Hard Water Map

Idaho Water Hardness by County

Water in Idaho has a median hardness of 130 mg/L as CaCO₃ — "hard" on the USGS scale — based on 33,016 measurements at 6,790 monitoring stations. Oneida County has the state's hardest measured water (320 mg/L); Clearwater County the softest (20). Rank: #23 of the 50 states and D.C.

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

Of Idaho's 44 measured counties, 15 very hard, 9 hard, 9 moderately hard, and 11 soft. The hardest are Oneida County (320 mg/L), Franklin County (290), and Bannock County (280). Clearwater County is the softest at 20 mg/L (soft).

About 46% of Idaho's roughly 1.9 million residents in counties with gate-passing hardness data live where tap water is hard or worse.

Statewide, Idaho's median works out to 7.6 grains per gallon and ranks #23 of the 50 states and D.C. Across 33,016 measurements at 6,790 stations — 12,687 of them since 2000 — the middle 80% of readings fall between 20 and 300 mg/L. The public record runs from 1917 to 2024.

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

Download the Idaho water hardness map (PNG) — Water hardness by county in Idaho: 44 counties mapped from USGS/EPA data, statewide median 130 mg/L (hard); darker counties have harder water.

By county

Water hardness by county

The table lists all 44 measured Idaho counties, hardest first. Their medians span 320 mg/L in Oneida County to 20 in Clearwater County — a 16× range.

County water hardness, hardest first — median mg/L as CaCO₃.
CountyMedianBandResultsSitesNewest
Oneida County320Very hard2771392011
Franklin County290Very hard1,021752015
Bannock County280Very hard6171342011
Twin Falls County270Very hard1,5213332023
Bear Lake County260Very hard709752023
Minidoka County239Very hard4811512020
Jerome County230Very hard5581562020
Power County226.5Very hard4811522018
Cassia County221.5Very hard6882612011
Bingham County201Very hard8772252019
Bonneville County199.5Very hard8961312018
Caribou County198Very hard7911352020
Elmore County190Very hard1,6632102023
Butte County188Very hard1,3613792023
Teton County185.5Very hard110512011
Gooding County170Hard8051662023
Jefferson County161.5Hard4601562018
Blaine County160Hard4221502023
Clark County151Hard196652012
Canyon County150Hard2,0093842019
Payette County150Hard253612011
Lincoln County143.5Hard214592017
Lemhi County140Hard3971012011
Lewis County127Hard64212016
Ada County113Moderately hard1,1794262019
Boundary County110Moderately hard381402023
Fremont County103.5Moderately hard3801242018
Latah County103Moderately hard263622016
Washington County100Moderately hard5531052019
Bonner County86Moderately hard7921172019
Gem County78.5Moderately hard256872022
Madison County74.3Moderately hard411462023
Idaho County60.2Moderately hard503822020
Adams County54.9Soft158532011
Custer County54Soft1,0942192018
Camas County50Soft134622011
Owyhee County46.8Soft7962632024
Shoshone County40Soft5,4597922023
Nez Perce County39Soft340442011
Boise County29Soft253562023
Valley County26Soft5741702022
Benewah County23Soft470692023
Kootenai County22.3Soft1,9621722023
Clearwater County20Soft187312011

ZIP codes

ZIP codes in Idaho

The ZIP codes below are grouped by the Idaho 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.

Oneida County · 320 mg/L · Very hard

83243†83252†

Franklin County · 290 mg/L · Very hard

83228†83232†83237†83263†83283†83286†

Bannock County · 280 mg/L · Very hard

83201†83202†8320983214†83234†83245†83246†83250†83281†

Twin Falls County · 270 mg/L · Very hard

83301†83302†83316†83321†8332883332†83334†83341†

Bear Lake County · 260 mg/L · Very hard

83220†83223†83233†83238†83239†83254†83261†83272†83287†

Minidoka County · 239 mg/L · Very hard

83336†83347†83350†

Jerome County · 230 mg/L · Very hard

83325†83335†83338†

Power County · 226.5 mg/L · Very hard

83204†83211†83212†83271†

Cassia County · 221.5 mg/L · Very hard

83311†83312†83318†83323†83342†83344†83346†

Bingham County · 201 mg/L · Very hard

8320383210†832158321883221†83236†83262†83274†83277†

Bonneville County · 199.5 mg/L · Very hard

83120†83285†83401†83402†83404†8340683427†83428†83443†83449†83454

Caribou County · 198 mg/L · Very hard

83217†83241†83276†

Elmore County · 190 mg/L · Very hard

83337†83623†83633†83647†8364883716†

Butte County · 188 mg/L · Very hard

83213†83244†83255†

Teton County · 185.5 mg/L · Very hard

83422†83424†83452†83455†

Gooding County · 170 mg/L · Hard

83314†83330†83355†

Jefferson County · 161.5 mg/L · Hard

83425†83431†83434†83435†83442†83444†83450†

Blaine County · 160 mg/L · Hard

83313†83320†83333†83340†83348†8335383354

Clark County · 151 mg/L · Hard

83423†83446†

Canyon County · 150 mg/L · Hard

8360583607†83626†83644†83651†8365683660†83676†83686†83687

Payette County · 150 mg/L · Hard

83619†83655†83661†

Lincoln County · 143.5 mg/L · Hard

83324†83349†83352†

Lemhi County · 140 mg/L · Hard

83253†83462†83463†83464†83465†83466†83467†83468†83469†

Lewis County · 127 mg/L · Hard

83523†83543†

Ada County · 113 mg/L · Moderately hard

83616†83634†83642836468366983702†837038370483705837068370983712†83713837148372083725

