Hard Water Map

Illinois Water Hardness by County

Water in Illinois has a median hardness of 285 mg/L as CaCO₃ — "very hard" on the USGS scale — based on 77,308 measurements at 4,491 monitoring stations. Saline County has the state's hardest measured water (449 mg/L); Pope County the softest (51). Rank: #5 of the 50 states and D.C.

Illinois water hardness map — what the numbers mean. Data verified 2026-07-22.

Of Illinois's 102 measured counties, 87 very hard, 13 hard, 1 moderately hard, and 1 soft. The hardest are Saline County (449 mg/L), Fulton County (390), and Kendall County (388.5). Pope County is the softest at 51 mg/L (soft).

About 100% of Illinois's roughly 12.7 million residents in counties with gate-passing hardness data live where tap water is hard or worse.

Statewide, Illinois's median works out to 16.6 grains per gallon and ranks #5 of the 50 states and D.C. Across 77,308 measurements at 4,491 stations — 42,890 of them since 2000 — the middle 80% of readings fall between 130 and 400 mg/L. The public record runs from 1901 to 2023.

See where Illinois sits nationally on the national map, or what its number means on the hardness scale.

Download the Illinois water hardness map (PNG) — Water hardness by county in Illinois: 102 counties mapped from USGS/EPA data, statewide median 285 mg/L (very hard); darker counties have harder water.

By county

Water hardness by county

The table lists all 102 measured Illinois counties, hardest first. Their medians span 449 mg/L in Saline County to 51 in Pope County — a 8.8× range.

County water hardness, hardest first — median mg/L as CaCO₃.
CountyMedianBandResultsSitesNewest
Saline County449Very hard1,210502022
Fulton County390Very hard1,6391192022
Kendall County388.5Very hard384242022
Boone County377Very hard409402022
Henry County377Very hard404232022
Jo Daviess County370Very hard748362022
Cass County368Very hard224112018
Stark County364Very hard637202022
Carroll County360Very hard18772017
Lee County357Very hard75172019
Ford County355Very hard4252021
Stephenson County351Very hard601322022
DeKalb County350Very hard306302022
Logan County350Very hard955272022
Winnebago County350Very hard1,356652022
Whiteside County349Very hard1,030222022
Knox County343Very hard409472020
Livingston County342Very hard565572022
Woodford County342Very hard126232020
Tazewell County341Very hard1,003352022
Bureau County340Very hard8871232022
McHenry County340Very hard1,6091462022
Moultrie County340Very hard817122022
Ogle County340Very hard876872022
Grundy County338Very hard769262023
McLean County329.5Very hard328632020
Iroquois County320Very hard909552022
LaSalle County319Very hard1,747572022
Henderson County318Very hard355142022
Kankakee County314Very hard908362022
Piatt County313Very hard482262023
Vermilion County313Very hard2,002872022
Kane County311Very hard1,0191012022
Rock Island County311Very hard489142022
Mason County310Very hard938162023
Menard County310Very hard362132018
Peoria County310Very hard950302022
Champaign County307Very hard1,133832022
Coles County303.5Very hard548292022
De Witt County302.5Very hard670352022
Warren County300Very hard194482020
Will County293Very hard2,7401462022
Cumberland County290Very hard455122022
Douglas County290Very hard698212022
DuPage County290Very hard1,6181172022
Lake County290Very hard2,7532742023
Mercer County290Very hard336132022
St. Clair County289Very hard1,1061202022
Putnam County287Very hard27782022
Schuyler County286Very hard467212022
Gallatin County284Very hard527252022
Marshall County284Very hard280122022
Scott County283.5Very hard956152022
Sangamon County282.5Very hard1,414452022
Calhoun County280Very hard31742022
Edgar County277.5Very hard254162021
Madison County273Very hard2,1061142023
Greene County272Very hard582212022
Clark County270Very hard5162021
Crawford County270Very hard1,120312022
Macon County270Very hard864352022
Brown County260Very hard15962021
Cook County260Very hard4,7045922022
Christian County258Very hard955382022
Jasper County258Very hard428132022
Wabash County256Very hard1442021
Perry County254Very hard162492021
McDonough County248Very hard604492022
Fayette County246Very hard1,272232022
Effingham County239Very hard430292022
Monroe County236Very hard137182022
Pike County234Very hard777302023
Lawrence County231Very hard357132022
Shelby County228Very hard942232022
Adams County223Very hard473152023
Hancock County223Very hard174142022
Hamilton County221Very hard50152021
Randolph County217Very hard1,149852022
Jersey County215Very hard42392023
Alexander County209Very hard891232023
Morgan County206Very hard79142022
Clay County200Very hard424162022
Montgomery County200Very hard421252022
Macoupin County190Very hard381372022
Jefferson County186Very hard1,003172022
Edwards County184Very hard51082022
Williamson County180Very hard1,059632022
Clinton County176Hard1,150342022
Washington County170Hard649242022
Jackson County167Hard1,866542022
Bond County166Hard73102022
Union County166Hard187522019
White County165.5Hard702162022
Marion County160Hard985332022
Hardin County159Hard42112020
Wayne County157Hard748272022
Richland County146Hard456172022
Pulaski County126Hard1,169322023
Massac County123Hard55172020
Franklin County120Hard1,044382022
Johnson County89.7Moderately hard437312022
Pope County51Soft544202022

