Hard Water Map

California Water Hardness by County

Water in California has a median hardness of 150 mg/L as CaCO₃ — "hard" on the USGS scale — based on 105,884 measurements at 20,318 monitoring stations. San Luis Obispo County has the state's hardest measured water (620 mg/L); Tuolumne County the softest (3.1). Rank: #20 of the 50 states and D.C.

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

Of California's 58 measured counties, 16 very hard, 10 hard, 19 moderately hard, and 13 soft. The hardest are San Luis Obispo County (620 mg/L), Santa Barbara County (506), and San Benito County (500). Tuolumne County is the softest at 3.1 mg/L (soft).

About 82% of California's roughly 39.2 million residents in counties with gate-passing hardness data live where tap water is hard or worse.

Statewide, California's median works out to 8.8 grains per gallon and ranks #20 of the 50 states and D.C. Across 105,884 measurements at 20,318 stations — 19,599 of them since 2000 — the middle 80% of readings fall between 32 and 550 mg/L. The public record runs from 1906 to 2025.

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

Download the California water hardness map (PNG) — Water hardness by county in California: 58 counties mapped from USGS/EPA data, statewide median 150 mg/L (hard); darker counties have harder water.

By county

Water hardness by county

The table lists all 58 measured California counties, hardest first. Their medians span 620 mg/L in San Luis Obispo County to 3.1 in Tuolumne County — a 200× range.

County water hardness, hardest first — median mg/L as CaCO₃.
CountyMedianBandResultsSitesNewest
San Luis Obispo County620Very hard1,0371792023
Santa Barbara County506Very hard8,2301,6762023
San Benito County500Very hard239582022
Ventura County470Very hard1,4213322022
Imperial County370Very hard4,2725392023
San Diego County300Very hard4,0831,0462022
Solano County260Very hard2681292020
San Francisco County250.5Very hard314772017
Santa Cruz County248Very hard1,1862082023
Alameda County240Very hard6,4593112022
Stanislaus County230Very hard3,2525422021
Monterey County220.5Very hard8822932023
Riverside County215Very hard6,3841,4742023
Orange County200Very hard1,9544952020
Inyo County182Very hard5081782023
Colusa County180Very hard8061982025
Los Angeles County179Hard4,1291,0572023
Santa Clara County170Hard1,7921362023
Yolo County170Hard1,1932452021
Contra Costa County160Hard661992022
Merced County160Hard2,6766092023
San Bernardino County153Hard11,1762,3822023
Glenn County140Hard8792042021
Lake County130Hard907372019
San Joaquin County130Hard3,6625992023
Kern County120Hard3,3221,4382023
Sonoma County117Moderately hard1,3411942022
Napa County109Moderately hard774872023
Kings County102Moderately hard3482822021
Amador County96.8Moderately hard19162022
Madera County90.6Moderately hard4202652023
Marin County86Moderately hard209512022
Fresno County85Moderately hard3,9191,5412023
Tehama County85Moderately hard2,0382412022
Calaveras County82Moderately hard248402021
Mendocino County78Moderately hard1,4231672020
Siskiyou County77Moderately hard1,1081272020
Humboldt County74Moderately hard2,5081242020
Sutter County72Moderately hard1,2781692021
Lassen County67Moderately hard424752023
Plumas County65Moderately hard179382020
San Mateo County65Moderately hard624672022
Yuba County62Moderately hard4211102021
Mono County61.9Moderately hard1,4451512022
Del Norte County61Moderately hard852322020
Modoc County59Soft442292020
Sacramento County52.3Soft2,8152892023
Butte County49Soft1,3012062021
Trinity County46Soft230232015
Shasta County44Soft3,0793112022
Nevada County36Soft1,418772023
Placer County35Soft1,0211522022
Sierra County34Soft19122016
Alpine County31Soft206242008
Tulare County30Soft1,6315112023
El Dorado County13.8Soft9071562022
Mariposa County7.4Soft1,1511272023
Tuolumne County3.1Soft394832018

ZIP codes

ZIP codes in California

The ZIP codes below are grouped by the California county they fall in. Each ZIP takes its 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.