Boundary County · 110 mg/L · Moderately hard

83805†83826†83845†83847†

Fremont County · 103.5 mg/L · Moderately hard

83420†83421†83429†83433†8343883445†

Latah County · 103 mg/L · Moderately hard

83537†83806†83823†83832†83834†83843†8384483855†83857†83871†83872†

Washington County · 100 mg/L · Moderately hard

83610†83645†83672†

Bonner County · 86 mg/L · Moderately hard

83803†83804†83809†83811†83813†83821†83822†83825†83836†8384083841†83848†8385283856†83860†83864†

Gem County · 78.5 mg/L · Moderately hard

83617†83636†83657†83670†

Madison County · 74.3 mg/L · Moderately hard

83436†8344083448†83451†83460

Idaho County · 60.2 mg/L · Moderately hard

83522†83525†83526†83530†83533†8353683539†83542†83547†83549†83552†83554†

Adams County · 54.9 mg/L · Soft

83612†83632†83643†83654†

Custer County · 54 mg/L · Soft

83226†83227†83235†83251†83278†

Camas County · 50 mg/L · Soft

83322†83327†

Owyhee County · 46.8 mg/L · Soft

83604†83624†83627†83628†83639†83641†83650†

Shoshone County · 40 mg/L · Soft

83802†83808†83812†83837†83839†83846†83849†83850†83867†83868†83873†83874†

Nez Perce County · 39 mg/L · Soft

83501†83524†83535†83540†83541†83545†83548†83555†

Boise County · 29 mg/L · Soft

83602†83622†83629†83631†83637†83666†

Valley County · 26 mg/L · Soft

83611†83615†83638†83677†

Benewah County · 23 mg/L · Soft

83824†83830†83851†83861†8386683870†

Kootenai County · 22.3 mg/L · Soft

8380183810†83814†8381583833†83835†83842†83854†8385883869†83876†

Clearwater County · 20 mg/L · Soft

83520†83544†83546†83553†83827†

† 244 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

Idaho's groundwater measures harder than its surface water — 170 mg/L across 14,303 samples versus 77.7 across 18,127, about 2.2× harder. Wells draw from groundwater, so well owners in Idaho tend to see the harder number — 170 mg/L, hard. Groundwater readings there run 43.5 to 348.5 mg/L across the middle 80% of samples. The comparison draws on 5,007 groundwater and 1,638 surface-water monitoring sites; 17 readings above 2,000 mg/L were logged but held out of the medians as outliers.

Groundwater (wells)

170 mg/L as CaCO₃ · 14,303 results at 5,007 stations.

Surface water (rivers, lakes)

77.7 mg/L as CaCO₃ · 18,127 results at 1,638 stations.

Groundwater measures about 2.2× harder than surface water here.

Chemistry

Water chemistry in Idaho

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

TDS

268 mg/L · n=3,869

268 mg/L — about 1.2× the national county median of 226, consistent with hard, mineral-rich water.

pH

7.9 pH · n=6,455

pH 7.9 — within the EPA secondary range of 6.5–8.5, on the alkaline side.

Alkalinity

155 mg/L as CaCO3 · n=14,733

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

Calcium : Magnesium

28.5 : 8 mg/L ≈ 3.56 : 1 · n=16,625

Calcium is about 68% of Idaho's hardness (28.5 mg/L Ca to 8 mg/L Mg), a little in line with 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 Idaho

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

  • Snake River Plain basin-fill aquifers — median 248 mg/L across 79 samples at 40 sites: very hard.
  • Basin and Range carbonate-rock aquifers — median 221 mg/L across 5 samples at 5 sites: very hard.
  • Snake River Plain basaltic-rock aquifers — median 212.5 mg/L across 770 samples at 194 sites: very hard.

Together these systems account for 854 measured samples at 239 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

1920s2020s230

Idaho's decade median moved from 65 mg/L in the 1920s to 72.6 in the 2020s. Over the series, the decade median peaked at 230 mg/L in the 1930s and bottomed at 65 in the 1920s. Decade medians reflect where and what agencies sampled in each era, not just changes in the water itself.

Scale

Where Idaho sits on the scale

On the USGS scale, Idaho's 130 mg/L is hard — 7.6 grains per gallon, above the 120 mg/L hard cutoff, below very hard.

SoftModerately hardHardVery hard60 · 3.5 gpg120 · 7.0 gpg180 · 10.5 gpg130 mg/L — Hard
Harder than 47% of the 2,977 measured county medians.

At 130 mg/L (7.6 gpg), water-softener owners in Idaho typically set their unit to this hardness — settings calculator.

Questions

Frequently asked questions

Is Idaho's water hard?
Idaho's median hardness is 130 mg/L as CaCO₃, "hard" on the USGS scale and below the national county median of 140 mg/L.
Which county in Idaho has the hardest water?
Oneida County, with a median of 320 mg/L as CaCO₃ across 277 measurements.
Is well water harder than city water in Idaho?
Groundwater measures harder here — about 2.2× the surface water — so wells tend to run harder (groundwater 170 vs surface water 77.7 mg/L).
How do I find my exact hardness in Idaho?
Search your ZIP on the lookup tool, read your utility's Consumer Confidence Report, or test at home with a kit.
Is Idaho's water harder than the national average?
No. Idaho's median of 130 mg/L is below the national county median of 140 mg/L; it ranks #23 of the 50 states and D.C.

Compare

States with similar hardness

Two states measure closest to Idaho's 130 mg/L: Kentucky at 120 mg/L and Minnesota at 140 mg/L. Idaho measures harder than 56% 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 — 33,016 Idaho measurements. Data last verified . Methodology.

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