How hard is your water, plant by plant?

WSSC Water hardness depends on the plant: the Potomac plant averaged 154 mg/L (9.0 grains, our arithmetic) in the 2025 report and the Patuxent plant averaged 70 mg/L (4.1 grains).

By the Your Water Numbers desk, not WSSC Water · Updated November 2, 2026 · How we check

Reading this: Find your plant first, then read across. Grains are mg/L ÷ 17.1, our arithmetic, and a blended area has no figure of its own here.

How hard is each plant’s water?

Potomac water is hard on average and Patuxent water is moderately hard. The USGS scale runs 0–60 soft, 61–120 moderately hard, 121–180 hard and over 180 very hard.

The yearly figures come from WSSC Water’s 2025 tap report. The utility’s general water page gives rounder typical figures, about 120–130 mg/L for Potomac and about 60–65 for Patuxent. We use the 2025 report because it’s dated and has a range.

What does the range tell you?

A Potomac reading can fall anywhere from 79 to 213 mg/L, a swing of 134 (our arithmetic). Patuxent moves only 28, from 58 to 86 (our arithmetic).

The two ranges overlap slightly. Potomac’s low of 79 sits under Patuxent’s high of 86. So one reading from a home kit won’t name your plant.

The figures we read give a range and no months, so we can’t say which season runs high. One test on one day is a snapshot.

Why do softener labels use grains?

Softener makers rate hardness in grains per gallon, and the report uses mg/L. Divide by 17.1 to convert. That makes 154 mg/L about 9.0 grains and 213 mg/L about 12.5, both our arithmetic.

If a salesperson quotes a hardness number for your street, ask for the unit and the source. Then compare it with the table above.

Do you need a softener at 9 grains?

Not automatically. Hardness has no limit line in the report figures we read. It shows up as scale and soap film, so this is a question of nuisance and upkeep.

• On Patuxent, at 4.1 grains on average, most homes have little reason to buy one.
• On a blend, test first. We have no published figure for your mix.
• On Potomac, at 9.0 grains with a high of 12.5, one can make sense if scale is costing you fixtures or heater upkeep.
• If you see no scale and don’t mind the feel of the water, you can skip it.

A softener deals with hardness. It isn’t the filter the lead guidance names, and it isn’t a PFAS answer.

Where does scale cost you most?

The first place to look is anything that heats water. Our water heater page covers the tank. Aerators and showerheads show it first, as in the photo above, and they’re the cheap place to check.

Questions on this page

Where each number comes from