ZIP codes

ZIP codes in Illinois

The ZIP codes below are grouped by the Illinois 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.

Saline County · 449 mg/L · Very hard

62917†62930†62935†62946†6296562977†

Fulton County · 390 mg/L · Very hard

61415†6142761431†61432†61433†61441†61459†61477†61482†61484†61501†6151961520†61524†61531†61542†61543†61544†61553†61563

Kendall County · 388.5 mg/L · Very hard

60447†60512†60536†6053860541†6054360545†60560

Boone County · 377 mg/L · Very hard

6100861011†61012†61038†61065†

Henry County · 377 mg/L · Very hard

6123361234†61235†61238†612416125461258†61262†61273†61274†61413†614196143461443†61468†61490†

Jo Daviess County · 370 mg/L · Very hard

61001†61025†61028†61036†61041†61059†61075†61085†61087†

Cass County · 368 mg/L · Very hard

62611†62612†62618†62622†62627†62691†

Stark County · 364 mg/L · Very hard

61421†61426†61449†61483†61491†

Carroll County · 360 mg/L · Very hard

61014†61046†61051†61053†61074†61078†61285†

Lee County · 357 mg/L · Very hard

60530†60553†61006†61021†61031†61042†61057†61310†61318†61324†61331†61353†61367†61378†

Ford County · 355 mg/L · Very hard

60919†6093360936†60946†60952†60957†60959†60962†61773†

Stephenson County · 351 mg/L · Very hard

6101361018†61019†61027†6103261039†61044†61048†61050†61060†61062†61067†6107061089†

DeKalb County · 350 mg/L · Very hard

60111†601126011560135†60145†60146†60150†60178†60520†6054860550†60552†60556

Logan County · 350 mg/L · Very hard

61723†61751†62512†62518†62519†62541†62543†62548†62634†62635†62643†6265662666†62671†

Winnebago County · 350 mg/L · Very hard

61016†61024†61063†61072†61073†61077†61079†61080†61088†61101†61102†611036110461107611086110961111611126111461115

Whiteside County · 349 mg/L · Very hard

61037†61071†61081†61230†61243†61250†61251†61252†61261†61270†61277†61283†

Knox County · 343 mg/L · Very hard

614016140261410†61414†61428†6143061436†61439†61448†61458†61467†61472†61474†61485†61488†61489†61572†

Livingston County · 342 mg/L · Very hard

6042060460†60920†60921†60929†60934†61311†61313†61319†61333†61739†61740†61741†61743†6176461769†61775†

