Hard water in Derry, NH
County estimate from 249 water samples in Rockingham County · July 2026
- NH typical
- 20 mg/L
- EPA health violations since 2016
- 0
- Softener for 3 people
- 24k grain
- Data
- County estimate
Water samples from around Derry put hardness at about 123 mg/L (7.2 gpg), which would be hard on the USGS scale. Treat this as a rough guide: it is the median of 249 samples taken across Rockingham County, mostly wells and streams, not a test of Derry's treated tap water. One more reason for caution: EPA lists Derry's water system as using surface water, which is often softer than the wells behind the county figure.
Derry sits 103 mg/L above the typical New Hampshire figure of 20. None of the 9 nearest cities we track has harder water. The range nearby runs from 13 mg/L in Concord to 123 in Portsmouth.
What 123 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 123 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 × 7.2 gpg = 1,618 grains a day. A 24,000-grain unit regenerates about every 14 days.
EPA record for Derry's water system
No health-based violations since 2016
Serves about 17,528 people from surface water.
- Health-based
- 0
- Monitoring or reporting
- 0
- Open now
- 0
EPA lists this system as serving Derry.
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 Derry water hard?
Yes. It measures about 123 mg/L (7.2 grains per gallon), which the USGS classes as hard.
What is the water hardness in Derry in grains per gallon?
About 7.2 gpg. One grain per gallon equals 17.1 mg/L, so 123 mg/L divided by 17.1 gives 7.2.
What size water softener do I need in Derry?
For 3 people using 75 gallons each per day, 7.2 gpg works out to about 1,618 grains a day. A 24,000-grain softener covers that with a regeneration about every 14 days.
Has the Derry water system had EPA violations?
EPA records show no health-based violations for Derry Water Dept since 2016.
Nearby cities
- Manchester16 mg/LSoft
- Hudson31 mg/LSoft
- Nashua31 mg/LSoft
- Concord13 mg/LSoft
- Portsmouth123 mg/LHard
- Dover64 mg/LModerately hard
- Rochester20 mg/LSoft
- Keene15 mg/LSoft
- Lebanon22 mg/LSoft
All 10 cities in New Hampshire
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.