Tool · water hardness

Water hardness converter

Water reports, test strips and softener manuals all quote hardness in different units. Enter a number in any of them to get every other unit, the U.S. Geological Survey hardness band, and how the water compares with the brewing-water figures on the DIY coffee water page.

Updated 2026-10-17 · sources listed at the end · runs in your browser; nothing is stored or sent

Hardness converter

Hardness value

Unit

ppm / mg/L CaCO₃171.2

gpg10.00

°dH (German)9.59

°fH (French)17.1

°e (Clark)12.01

mmol/L1.710

10 gpg × 17.118 = 171.18 mg/L as CaCO₃ · ÷ 17.848 = 9.59 °dH · ÷ 10 = 17.1 °fH · ÷ 14.254 = 12.01 °e · ÷ 100.09 = 1.710 mmol/L

USGS band: hard (121–180 mg/L). On the WQA-style grains scale used by Nebraska and Penn State Extension (7.0–10.5 gpg): hard.

Coffee: above the SCAA 2009 brewing range of 17–85 mg/L (target 68); inside the SCA 310-2021 test-water band of 50–175 ppm (calcium hardness); inside Wellinger’s 40–250 ppm window.

Every conversion goes through mg/L as CaCO₃ (“ppm”):
mg/L as CaCO₃ = value × factor
other unit = mg/L as CaCO₃ ÷ its factor

factors: gpg 17.118 · °dH 17.848 · °e 14.254 · °fH 10 · mmol/L 100.09

The factors, and where they come from

Hardness is mostly dissolved calcium and magnesium. So that the two can be added up, labs report them as the amount of calcium carbonate (CaCO₃) that would be equivalent; Nebraska Extension notes that concentrations “are generally converted to the equivalent concentration as calcium carbonate.” Each unit below is a different way of writing that same CaCO₃ figure.

Unit	Definition	mg/L CaCO₃	Other published values
ppm, mg/L	1 mg CaCO₃ per liter	1	also called the American degree (Wikipedia)
gpg	1 grain (64.8 mg) CaCO₃ per US gallon (3.785 L)	17.118	17.12 and “17.1” (Wikipedia, Grain per gallon); 17.1 (Nebraska table); “approximately 17” (Penn State)
°dH, dGH	10 mg calcium oxide (CaO) per liter	17.848	17.85 (Hard water table); 17.8 (chemie.de)
°e, °Clark	1 grain CaCO₃ per imperial gallon (4.55 L)	14.254	14.25 (table); 14.3 (chemie.de)
°fH	10 mg CaCO₃ per liter	10	—
mmol/L	1 mmol of calcium + magnesium ions per liter	100.09	100.1 (table); also 40.08 mg/L as Ca²⁺

The rounded versions barely matter: 10 gpg is 171.18 mg/L at 17.118, 171.2 at 17.12, 171 at 17.1 and 170 at 17 — less than 1% apart. The 17.118 factor is the one the DIY coffee water page uses for the SCAA’s “4 grains.” One naming clash: Wikipedia’s grain-per-gallon article says that unit is “also called Clark degree,” while its hard-water article defines the Clark degree per imperial gallon (14.254 ppm). This tool follows the hard-water definition.

Conversion reference table

Common values, computed with the factors above. Rows include the USGS band edges and the coffee figures used below.

mg/L CaCO₃	gpg	°dH	°fH	°e	mmol/L	USGS band
17.1	1.00	0.96	1.7	1.20	0.171	soft
40	2.34	2.24	4.0	2.81	0.400	soft
50	2.92	2.80	5.0	3.51	0.500	soft
60	3.51	3.36	6.0	4.21	0.599	soft (upper edge)
68	3.97	3.81	6.8	4.77	0.679	moderately hard
85	4.97	4.76	8.5	5.96	0.849	moderately hard
120	7.01	6.72	12.0	8.42	1.199	moderately hard (upper edge)
150	8.76	8.40	15.0	10.52	1.499	hard
175	10.22	9.81	17.5	12.28	1.748	hard
180	10.52	10.09	18.0	12.63	1.798	hard (upper edge)
250	14.60	14.01	25.0	17.54	2.498	very hard

Hardness bands

The USGS Water Science School gives “general guidelines”: 0 to 60 mg/L as CaCO₃ is soft, 61 to 120 moderately hard, 121 to 180 hard, and more than 180 very hard. Those whole-number edges leave small gaps (60 to 61, for example); the tool puts anything above 60 in the next band up, and likewise at 120 and 180.