Woodford County · 342 mg/L · Very hard

61516†6153061545†61548†61561†61570†61725†61729†61738†61742†61760†61771†

Tazewell County · 341 mg/L · Very hard

61534†61535†61546†615506155461564†61568†61571616106161161721†617336173461747†61755†61759†

Bureau County · 340 mg/L · Very hard

61312†61314†61315†61317†6132061322†6132361328†61329†61330†61337†61338†61344†61345†61346†61349†61356†6135961361†61362†61368†61374†61376†61379†

McHenry County · 340 mg/L · Very hard

60012†6001360014600216003360034†6005060051†6007160072†60081†60097†600986010260142†60152†6015660180†

Moultrie County · 340 mg/L · Very hard

61911†61914†61928†61937†61951†

Ogle County · 340 mg/L · Very hard

6011360129†61007†6101061015†61020†61030†61043†61047†61049†61052†61054†61061†610646106861084†61091†

Grundy County · 338 mg/L · Very hard

60407†6041660424†60437†60444†6045060474†60479†

McLean County · 329.5 mg/L · Very hard

61701617046170561720†61722†61724†61726†61728†61730†61731†61732†61736†61737†617446174561748†6175261753†61754†6176161770†61772†61774†61776†

Iroquois County · 320 mg/L · Very hard

60911†60912†60918†60924†60926†60927†60928†60930†60931†6093860939†60945†60948†60951†6095360955†6095660966†60967†60968†60970†60973†60974†

LaSalle County · 319 mg/L · Very hard

60470†60518†60531†60537†60549†60551†60557†61301†61316†61321†61325†6133261334†61341†61342†61348†61350†6135461358†61360†61364†61370†613716137261373†61377†

Henderson County · 318 mg/L · Very hard

61418†61425†61437†61454†61460†61469†6147161480†

Kankakee County · 314 mg/L · Very hard

60901†60910†6091360914†6091560917†60922†60935†60940†60941†60950†60954†60958†60961†60964†60969†

Piatt County · 313 mg/L · Very hard

61813†61818†6183061839†61854†61855†6185661884†61913†61929†61936†

Vermilion County · 313 mg/L · Very hard

609326094260960†60963†61810†61811†61812†61814†61817†61831†618326183361834†61841†61844†61846†61848†6185061857†61858†6186561870†61876†61883

Kane County · 311 mg/L · Very hard

60109†60110†6011860119†6012360124†6013460136†60140†60144†60151†60174601756017760183†60505605066051060511†60539†6054260554†

Rock Island County · 311 mg/L · Very hard

6120161232†61236†61237†6123961240†61242†6124461256†61257†61259†612646126561275†61278†6128261284

Mason County · 310 mg/L · Very hard

61532†61567†62617†62633†6264462655†62664†62682†

Menard County · 310 mg/L · Very hard

62613†62642†6267362675†62688†

Peoria County · 310 mg/L · Very hard

61451†61517†61523†61525†61526†61528†61529†615336153661539†61547†6155261559†6156261569616026160361604†61605616066160761614616156161661625

Champaign County · 307 mg/L · Very hard

60949†6180161802†6181561816†618206182161822†61840†61843†61845†61847†61849†61851†61852†61853†61859†61862†61863†61864†6186661871†61872†61873†6187461875†61877†61878†61880†

Coles County · 303.5 mg/L · Very hard

61912†61920†61931†619386194362440†

De Witt County · 302.5 mg/L · Very hard

6172761735†61749†61750†61777†61778†61842†61882†

Warren County · 300 mg/L · Very hard

61417†61423†61435†61447†61453†6146261473†61478†

Will County · 293 mg/L · Very hard

6040160403†604046040860410†60417†60421†604236043160432†60433†6043560436†604406044160442†604466044860449†6045160468†60481†60484†60490604916050360544605646058560586

Cumberland County · 290 mg/L · Very hard

62428†62436624476246862469

Douglas County · 290 mg/L · Very hard

61910†61919†61930†61941†61942†6195361956†

DuPage County · 290 mg/L · Very hard

6010160103601066010860126601336013760139601436014860157†601726018160184†601856018760188601896019060191605026050460514605156051660517605196052160523†6052760532605406055560559605616056360565

