Hard water in Lewiston, ID
County estimate from 77 water samples in Nez Perce County · July 2026
- ID typical
- 118 mg/L
- EPA health violations since 2016
- 0
- Softener for 3 people
- 24k grain
- Data
- County estimate
Water samples from around Lewiston put hardness at about 122 mg/L (7.1 gpg), which would be hard on the USGS scale. Treat this as a rough guide: it is the median of 77 samples taken across Nez Perce County, mostly wells and streams, not a test of Lewiston's treated tap water. One more reason for caution: EPA lists Lewiston's water system as using surface water, which is often softer than the wells behind the county figure.
At 122 mg/L, Lewiston is close to the typical Idaho figure of 118 mg/L. Of the 10 nearest cities we track, 8 have softer water and 2 have harder. The range nearby runs from 49 mg/L in Sandpoint to 170 in Caldwell.
What 122 mg/L means at home
Expect white crust on faucets and showerheads, plus spots on dishes that come out of the dishwasher.
Kettles collect scale quickly. Water heaters build up sediment at the bottom of the tank, which makes them work harder over time.
Soap and detergent need a little more to lather, and soap scum shows up in the tub.
Hardness is not a health problem, and EPA sets no limit for it. It is a matter of scale and soap.
What to do about it
At 122 mg/L most of the trouble is scale, and only a few things deal with it.
Ion-exchange water softener
The standard whole-house fix. It swaps calcium and magnesium for sodium or potassium and needs salt plus a drain. Size it with the calculator below.
Salt-free conditioner
No salt and no drain line. It changes how minerals form scale instead of removing them, so water still tests hard.
Reverse osmosis or carbon filter
Under-sink reverse osmosis removes minerals from drinking water only. Plain carbon filters improve taste but do not soften.
3 people × 75 gallons a day × 7.1 gpg = 1,604 grains a day. A 24,000-grain unit regenerates about every 14 days.
EPA record for Lewiston's water system
No health-based violations since 2016
Serves about 15,001 people from surface water.
- Health-based
- 0
- Monitoring or reporting
- 42
- Open now
- 0
EPA lists this system as serving Lewiston.
Health-based means a contaminant above its legal limit or a missed treatment step. Monitoring and reporting violations are late or missing tests and notices. Source: EPA ECHO, 2026Q3 export, violations with periods starting 2016 or later. Full EPA record.
Questions
Is Lewiston water hard?
Yes. It measures about 122 mg/L (7.1 grains per gallon), which the USGS classes as hard.
What is the water hardness in Lewiston in grains per gallon?
About 7.1 gpg. One grain per gallon equals 17.1 mg/L, so 122 mg/L divided by 17.1 gives 7.1.
What size water softener do I need in Lewiston?
For 3 people using 75 gallons each per day, 7.1 gpg works out to about 1,604 grains a day. A 24,000-grain softener covers that with a regeneration about every 14 days.
Has the Lewiston water system had EPA violations?
EPA records show no health-based violations for Lewiston City of since 2016. There were also 42 monitoring or reporting violations, which are about late or missing tests and notices.
Nearby cities
- Moscow111 mg/LModerately hard
- Coeur d'Alene118 mg/LModerately hard
- Post Falls116 mg/LModerately hard
- Hayden118 mg/LModerately hard
- Rathdrum110 mg/LModerately hard
- Sandpoint49 mg/LSoft
- Star124 mg/LHard
- Eagle90 mg/LModerately hard
- Caldwell170 mg/LHard
- Garden City90 mg/LModerately hard
Hardness data: TapWaterData US Water Hardness (county estimate, July 4, 2026), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.