness data live where tap water is hard or worse.
Statewide, Washington's median works out to 1.9 grains per gallon and ranks #43 of the 50 states and D.C. Across 78,339 measurements at 7,030 stations — 35,241 of them since 2000 — the middle 80% of readings fall between 12.4 and 128 mg/L. The public record runs from 1916 to 2025.
See where Washington sits nationally on the national map, or what its number means on the hardness scale.
Download the Washington water hardness map (PNG) — Water hardness by county in Washington: 37 counties mapped from USGS/EPA data, statewide median 32.4 mg/L (soft); darker counties have harder water.
By county
Water hardness by county
The table lists all 37 measured Washington counties, hardest first. Their medians span 190 mg/L in Island County to 17 in Cowlitz County — a 11.2× range. 2 more Washington counties lack gate-passing data.
| County | Median | Band | Results | Sites | Newest |
|---|---|---|---|---|---|
| Island County | 190 | Very hard | 150 | 115 | 2014 |
| San Juan County | 166.5 | Hard | 92 | 81 | 2000 |
| Grant County | 160 | Hard | 1,582 | 387 | 2023 |
| Lincoln County | 126 | Hard | 332 | 99 | 2022 |
| Franklin County | 120 | Hard | 817 | 225 | 2023 |
| Adams County | 110 | Moderately hard | 311 | 134 | 2023 |
| Whitman County | 100 | Moderately hard | 1,061 | 140 | 2016 |
| Benton County | 94 | Moderately hard | 1,520 | 286 | 2023 |
| Ferry County | 93 | Moderately hard | 363 | 69 | 2014 |
| Okanogan County | 88 | Moderately hard | 1,417 | 183 | 2023 |
| Spokane County | 88 | Moderately hard | 1,497 | 231 | 2016 |
| Garfield County | 81 | Moderately hard | 59 | 15 | 2016 |
| Pend Oreille County | 79 | Moderately hard | 192 | 25 | 1999 |
| Douglas County | 78.7 | Moderately hard | 115 | 45 | 2016 |
| Yakima County | 74.9 | Moderately hard | 2,607 | 485 | 2023 |
| Stevens County | 73.6 | Moderately hard | 1,311 | 137 | 2012 |
| Kitsap County | 63 | Moderately hard | 542 | 394 | 2021 |
| Walla Walla County | 58.2 | Soft | 788 | 89 | 2025 |
| Asotin County | 57.5 | Soft | 56 | 17 | 2016 |
| Clark County | 56 | Soft | 372 | 138 | 2024 |
| Thurston County | 43 | Soft | 827 | 465 | 2016 |
| Pierce County | 40.2 | Soft | 1,809 | 443 | 2016 |
| King County | 39.9 | Soft | 2,201 | 558 | 2017 |
| Kittitas County | 39 | Soft | 707 | 101 | 2016 |
| Clallam County | 38 | Soft | 373 | 63 | 2008 |
| Mason County | 34.4 | Soft | 463 | 123 | 2023 |
| Klickitat County | 34 | Soft | 433 | 172 | 2016 |
| Whatcom County | 32 | Soft | 2,477 | 435 | 2025 |
| Jefferson County | 31 | Soft | 529 | 40 | 2009 |
| Snohomish County | 26 | Soft | 48,503 | 548 | 2016 |
| Skagit County | 24 | Soft | 947 | 133 | 2015 |
| Skamania County | 23 | Soft | 329 | 48 | 2009 |
| Chelan County | 22 | Soft | 322 | 50 | 2010 |
| Lewis County | 21 | Soft | 761 | 248 | 2015 |
| Pacific County | 21 | Soft | 218 | 78 | 2010 |
| Wahkiakum County | 18 | Soft | 70 | 7 | 2001 |
| Cowlitz County | 17 | Soft | 1,026 | 125 | 2012 |
| 4 additional results from sites without county attribution (not shown). | |||||
ZIP codes
ZIP codes in Washington
The ZIP codes below are grouped by the Washington county each mostly falls within (by land area). Each ZIP takes that county's measured median — shown in the group header — so this is a ZIP-to-county guide, not a separate reading for every ZIP. To check the water system serving a specific address, use the lookup.
Key: a † after a ZIP means it has no mapped community water system — usually a private-well area. Plain ZIPs are served by a mapped community water system.