San Luis Obispo County · 620 mg/L · Very hard

9340193402†93405†93407†93408†934099341093420†9342293424†93428†93430†93432†9343393442†9344493445†93446†93449†93452†93453†93461†93465

Santa Barbara County · 506 mg/L · Very hard

93013†93067931019310393105†931069310893109931109311193117†93254†93427†93429†93434†93436†9343793440†93441†93454†93455†93458†93460†93463

San Benito County · 500 mg/L · Very hard

95023†95043†95045†95075†

Ventura County · 470 mg/L · Very hard

9132091360913619136291377930019300393004930109301293015930219302293023†9303093033930359303693040†93041930429304393060†9306393064930659306693225†93252†

Imperial County · 370 mg/L · Very hard

92222†92227†92231†92233†92243†92249†92250†92251†92257†92259†92266†92273†92275†92281†92283†

San Diego County · 300 mg/L · Very hard

919019190291905†91906†919109191191913919149191591916†91917†91931†9193291934†91935†91941919429194591948†9195091962†91963†919779197891980†9200392004†9200792008920099201092011920149201992020920219202492025†9202692027†920289202992036†9203792040†920549205592056920579205892059†9206092061920649206592066†920679206992070†9207192075920789208192082920839208492086†9209192092920939209692101†9210292103921049210592106921079210892109921109211192113921149211592116921179211892119†9212092121921229212392124921269212792128921299213092131†921329213492135921369213992140921459214792154921559216192173921799218292672

Solano County · 260 mg/L · Very hard

94510†94512†9453394534†9453594571†94585†94589945909459194592†95620†9562595687†95688

San Francisco County · 250.5 mg/L · Very hard

94102941039410494105941079410894109941109411194112941149411594116941179411894121941229412394124941279412994130941319413294133941349415894188

Santa Cruz County · 248 mg/L · Very hard

95003†9500595006950079501095017†95018†95019950339504195060†950629506495065†95066†95073†95076

Alameda County · 240 mg/L · Very hard

94501945029453694538945399454194542945449454594546945509455194552†945559456094566945689457794578945799458094586945879458894601946029460394605946069460794608946099461094611946129461394618946199462194702947039470494705947069470794709947109472095391

Stanislaus County · 230 mg/L · Very hard

95230†95307†95313†95316†95319†95323†95326†95328953509535195354953559535695357953589536095361†95363†953679536895380†9538295385†95386†95387†

Monterey County · 220.5 mg/L · Very hard

93426†93450†93451†93901939059390693907†93908†93920†9392193923†93924†93925†93926†93927†93928†93930†93932†9393393940†9394393944939509395393954†9395593960†9396295004†95012†95039†

Riverside County · 215 mg/L · Very hard

917529220192203922109221192220†9222392225†92230922349223692239†922409224192253922549225892260922629226492270922749227692282923209250192503925049250592506925079250892509925189252192530†9253292536†92539†9254392544†925459254892549†9255192553925559255792561†92562925639256792570925719258292583925849258592586925879259092591925929259592596928609287992880†928819288292883

Orange County · 200 mg/L · Very hard

906209062190623†906309063190680907209074090742907439260292603926049260692610926129261492617926189262092624926259262692627926299263092637926469264792648926499265192653926559265692657926609266192662926639267392675926769267792678†92679926839268892691926929269492697927019270392704927059270692707927089278092782928019280292804928059280692807928089282192823928319283292833928359284092841928439284492845928619286592866928679286892869928709288692887