Lake County · 290 mg/L · Very hard

6000260015†6002060030600316003560040600416004260044†600456004660047†60048†60060†6006160064600696007360075†60083†60084600856008760088600896009660099

Mercer County · 290 mg/L · Very hard

61231†61260†61263†61272†61276†61279†61281†61412†61442†61465†61466†61476†61486†

St. Clair County · 289 mg/L · Very hard

62059†6220162203622046220562206622076220862220†62221†6222362225†622266223262239†62240622436225462255†62257†62258†6226062264†622696228262285†62289

Putnam County · 287 mg/L · Very hard

61326†61327†61335†61336†6134061363†61560†

Schuyler County · 286 mg/L · Very hard

61452†62319†62344†62624†62639†62681†

Gallatin County · 284 mg/L · Very hard

62871†62934†62954†62979†62984†

Marshall County · 284 mg/L · Very hard

61369†61375†61424†61479†61537†61540†6154161565†

Scott County · 283.5 mg/L · Very hard

62610†62621†62663†62694†

Sangamon County · 282.5 mg/L · Very hard

62515†62520†6253062536†62539†62545†62558†62561†62563†6261562625†6262962661†62670†62677†62684†62689†62693†6270162702627036270462706627076271162712†

Calhoun County · 280 mg/L · Very hard

62006†62013†62036†62045†62047†62053†

Edgar County · 277.5 mg/L · Very hard

61917†61924†6193261933†619406194461949†61955

Madison County · 273 mg/L · Very hard

62001†62002†620106201862021†62024†62025†620266203462035†620406204662048†6205862060620616206262067†62074†6208462087†620906209562097†622346224962281†62294

Greene County · 272 mg/L · Very hard

6201662027†62044†62050†62054†6207862081†62082†62092†62098†

Clark County · 270 mg/L · Very hard

62420†62423†6244162442†62474†62477†62478†

Crawford County · 270 mg/L · Very hard

6241362427†62433†6244962451†62454†62464†

Macon County · 270 mg/L · Very hard

6175661925†62501†62514†62521†62522†6252362526†62532†6253562537†62544†6254962551†62554†62573†

Brown County · 260 mg/L · Very hard

6235362375†62378†

Cook County · 260 mg/L · Very hard

600046000560007†6000860010†6001660018600226002560026600296004360053†6005660062600676006860070600746007660077600906009160093†601046010760120601306013160141†6015360154601556016060162601636016460165†6016960171†601736017660192601936019460195602016020260203602086030160302603046030560402604066040960411†60415604186041960422604256042660428604296043060438604396044360445604526045360455604566045760458†6045960461604626046360464604656046660467604696047160472604736047560476†604776047860480†604826048760501†605136052560526†6053460546605586060160602606036060460605606066060760608606096061060611606126061360614606156061660617606186061960620606216062260623606246062560626606286062960630606316063260633606346063660637606386063960640606416064260643606446064560646606476064960651606526065360654606556065660657606596066060661607066070760712607146080360804†6080560827

Christian County · 258 mg/L · Very hard

6208362510†62513†62517†62531†6254062546†62547†6255562556†6255762567†6256862570†

Jasper County · 258 mg/L · Very hard

62432624456244862459624756247962480†62481

Wabash County · 256 mg/L · Very hard

62410†62811†6285262863†

Perry County · 254 mg/L · Very hard

62237†62238†62274628326288862997

McDonough County · 248 mg/L · Very hard

61411†614166142061422†61438†61440†61455†61470†61475†62326†

Fayette County · 246 mg/L · Very hard

620116208062414†62418†6245862471†62838†6288062885†

Effingham County · 239 mg/L · Very hard

62401624116242462443624616246762473

Monroe County · 236 mg/L · Very hard

62236†62244†62248622566227862279†62295†62298†

Pike County · 234 mg/L · Very hard

62312†62314†62323†62340†62343†62345†62352†62355†62356†62357†62361†62362†62363†62366†62370†

