Hard water costs you most in the water heater, because heat is what turns dissolved calcium and magnesium into solid scale. The cost is not one bill. It is a slow loss of capacity and efficiency, a shorter service life, and, on tankless units, a yearly descaling job. If your water is soft, you can mostly ignore this. If it is hard or very hard, plan for it.
To see where you stand, use the lookup page with your ZIP or postcode, or open your city page below. The calculator turns your hardness and household size into a rough sizing for a softener or scale inhibitor.
Why a water heater is where scale shows up first
The USGS describes the process plainly: when hard water is heated, such as in a home water heater, solid deposits of calcium carbonate can form. The same page notes that scale can lower the efficiency of electric water heaters.
Heat is the trigger. Calcium carbonate dissolves less well as water warms. So the surface that gets hottest, the heating element or the heat exchanger, collects the deposit first.
In practice this looks different by heater type.
- Tank heaters. Scale and loose sediment settle at the bottom of the tank. On an electric unit it coats the element. On a gas unit it sits between the flame and the water.- Tankless heaters. The heat exchanger has narrow passages and little water volume. The Building America Solution Center (PNNL, funded by the US Department of Energy) says scale can build up on the heat exchanger piping, especially in hard water areas. It states that this reduces capacity and efficiency, and in extreme cases can cause complete failure.
- Pipes and fittings downstream. Hot-side taps, mixing valves and shower heads clog sooner than cold-side ones.
We do not give a percentage for efficiency loss here. The sources we trust say scale lowers efficiency but do not give one number that fits every heater, and a figure from a vendor page would not meet our data rules.
What counts as hard
Hardness is stored on this site in mg/L as calcium carbonate. The USGS classes are:
- soft: 0 to 60 mg/L
- moderately hard: 61 to 120 mg/L
- hard: 121 to 180 mg/L
- very hard: more than 180 mg/L
US readers often see grains per gallon (gpg). One gpg is about 17.1 mg/L. UK suppliers publish mg/L, and sometimes Clark degrees.
The table shows how different the starting point is. Figures come from our city pages. Where a city has several supply zones, the page shows a median and a range, and your street may differ.
| City | Hardness | USGS class |
|---|---|---|
| Charlotte, US | 1.9 gpg (32 mg/L) | soft |
| New York, US | 1.9 gpg (32 mg/L) | soft |
| Austin, US | 5.4 gpg (93 mg/L) | moderately hard |
| Houston, US | 6.4 gpg (110 mg/L) | moderately hard |
| Indianapolis, US | 18.5 gpg (317 mg/L) | very hard |
| Sheffield, UK | 42.0 mg/L | soft |
| Hastings, UK | 81.4 mg/L | moderately hard |
| Leeds, UK | 135.6 mg/L | hard |
| Southampton, UK | 297.1 mg/L | very hard |
| Margate, UK | 332.1 mg/L | very hard |
| Kingston upon Hull, UK | 362.1 mg/L | very hard |
Indianapolis, Hull, Margate and Southampton are the cases where a heater without any protection is most exposed. The utility range for Indianapolis runs from 8.7 to 26.9 gpg across the system, so the figure at your tap depends on where you live. Hull has a median of 362.1 mg/L, with a range of 243.8 to 374.6 mg/L across the city.
The pattern follows geology. Chalk and limestone aquifers give hard water, and the highest UK figures above are in chalk country in Kent, Hampshire and East Yorkshire. Surface water from upland catchments tends to be soft. The sources behind each figure are listed under Sources, and each city page names its utility.
What scale actually costs
There are four costs. We can describe them, but we cannot put a dollar or pound figure on them for your home without your heater model and your tariff.
- Energy. A scale layer sits between the heat source and the water. The USGS and PNNL both say efficiency drops. How much depends on thickness, heater type and fuel.
- Capacity. On tankless units, narrowed passages reduce flow and can trip error codes. On tanks, sediment reduces usable volume.
- Lifespan. PNNL warns of failure in extreme cases for tankless heaters. For tank heaters we have no source that quantifies the effect on service life, so we make no claim.
- Maintenance. PNNL says many tankless manufacturers call for a yearly professional cleaning of the heat exchanger with a mild acid solution, with vinegar given as an example. It recommends fitting flush-kit valves at installation so the unit can be isolated, and says these add a small cost.
Check your heater manual. Many warranties state a maximum water hardness, and a few set conditions on descaling. Read that section before you buy.
Three ways to deal with it
Flush and descale on a schedule. This is the cheapest start. A tank can be drained through its drain valve. A tankless unit needs the isolation valves and a pump. Follow the manual and, if you are unsure, call a plumber.
Scale inhibitor. A polyphosphate or similar cartridge before the heater. It does not remove hardness. We have no independent test data we trust for these products, so we do not quote a performance figure. Check what the heater manufacturer accepts.
Ion-exchange softener. It removes calcium and magnesium, so the heater sees soft water. It needs salt, a drain and some space. It is the usual answer in very hard areas.
Which one fits depends on your hardness. As a rough rule, soft water needs nothing. Moderately hard water is manageable with flushing. Hard and very hard water is where inhibitors or softeners start to pay back. The calculator will size a softener from your figure.
Check your own street
Citywide medians hide differences. Philadelphia, for example, is supplied by three treatment plants with different hardness, and Columbus by three plants too. Look at your city page, then ask your utility for the figure at your address if the range is wide. See Philadelphia for how the range is shown.
FAQ
Does soft water mean I never need to flush the heater? No. Sediment can come from other sources, such as rust or fine silt. Soft water slows scale, but the manual’s maintenance schedule still applies.
Is a tankless heater more sensitive to hard water than a tank? Its narrow heat exchanger leaves less room for deposits, and PNNL highlights scale as a known maintenance issue for these units. Manufacturers publish their own hardness limits. Check yours.
Can I rely on a vinegar flush alone in a very hard area? It removes existing scale, and PNNL lists vinegar as an example of a mild acid for annual cleaning. In very hard water the scale returns, so you flush more often or add treatment upstream. We cannot say how often for your heater without its manual.
Sources
- USGS, Water Science School, Hardness of water: https://www.usgs.gov/special-topics/water-science-school/science/hardness-water
- PNNL Building America Solution Center, Gas-Fired Tankless Water Heaters: https://basc.pnnl.gov/resource-guides/gas-fired-tankless-water-heaters
- NYC DEP, 2025 Drinking Water Supply and Quality Report: https://www1.nyc.gov/assets/dep/downloads/pdf/water/drinking-water/drinking-water-supply-quality-report/2025-drinking-water-supply-quality-report.pdf
- Citizens Energy Group, 2025 Drinking Water Report: https://info.citizensenergygroup.com/hubfs/site/water/2025%20Citizens%20Water%20Quality%20Report.pdf
- Yorkshire Water drinking water quality, Stream open data: https://www.arcgis.com/home/item.html?id=fb641f56af3c418298dcdd052e40eb78
- Southern Water drinking water quality, Stream open data: https://www.arcgis.com/home/item.html?id=c5bf5e8411364c308272137b08177872