Most of this region runs moderately hard, roughly 3 to 10 grains per gallon depending on which utility you are on, which source it drew from that week, and the season. Hardness is not a regulated contaminant, so utilities are not required to report it the way they report the things on your water report. Some publish it clearly. Several do not publish it at all.
What each utility publishes
| Utility | Grains per gallon | mg/L (ppm) | Source |
|---|---|---|---|
| Fairfax Water Potomac + Occoquan |
5 to 10 | 84 to 170 | Fairfax Water's own FAQ, which classes it moderately hard to hard and notes it varies with source and rainfall |
| DC Water Potomac, Washington Aqueduct |
3 to 9 | 70 to 120 | DC Water, which also notes hardness peaks in warmer months |
| Arlington County and Falls Church | 3 to 9 | 70 to 120 | Neither treats its own water. Both buy from the Washington Aqueduct, the same plant as DC, so DC Water's published range applies |
| Prince William Water East, West and Carter's Grove |
3.5 to 7.0 | 60 to 120 | Prince William Water, classed moderately hard |
| Prince William Water Bull Run Mountain, Evergreen, Hoadly Manor |
7.0 to 10.5 | 120 to 180 | Prince William Water, classed hard. Same county, a different answer, which is the point |
| Loudoun Water | Not published | Not published | Its FAQ discusses hardness without giving a figure. Loudoun buys most of its water from Fairfax Water, so the Fairfax range is the closest published proxy. Ask for the annual water quality report |
| WSSC Water Potomac and Patuxent plants |
Varies by plant | Varies by plant | Potomac-sourced and Patuxent-sourced water differ materially, and which one reaches you depends on where you are. The annual water quality report is the place to look |
| Virginia American Water Alexandria, Potomac |
Not published prominently | Not published prominently | Potomac-sourced, so it sits in the same general band as its neighbours, but no headline figure is published |
Converting between the two units
Utilities report in mg/L, which is the same as parts per million. Softeners are sized in grains per gallon. Divide mg/L by 17.1 to get grains per gallon. So 120 mg/L is about 7 gpg, and 170 mg/L is about 10.
The bands most utilities and the U.S. Geological Survey use: under 1 gpg is soft, 1 to 3.5 is slightly hard, 3.5 to 7 is moderately hard, 7 to 10.5 is hard, and above 10.5 is very hard. What each band actually feels like at the tap, with a scale you can move.
Why your utility may not publish it
Hardness is not a health contaminant, so it is not on the required list. The annual water quality report has to cover regulated substances, and hardness is a courtesy inclusion. That is why the answer above ranges from a clear published number to nothing at all, and why the only figure that describes your house is one measured at your tap.
It also moves. DC Water's note about warmer months is true across the region: a river-sourced system concentrates minerals when flows are low, so the same address can read differently in August than in February.
What the number is actually for
- Sizing a softener. Grains per gallon times gallons used is the load a softener has to strip. It is the input to every cost estimate: what a softener costs to run.
- Predicting water heater life. Scale is what turns a heater rated for 8 to 12 years into one that fails at 6: how that happens inside the tank.
- Deciding whether to bother. At the bottom of the moderate band many homes do nothing and are fine. At the top of hard, the equipment cost shows up whether or not you address it.
Get the only number that describes your house
A SwiftPro technician measures hardness at your tap in grains per gallon, checks your pressure and your water heater, and tells you whether treatment is worth it for your numbers. Free, about 30 minutes, no obligation, Northern Virginia.