Lawrence County · 231 mg/L · Very hard

62417†62439†62460†62466†

Shelby County · 228 mg/L · Very hard

6195762422†62431†624386244462462624636246562534†62550†625536256562571

Adams County · 223 mg/L · Very hard

623016230562320†62324†62325†62338†62339†62346†62347†62348†62349†62351†62359†62360†62365†62376†

Hancock County · 223 mg/L · Very hard

61450†62311†62313†62316†62321†62330†6233462336†62341†62354†62358†62367†62373†62374†62379†62380†

Hamilton County · 221 mg/L · Very hard

6281762828†6282962859

Randolph County · 217 mg/L · Very hard

62217†62233†62241†62242†62261†62272†62277†6228662288†6229262297†

Jersey County · 215 mg/L · Very hard

62012†62022†6202862030†62031†62037†6205262063†

Alexander County · 209 mg/L · Very hard

62914†62957†62962†62969†62988†62990†

Morgan County · 206 mg/L · Very hard

62601†62628†62631†62638†6265062665†62668†62692†62695†

Clay County · 200 mg/L · Very hard

62426†6243462824†62839†628586287962899†

Montgomery County · 200 mg/L · Very hard

620156201762032620496205162056620756207662077†6208962091†6209462533†62538†62560†62572†

Macoupin County · 190 mg/L · Very hard

620096201462023†62033†62069†62079†620856208862093†6262662630†62640†62649†62667†6267262674†62685†62690

Jefferson County · 186 mg/L · Very hard

62810†62814†62816†62830†62846†6286462872†62883†62889†62893†6289462898†

Edwards County · 184 mg/L · Very hard

6247662806†6281562818†

Williamson County · 180 mg/L · Very hard

62841†62902†6291562918†62921†62922†6293362948629496295162959†62974†

Clinton County · 176 mg/L · Hard

62215†62216†62218†6221962230†62231†62245†6225062265†62266†62293†

Washington County · 170 mg/L · Hard

62214†62263†62268†62271†62803†62808†62831†6284862876†62877†

Jackson County · 167 mg/L · Hard

62280†6290162903†62907†62916†62924†62927†62932†62940†62942†62950†62958†6296662975†62994†

Bond County · 166 mg/L · Hard

62019†62086†62246†62253†62262†62273†62275†62284†

Union County · 166 mg/L · Hard

62905†62906†62920†62926†62952†62961†62998†

White County · 165.5 mg/L · Hard

62820†62821†62827†62835†62844†62861†62862†62867†62869†62887†

Marion County · 160 mg/L · Hard

6280162807†62849†62853†62854†62870†62875†6288162882†62892†

Hardin County · 159 mg/L · Hard

62919†62931†62982†

Wayne County · 157 mg/L · Hard

62446†62809†6282362833†62837†62842†62843†62850†62851†62878†62886†62895†

Richland County · 146 mg/L · Hard

6241962421†62425624506245262868

Pulaski County · 126 mg/L · Hard

62941†62963†62964†62970†62976†62992†62996†

Massac County · 123 mg/L · Hard

62908†62910†6295362956†62960†

Franklin County · 120 mg/L · Hard

6281262819†6282262825†62836†6285662860†62865†6287462884†62890†628916289662897†62983†62999†

Johnson County · 89.7 mg/L · Moderately hard

62912†62923†6293962943†62967†62972†62995

Pope County · 51 mg/L · Soft

62928†62938†62947†62985†62987†

† 871 ZIP codes are not served by a mapped community water system — most are private-well areas. See well water hardness.

Check any ZIP's utility and hardness →

Source

Groundwater vs surface water

Illinois's groundwater measures harder than its surface water — 307 mg/L across 3,461 samples versus 284 across 73,634, about 1.1× harder. Wells draw from groundwater, so well owners in Illinois tend to see the harder number — 307 mg/L, very hard. Groundwater readings there run 8 to 680.6 mg/L across the middle 80% of samples. The comparison draws on 1,344 groundwater and 3,044 surface-water monitoring sites; 137 readings above 2,000 mg/L were logged but held out of the medians as outliers.