Inyo County · 182 mg/L · Very hard

92328†92384†92389†93513†93522†93526†93530†93545†93549†

Colusa County · 180 mg/L · Very hard

95912†95932†95950†95955†95970†95979†95987†

Los Angeles County · 179 mg/L · Hard

900019000290003900049000590006900079000890010900119001290013900149001590016900179001890019900209002190022900239002490025900269002790028900299003190032900339003490035900369003790038900399004090041900429004390044900459004690047900489004990052900569005790058900599006190062900639006490065900669006790068900699007190073900779007990089900949009590201902109021190212902209022190222†902309023290240902419024290245902479024890249902509025490255902609026290263902659026690270902729027490275902779027890280902909029190292†902939030190302903039030490305904019040290403904049040590501905029050390504905059050690601†9060290603906049060590606906389063990640906509066090670907019070390704907069071090712907139071590716907179072390731†9073290744907459074690747907559080290803908049080590806908079080890810908139081490815908229083190840910019100691007910089101091011910169102091024910309104091042†9104691101911039110491105911069110791108911259120191202912039120491205912069120791208912109121491301913029130391304913069130791311†91316913219132491325913269133091331913359134091342†91343913449134591350913519135291354913559135691364913679137191381†91384†91387†91390†914019140291403914059140691411914239143691501915029150491505915069160191602916049160591606916079160891702†91706917119172291723917249173191732917339174091741917449174591746†9174891750917549175591759†917659176691767917689177091773917759177691780†91789917909179191792918019180392397†935109353293534935359353693543†93544†93550†935519355293553†93563†93591

Santa Clara County · 170 mg/L · Hard

94022940249404094041940439408594086940879408994301943039430494305943069500295008950139501495020950309503295035950379504695050950519505395054950709511095111951129511395116951179511895119951209512195122951239512495125951269512795128951299513095131951329513395134951359513695138951399514095148

Yolo County · 170 mg/L · Hard

9560595606†95607†95612†95616†9561895627†95637†95645†95653†95679†95691†95694†95695†9569795698†95776†95937

Contra Costa County · 160 mg/L · Hard

94505†94506945079450994511†9451394514†9451694517†945189451994520945219452394525†945269452894530945319454794548†9454994553†94556†9456194563945649456594569†94572†94575945829458394595945969459794598†947089480194803948049480594806

Merced County · 160 mg/L · Hard

93620†93635†9366595301†95303†95312†9531595317†95322†95324†95333†95334†95340†95341†95348†95365†95369†95374†95388†

San Bernardino County · 153 mg/L · Hard

9170191708917099171091730917379173991761917629176391764917849178692242†9225292256†92267†92268†92277†92278†92280†9228492285†92301†92304†92305†92307†9230892309†92310†92311†923139231492315†9231692317923219232292324923259232792332†92333†92335923369233792338†92339†92341†9234292344923459234692347†92350923529235492356†92358†9235992363†92364†92365†92368†92369923719237292373†923749237692377923789238292385†923869239192392923949239592398†923999240192404†9240592407924089241092411924159355893562†

Glenn County · 140 mg/L · Hard

95913†95920†95939†95943†95951†95963†95988†

Lake County · 130 mg/L · Hard

95422†95423†95426†95435†95443†95451†95453†95457†95458†95461†9546495467†95485†95493†

San Joaquin County · 130 mg/L · Hard

9520295203952049520595206†9520795209952109521195212†95215†95219†95220†95227†95231†95236†9523795240†95242†95253†95258†95304†95320953309533695337953669537695377†95686†

Kern County · 120 mg/L · Hard

93203†93205†93206†93215†93220†93222†9322493226†93240†93241†93243†93249†93250†9325193255†932639326893276†93280†93283†93285†93287†93301933049330593306†93307†93308†9330993311†9331293313†9331493501935059351693518†9351993523935249352893531†93554935559356093561†93581

Sonoma County · 117 mg/L · Moderately hard

94515†94922†94923†94928949319495194954†94972†95401†95403954049540595407†95409†95412†9541995421†95425†9543095431†9543695439†95441†95442†95444†9544695448†95450†95452†9546295465†95471†95472†95476†95480†954869549295497

Napa County · 109 mg/L · Moderately hard

945039450894558†9455994562†94567†9457394574†9457694599†

Kings County · 102 mg/L · Moderately hard

9320293204†93212†93230†93239†932459324693266†

Amador County · 96.8 mg/L · Moderately hard

9560195629†95640956429566595666†95669†9567595685†95689†95699

Madera County · 90.6 mg/L · Moderately hard

93601†93604†93610†93614†93626†93636†93637†93638†93643†93644†93645†93653†93669†

Marin County · 86 mg/L · Moderately hard

9490194903949049492094924†9492594929†949309493394937†94938†9493994940949419494594946†94947949499495094952†94956†949579496094963949649496594970†94971†94973

