How hard is the water in Mercer County, NJ?
7 grains per gallon
Optional: a softener helps, but plenty of homes skip it.
The median hardness measured in Mercer County, NJ is 120 mg/L as calcium carbonate, which is 7 gpg. That is moderately hard on the USGS scale. It comes from 111 samples at 50 sites between 1990 and 2025, drawn from groundwater wells, which is what most taps and private wells in the county draw on.
How much it varies here
Half the sites came in between 3.6 and 11.8 gpg. That spread crosses a decision line, so some homes here land on a different answer. A $10 test strip on your own tap settles it. Confidence in the county figure: high (how it is measured).
What to do next
At 7 gpg this is your call. Look for white film on glasses and crust on the shower head: if you see little of either, you can live without one. If you do want one, a 24,000-grain unit is enough, so ask for that size rather than a bigger one. Because Mercer County straddles a decision line, check your own tap with a $10 test strip before you spend anything.
More on when a softener is not worth it.
From 0.3 to 24.5 gpg
The softest single sample on record in Mercer County was 5.2 mg/L and the hardest 419 mg/L, an 81-fold gap. The top quarter of sites read above 11.8 gpg and the bottom quarter below 3.6 gpg.
Wells 7 gpg, rivers and lakes 3.7 gpg
111 groundwater samples from 50 wells put the median at 119.5 mg/L. 1,470 surface samples from 50 sites put it at 63.2 mg/L. In Mercer County the wells come out 3.3 gpg harder. Groundwater sits in contact with rock for years and picks up calcium and magnesium as it goes, so wells usually read harder. A utility here that draws from the river can deliver noticeably different water from a private well next door.
What a softener would cost to run
For a household of 3, a 24,000-grain unit covers a week between regenerations. Expect about 152 lb of salt a year (4 bags of 40 lb, about $27), and roughly 2,346 gallons of regeneration water. Installed, most units run $1,000 to $4,500; spread over 15 years with salt, that is about $127 a year.
Sizing at 7 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 420 | 24,000 | 51 lb |
| 2 | 840 | 24,000 | 102 lb |
| 4 | 1,680 | 24,000 | 204 lb |
| 6 | 2,520 | 24,000 | 307 lb |
Mercer County against the rest of New Jersey
Among the 21 New Jersey counties with data, Mercer County is number 11 of 21 counting from the hardest. Across the whole country it is harder than 39% of counties.
Softest in New Jersey: Ocean County at 0.8 gpg. Hardest: Hudson County at 17.2 gpg. Closest to Mercer County: Morris County (7.4), Hunterdon County (8.3), Passaic County (8.6).