Water-treatment guides often use a five-step grains scale instead. Nebraska Extension attributes its version to the U.S. Department of the Interior and the Water Quality Association; Penn State and UF/IFAS Extension print the same steps:

Class	gpg	mg/L (as printed)	USGS equivalent
Soft	under 1	0–17.1 (Nebraska) · under 17 (Penn State)	soft
Slightly hard	1–3.5	17.1–60	soft
Moderately hard	3.5–7.0	60–120	moderately hard
Hard	7.0–10.5	120–180	hard
Very hard	over 10.5	180 and over	very hard

The printed ranges share their end points, so the tool counts each edge in the higher class (exactly 7.0 gpg reads as hard). German-language sources use yet another scale: chemie.de lists the detergent-law ranges in force since 2007 as soft below 8.4 °dH, medium 8.4–14 and hard above 14.

Soft — 0–60 mg/L as CaCO₃ (USGS).
Moderately hard — 61–120 mg/L (USGS).
Hard — 121–180 mg/L (USGS).
Very hard — more than 180 mg/L (USGS); the bar continues past 300.
SCAA 2009 brewing water — hardness 17–85 mg/L, diamond at the 68 mg/L (4-grain) target.
SCA Standard 310-2021 test water — calcium hardness 50–175 ppm.
Wellinger (SCA 25) — below 40 ppm total hardness tastes under-extracted, above 250 over-extracted.
Grains per gallon — the same scale at 1 gpg = 17.118 mg/L.
Your value — moves with the converter above (starts at 10 gpg = 171.2 mg/L).

Drawn by pourxlab.com for this page from the USGS hardness bands (source 1), the Wikipedia gpg factor (source 3) and the SCA figures summarized on the DIY coffee water page (sources 11–13). Horizontal axis to scale.

Reading a coffee result

The brewing figures in the converter come from the DIY coffee water page, which explains why they disagree. The SCAA’s 2009 table asks for 68 mg/L (4 grains) within 17–85; SCA Standard 310-2021 tests brewers with water of 50–175 ppm calcium hardness; and Dr. Marco Wellinger, writing in SCA’s 25 Magazine, puts the useful window for total hardness at roughly 40–250 ppm. Water in the USGS “hard” band (121–180 mg/L) is above the SCAA range, inside Wellinger’s window, and inside the 310-2021 band up to 175. Wellinger also found that hardness between 20 and 250 ppm had an insignificant effect on extraction efficiency, while alkalinity strongly shapes perceived acidity; see water temperature & chemistry.

“ppm” is not always the same number

ppm vs mg/L. Treating 1 ppm as 1 mg/L assumes water weighs 1.00 g per mL, a standard assumption for dilute solutions (Wikipedia, Parts-per notation). Penn State states the equivalence outright for hardness reports.
As CaCO₃ vs as calcium. One mmol/L of hardness is 100.09 mg/L written as CaCO₃ but only 40.08 mg/L written as calcium ions. A report line reading “calcium, 40 mg/L” is roughly 100 mg/L of calcium hardness as CaCO₃ — before any magnesium. Convert only numbers labeled “as CaCO₃” (or plain “hardness”).
Hardness vs TDS. A TDS pen also reports “ppm,” but of all dissolved solids, not hardness; the SCA lists TDS and hardness as separate targets.

Using it with a water report

Find the hardness line in your utility’s annual water quality report (how to get one is on the water page) and note its unit.
Enter the number and unit above; read the USGS band and the coffee comparison.
Remember the scale. The USGS national hardness map is built from 1975 monitoring data, and the USGS says it is not designed for decisions about an individual home; your local utility is the source for that.
If the water is hard, expect mineral scale in kettles and brewers. USGS notes buildup happens “in your home coffee maker too.”

Is there a legal limit?

Not federally. The EPA sets non-mandatory secondary standards for 15 contaminants covering aesthetic, cosmetic and technical effects; hardness is not one of them. It appears only as a noticeable effect of total dissolved solids, whose secondary level is 500 mg/L, and the EPA says these levels are not federally enforceable. According to a USGS domestic-well study, hard water was prevalent in the east-central and western United States.