Island County · 190 mg/L · Very hard
98236†9823998249†98253†98260982779827898282†
San Juan County · 166.5 mg/L · Hard
98222†98243†98245†98250†98261†9827998280†98286†98297†
Grant County · 160 mg/L · Hard
98823†98824†98832†9883798848†98851†98853†98857†98860†99123†99135†99321†99349†99357†
Lincoln County · 126 mg/L · Hard
99008†99029†99032†99103†99117†99122†99134†99144†99147†99154†99159†99185†
Franklin County · 120 mg/L · Hard
99301†99326†99330†99335†99343†
Adams County · 110 mg/L · Moderately hard
99105†9916999341†9934499371†
Whitman County · 100 mg/L · Moderately hard
99017†99033†9910299111†99113†99125†99128†99130†99136†99143†99149†9915899161†99163†99170†99171†99174†99176†99179†99333†
Benton County · 94 mg/L · Moderately hard
98944†99320†9933699337†99338†99345†99346†99350†993529935399354
Ferry County · 93 mg/L · Moderately hard
99118†99121†99138†99140†99141†99146†99150†99160†99166†
Okanogan County · 88 mg/L · Moderately hard
98812†98814†98819†98827†98829†98833†98834†98840†98841†98844†98846†98849†98855†98856†98859†98862†99116†9912499155†
Spokane County · 88 mg/L · Moderately hard
9900199003†99004†9900599006†99009†9901199012†990149901699018†990199902099021†9902299023†99025990269902799030†99031†99036†990379903999201992029920399204992059920699207992089921299216†992179921899223992249925199258
Garfield County · 81 mg/L · Moderately hard
99347†
Pend Oreille County · 79 mg/L · Moderately hard
99119†99139†99152†99153†99156†99180†
Douglas County · 78.7 mg/L · Moderately hard
98802†98813†98830†98843†98845†98850†98858†99115†99133†
Yakima County · 74.9 mg/L · Moderately hard
98901†9890298903†98908†989219892398930†98932†98933†98935†98936†98937†98938†98939†98942†98947†98948†98951†98952†98953†
Stevens County · 73.6 mg/L · Moderately hard
99013†99034†99040†99101†99109†99110†99114†99126†99129†99131†99137†99148†9915199157†99167†99173†99181†
Kitsap County · 63 mg/L · Moderately hard
98110983109831198312†9831498315983379834098342983459834698353†98359983649836698367†98370†98380†9838398392
Walla Walla County · 58.2 mg/L · Soft
99323†9932499329†99348†99360†99361†99362†99363†
Asotin County · 57.5 mg/L · Soft
99401†99402†99403†
Clark County · 56 mg/L · Soft
986019860498606986079862998642986609866198662986639866498665986759868298683986849868598686
Thurston County · 43 mg/L · Soft
9850198502†985039850598506†98512†985139851698530†9855698576†98579†98589†98597†
Pierce County · 40.2 mg/L · Soft
98303†98304†9832198323†9832798328†9832998330†983329833398335†983389834998351†9835498360†9837198372983739837498375983859838798388983909839198394†98396†9840298403984049840598406984079840898409984169841898421984229842498430984339843898439984439844498445984469844798465984669846798498984999855898580†
King County · 39.9 mg/L · Soft
9800198002980039800498005980069800798008980109801198014†9801998022†98023980249802598027†980289802998030980319803298033980349803898039980409804298045980479805098051†9805298053980559805698057980589805998065†98070†980729807498075980779809298101981029810398104981059810698107981089810998112981159811698117981189811998121981229812598126981339813498136981449814698148981549815598158†9816498166981689817498177981789818898195981989819998224†98288†
Kittitas County · 39 mg/L · Soft
98068†98922†98925†98926†9893498940†989419894398946†98950†
Clallam County · 38 mg/L · Soft
98305†98326†98350†9835798362†98363†98381†98382†
Mason County · 34.4 mg/L · Soft
98524†98528†98546†98548†98555†98560†9858498588†98592†
Klickitat County · 34 mg/L · Soft
98602†98613†98617†98619†9862098623†98628†98635†98650†98670†98672†98673†99322†99356†
Whatcom County · 32 mg/L · Soft
98220†9822598226†98229†9823098240†98244†982479824898262†9826498266†982769828198283†98295†
Jefferson County · 31 mg/L · Soft
98320†98325†98331†98339†98358†98365†98368†98376†
Snohomish County · 26 mg/L · Soft
9801298020980219802698036980379804398087982019820398204982079820898223†98241†98251†98252†98256†982589827098271†9827298275982909829298294†98296
Skagit County · 24 mg/L · Soft
9822198232982339823598237†982389825598257†9826398267†9827398274†98284†
Skamania County · 23 mg/L · Soft
98605†98610†98616†9863998648†9865198671
Chelan County · 22 mg/L · Soft
9880198811†98815†98816†98817†98821†98822†98826†98828†98831†98836†98847†98852†
Lewis County · 21 mg/L · Soft
98336†98355†98356†98361†98377†98531†9853298533†98538†98539†98542†98544†98564†9856598570†98572†98582†98585†98591†98593†98596†
Pacific County · 21 mg/L · Soft
98527†98547†98577†98586†98590†98614†98621†98624†9863198638†9864098641†98644
Wahkiakum County · 18 mg/L · Soft
98612†9864398647†
Cowlitz County · 17 mg/L · Soft
98581†98603†9860998611†98625†98626†98632†98645†98649†98674†
† 321 ZIP codes are not served by a mapped community water system — most are private-well areas. See well water hardness.