Fresno County · 85 mg/L · Moderately hard

93210†93234†93242†93602†93605†93606†93608†93609†93611936129361693619†93621†9362293624†93625†93627†93628†93630†93634†93640†93641†93648†9365093651†93652†93654†93656†93657†93660†93662†93664†93667†9366893675†937019370293703†93704†9370593706†9371093711†93720937219372293723†93725†9372693727937289373093737†9374093741

Tehama County · 85 mg/L · Moderately hard

9602196022†96029†96035†96055†96059†96061†96063†96074†96075†96078†96080†96090†96092†

Calaveras County · 82 mg/L · Moderately hard

9522195222†95224†95225†95226†95228†95232†95233†95245†95246†9524795248†95249†95251†952529525495255†95257†

Mendocino County · 78 mg/L · Moderately hard

95410†95415†95417†95420†95427†95428†95429†95432†95437†95445†95449†95454†95456†95459†95460†95463†95466†95468†95469†95470†95482†95488†95490†95494†95585†95587†

Siskiyou County · 77 mg/L · Moderately hard

95568†96014†96023†96025†96027†96031†96032†96034†96037†96038†96039†96044†96050†96057†96058†96064†9606796085†96086†96094†96097†

Humboldt County · 74 mg/L · Moderately hard

955019550395511†95514†95519955219552495525†95526†95528†95536†955379554095542†95545†95546†95547†95549†9555095551†95553†95554†95555†95556†95558†95559†95560†95562†9556495565†95569†95570†95571†95573†95589†

Sutter County · 72 mg/L · Moderately hard

95659†95668†95674†95676†95953†95957†95982†95991†95993†

Lassen County · 67 mg/L · Moderately hard

96006†96009†96056†96068†96109†96113†96114†96117†96119†96121†96123†96128†96130†96132†96136†96137†

Plumas County · 65 mg/L · Moderately hard

95915†95923†95934†95947†95956†95971†95981†95983†95984†96020†96103†96105†96106†9612296129†96135†

San Mateo County · 65 mg/L · Moderately hard

94002940059401094014†940159401894019†94020†94021†940259402794028940309403794038†9404494060†940619406294063940659406694070940749408094128†94401944029440394404

Yuba County · 62 mg/L · Moderately hard

9569295901†95903959189591995922†9592595935959619596295972†95977†

Mono County · 61.9 mg/L · Moderately hard

93512†93514†93517†93529†93541†93546†96107†96133†

Del Norte County · 61 mg/L · Moderately hard

95531†95538†95543†95548†95567†

Modoc County · 59 mg/L · Soft

96015†96054†96101†96104†96108†96110†96112†96115†96116†9613497635†

Sacramento County · 52.3 mg/L · Soft

956089561095615†956219562495626†956289563095632†95638†95639†95641†9565295655956609566295670956719567395680†9568395690†95693†9574295757†957589581195814958159581695817958189581995820958219582295823958249582595826958279582895829958309583195832†95833958349583595837†9583895841958429584395864

Butte County · 49 mg/L · Soft

95914†95916†95917†9592695928†9592995930†95938†95941†95942†95948†95954†95958†9596595966†9596895969†95973†95974†95978

Trinity County · 46 mg/L · Soft

95527†95552†95563†95595†96010†96024†96041†96046†96048†96052†96076†96091†96093†

Shasta County · 44 mg/L · Soft

9600196002960039600796008†96011†96013†96016†96017†9601996028†96033†96040†96047†96051†96062†96065†96069†96071†9607396084†96087†96088†96096†

Nevada County · 36 mg/L · Soft

95945†95946†95949†95959†95975†9598696111†96161

Placer County · 35 mg/L · Soft

95602†956039560495631†95648†9565095658†9566195663956779567895681†95701†95703†95713†95714†95715†95717†9572295728†95736†9574695747†9576596140†96141†96143†96145†96146†96148

Sierra County · 34 mg/L · Soft

95910†95936†95944†95960†96118†96124†96125†96126†

Alpine County · 31 mg/L · Soft

95223†95646†96120†

Tulare County · 30 mg/L · Soft

93201†93207†93208†93218†93219†93221†9322393227†9323593237†93238†93244†9324793256†93257†93258†93260†9326193262†93265†9326793270†93271†93272†9327493277†93286†93291†9329293527†93603†93615†93618†93631†9363393646†93647†936669367093673†