Groundwater (wells)

307 mg/L as CaCO₃ · 3,461 results at 1,344 stations.

Surface water (rivers, lakes)

284 mg/L as CaCO₃ · 73,634 results at 3,044 stations.

Groundwater measures about 1.1× harder than surface water here.

Chemistry

Water chemistry in Illinois

Beyond the hardness number, Illinois's water carries a fuller chemical signature — here is how its measured parameters compare to national benchmarks.

TDS

249 mg/L · n=5,443

249 mg/L — about 1.1× the national county median of 226, consistent with hard, mineral-rich water.

pH

7.8 pH · n=19,948

pH 7.8 — within the EPA secondary range of 6.5–8.5, on the alkaline side.

Alkalinity

182 mg/L as CaCO3 · n=34,451

182 mg/L — substantial buffering capacity, which resists pH swings.

Calcium : Magnesium

63.5 : 27.6 mg/L ≈ 2.3 : 1 · n=81,024

Calcium is about 58% of Illinois's hardness (63.5 mg/L Ca to 27.6 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 Illinois

3 aquifer systems supply much of Illinois's measured groundwater.

  • Silurian-Devonian aquifers — median 365 mg/L across 99 samples at 38 sites: very hard.
  • Sand and gravel aquifers (glaciated regions) — median 355 mg/L across 1,088 samples at 479 sites: very hard.
  • Mississippian aquifers — median 235 mg/L across 16 samples at 6 sites: very hard.

Together these systems account for 1,203 measured samples at 523 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

1910s2020s418

Illinois's decade median moved from 418 mg/L in the 1910s to 262 in the 2020s. Over the series, the decade median peaked at 418 mg/L in the 1910s and bottomed at 140 in the 1950s. Decade medians reflect where and what agencies sampled in each era, not just changes in the water itself.

Scale

Where Illinois sits on the scale

On the USGS scale, Illinois's 285 mg/L is very hard — 16.6 grains per gallon, well above the 180 mg/L very-hard cutoff — among the hardest water measured in the US.

SoftModerately hardHardVery hard60 · 3.5 gpg120 · 7.0 gpg180 · 10.5 gpg285 mg/L — Very hard
Harder than 80% of the 2,977 measured county medians.

At 285 mg/L (16.6 gpg), water-softener owners in Illinois typically set their unit to this hardness — settings calculator.

Questions

Frequently asked questions

Is Illinois's water hard?
Illinois's median hardness is 285 mg/L as CaCO₃, "very hard" on the USGS scale and 2.04× the national county median of 140 mg/L. About 100% of residents in measured counties have hard-or-worse water.
Which county in Illinois has the hardest water?
Saline County, with a median of 449 mg/L as CaCO₃ across 1,210 measurements.
Is well water harder than city water in Illinois?
Groundwater measures harder here — about 1.1× the surface water — so wells tend to run harder (groundwater 307 vs surface water 284 mg/L).
How do I find my exact hardness in Illinois?
Search your ZIP on the lookup tool, read your utility's Consumer Confidence Report, or test at home with a kit.
Is Illinois's water harder than the national average?
Yes. Illinois's median of 285 mg/L is 2.04× the national county median of 140 mg/L, and Illinois ranks #5 of the 50 states and D.C.

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States with similar hardness

Two states measure closest to Illinois's 285 mg/L: Indiana at 280 mg/L and Arizona at 300 mg/L. Illinois measures harder than 92% of states. Browse the full ranking on the statistics page.

More: hardness scale · check your address · national statistics · the national map.

By . Source: USGS/EPA Water Quality Portal — 77,308 Illinois measurements. Data last verified . Methodology.

Cite this page: “Illinois Water Hardness by County.” Hard Water Map. Retrieved 2026-07-22, https://hardwatermap.com/water-hardness/illinois/