Source
Groundwater vs surface water
Washington's groundwater measures harder than its surface water — 95 mg/L across 9,267 samples versus 29 across 68,438, about 3.3× harder. Wells draw from groundwater, so well owners in Washington tend to see the harder number — 95 mg/L, moderately hard. Groundwater readings there run 35.7 to 250 mg/L across the middle 80% of samples. The comparison draws on 5,402 groundwater and 1,568 surface-water monitoring sites; 634 readings above 2,000 mg/L were logged but held out of the medians as outliers.
Groundwater (wells)
95 mg/L as CaCO₃ · 9,267 results at 5,402 stations.
Surface water (rivers, lakes)
29 mg/L as CaCO₃ · 68,438 results at 1,568 stations.
Groundwater measures about 3.3× harder than surface water here.
Chemistry
Water chemistry in Washington
Beyond the hardness number, Washington's water carries a fuller chemical signature — here is how its measured parameters compare to national benchmarks.
TDS
170 mg/L · n=1,925
170 mg/L — below the national county median of 226.
pH
7.6 pH · n=9,692
pH 7.6 — within the EPA secondary range of 6.5–8.5, on the alkaline side.
Alkalinity
79.5 mg/L as CaCO3 · n=5,097
79.5 mg/L — modest buffering capacity.
Calcium : Magnesium
20.7 : 8.2 mg/L ≈ 2.52 : 1 · n=3,745
Calcium is about 60% of Washington's hardness (20.7 mg/L Ca to 8.2 mg/L Mg), a little more magnesium-rich than the national ~68% calcium share.
Parameter medians may include a small share of quality-control samples; the hardness lane is fully QC-filtered.
Aquifers
Major aquifers under Washington
3 aquifer systems supply much of Washington's measured groundwater.
- Columbia Plateau basin-fill aquifers — median 240 mg/L across 398 samples at 141 sites: very hard.
- Columbia Plateau basaltic-rock aquifers — median 137.5 mg/L across 250 samples at 110 sites: hard.
- Puget Sound aquifer system — median 56.8 mg/L across 185 samples at 86 sites: soft.
Together these systems account for 833 measured samples at 337 sites. Aquifer medians pool samples across a whole system and can mask local variation — see the national aquifer map for how these compare across states.
Trend
Hardness over time
Washington's decade median moved from 83 mg/L in the 1940s to 109 in the 2020s. Over the series, the decade median peaked at 109 mg/L in the 2020s and bottomed at 26.1 in the 1990s. Decade medians reflect where and what agencies sampled in each era, not just changes in the water itself.
Scale
Where Washington sits on the scale
On the USGS scale, Washington's 32.4 mg/L is soft — 1.9 grains per gallon, below the 60 mg/L soft cutoff — among the softest water measured in the US.
Questions
Frequently asked questions
- Is Washington's water hard?
- Washington's median hardness is 32.4 mg/L as CaCO₃, "soft" on the USGS scale and below the national county median of 140 mg/L.
- Which county in Washington has the hardest water?
- Island County, with a median of 190 mg/L as CaCO₃ across 150 measurements.
- Is well water harder than city water in Washington?
- Groundwater measures harder here — about 3.3× the surface water — so wells tend to run harder (groundwater 95 vs surface water 29 mg/L).
- How do I find my exact hardness in Washington?
- Search your ZIP on the lookup tool, read your utility's Consumer Confidence Report, or test at home with a kit.
- Is Washington's water harder than the national average?
- No. Washington's median of 32.4 mg/L is below the national county median of 140 mg/L; it ranks #43 of the 50 states and D.C.
Compare
States with similar hardness
Two states measure closest to Washington's 32.4 mg/L: Hawaii at 37.8 mg/L and North Carolina at 27 mg/L. Washington measures harder than 16% of states. Browse the full ranking on the statistics page.
More: hardness scale · check your address · national statistics · the national map.
By Marko Visic, MPharm, BSc Physics. Source: USGS/EPA Water Quality Portal — 78,339 Washington measurements. Data last verified . Methodology.
Cite this page: “Washington Water Hardness by County.” Hard Water Map. Retrieved 2026-07-22, https://hardwatermap.com/water-hardness/washington/