El Dorado County · 13.8 mg/L · Soft

95613956149561995623†95633956349563595636†95651†956649566795672†9568295684†9570995720†95721†95726†95735†9576296142†96150†96155†

Mariposa County · 7.4 mg/L · Soft

93623†95306†95311†95318†95325†95338†95345†95389†

Tuolumne County · 3.1 mg/L · Soft

95305†95309†95310†95321†95327†95329†95335†9534695364†95370†95372†95375†95379†95383†

† 738 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

California's groundwater measures harder than its surface water — 220 mg/L across 53,503 samples versus 92 across 51,905, about 2.4× harder. Wells draw from groundwater, so well owners in California tend to see the harder number — 220 mg/L, very hard. Groundwater readings there run 54 to 718 mg/L across the middle 80% of samples. The comparison draws on 17,636 groundwater and 2,640 surface-water monitoring sites; 1,712 readings above 2,000 mg/L were logged but held out of the medians as outliers.

Groundwater (wells)

220 mg/L as CaCO₃ · 53,503 results at 17,636 stations.

Surface water (rivers, lakes)

92 mg/L as CaCO₃ · 51,905 results at 2,640 stations.

Groundwater measures about 2.4× harder than surface water here.

Chemistry

Water chemistry in California

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

TDS

428 mg/L · n=19,438

428 mg/L — about 1.9× the national county median of 226, consistent with hard, mineral-rich water.

pH

7.8 pH · n=14,067

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

Alkalinity

134 mg/L as CaCO3 · n=31,858

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

Calcium : Magnesium

32.6 : 11 mg/L ≈ 2.96 : 1 · n=21,599

Calcium is about 64% of California's hardness (32.6 mg/L Ca to 11 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 California

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

  • California Coastal Basin aquifers — median 310 mg/L across 19,423 samples at 5,003 sites: very hard.
  • Central Valley aquifer system — median 190 mg/L across 11,387 samples at 6,043 sites: very hard.
  • Basin and Range basin-fill aquifers — median 130 mg/L across 15,431 samples at 3,464 sites: hard.

Together these systems account for 46,241 measured samples at 14,510 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

1910s2020s440

California's decade median moved from 140 mg/L in the 1910s to 145 in the 2020s. Over the series, the decade median peaked at 440 mg/L in the 1920s and bottomed at 120 in the 1960s. Decade medians reflect where and what agencies sampled in each era, not just changes in the water itself.

Scale

Where California sits on the scale

On the USGS scale, California's 150 mg/L is hard — 8.8 grains per gallon, above the 120 mg/L hard cutoff, below very hard.

SoftModerately hardHardVery hard60 · 3.5 gpg120 · 7.0 gpg180 · 10.5 gpg150 mg/L — Hard
Harder than 52% of the 2,977 measured county medians.

At 150 mg/L (8.8 gpg), water-softener owners in California typically set their unit to this hardness — settings calculator.

Questions

Frequently asked questions

Is California's water hard?
California's median hardness is 150 mg/L as CaCO₃, "hard" on the USGS scale and 1.07× the national county median of 140 mg/L. About 82% of residents in measured counties have hard-or-worse water.
Which county in California has the hardest water?
San Luis Obispo County, with a median of 620 mg/L as CaCO₃ across 1,037 measurements.
Is well water harder than city water in California?
Groundwater measures harder here — about 2.4× the surface water — so wells tend to run harder (groundwater 220 vs surface water 92 mg/L).
How do I find my exact hardness in California?
Search your ZIP on the lookup tool, read your utility's Consumer Confidence Report, or test at home with a kit.
Is California's water harder than the national average?
Yes. California's median of 150 mg/L is 1.07× the national county median of 140 mg/L, and California ranks #20 of the 50 states and D.C.

Compare

States with similar hardness

Two states measure closest to California's 150 mg/L: Nevada at 150 mg/L and Florida at 151 mg/L. California measures harder than 62% 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 — 105,884 California measurements. Data last verified . Methodology.

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