Quick answer
Water hardness is usually reported in three ways: milligrams per liter (mg/L), parts per million (ppm), and grains per gallon (gpg)2. In typical water samples, mg/L and ppm are treated as the same number, as a practical equivalence rather than an exact identity2. Grains per gallon use a different, larger scale: 1 gpg = 17.118 mg/L2. To convert hardness:
- gpg = mg/L ÷ 17.118
- mg/L = gpg × 17.118
Once you know the number in mg/L, compare it to the reference table below. If you need a reading, see how to test water hardness. For condition context, see what is hard water. A treatment-planning task may use the reading as one input to water-softener sizing, but hardness alone does not determine system size.
What the units mean
Hardness is a measure of dissolved calcium and magnesium (with minor contributions from a few other polyvalent ions), and it is customarily expressed as an equivalent concentration of calcium carbonate (CaCO3)1. Reporting it “as CaCO3” provides a common equivalent basis; it does not mean the water literally contains that concentration of calcium carbonate1.
- mg/L (milligrams per liter) — a mass-per-volume concentration unit. Laboratory and regulatory results, including Standard Methods, commonly report total hardness in mg/L as CaCO32.
- ppm (parts per million) — a parts-per-million concentration. In water-hardness reporting, 1 ppm is generally treated as equal to 1 mg/L2. This is a practical water-reporting convention, not a universal identity2.
- gpg (grains per gallon) — an older unit, common on softener specifications and in consumer literature. One grain of hardness in a gallon corresponds to 17.118 mg/L2.
Standard Methods 2340C is identified as an EDTA titrimetric hardness method5. Once a hardness result exists, mg/L, ppm and gpg are different representations of that result under the stated conversion convention2.
Converting between units
The conversion is a single fixed ratio: 1 grain per gallon equals 17.118 mg/L2. From that:
| Convert | Formula | Example |
|---|---|---|
| mg/L (or ppm) → gpg | gpg = mg/L ÷ 17.118 |
A result of 150 mg/L → 150 ÷ 17.118 = 8.8 gpg |
| gpg → mg/L (or ppm) | mg/L = gpg × 17.118 |
A spec of 10 gpg → 10 × 17.118 = 171 mg/L |
Practical note on rounding. The conversion constant used here is 17.1182. NDSU publishes a rounded factor of 173, while 17.1 is a useful reader-facing shorthand. For comparisons, sizing inputs or records, calculate with 17.118 and round only the displayed result2.
A note on “ppm vs mg/L.” In typical water-hardness reporting, sources treat the two as numerically equivalent for practical use2. Keep that statement bounded to water-hardness reports rather than treating it as a universal identity2.
Worked examples
These static examples apply the MGS conversion factor and band table; they show the arithmetic rather than an interactive tool2.
- Lab result: 150 mg/L as CaCO3.
150 ÷ 17.118 = 8.8 gpg. In the reference table below, 150 mg/L falls in the hard band (121–180 mg/L)2. - Water-test result: 250 mg/L.
250 ÷ 17.118 = 14.6 gpg. This is in the very-hard band (>180 mg/L)2. - Softener spec: 10 gpg.
10 × 17.118 = 171 mg/L. This sits near the top of the hard band2. - Moderate reading: 5 gpg.
5 × 17.118 = 86 mg/L. This is moderately hard (61–120 mg/L)2.
Hardness reference bands
The reference bands below use the four-tier scheme from the Maryland Geological Survey2, which also supplies the exact conversion factor used above.
| Hardness as CaCO3 (mg/L) | ≈ ppm | ≈ gpg | Classification |
|---|---|---|---|
| 0 – 60 | 0 – 60 | 0 – 3.5 | Soft |
| 61 – 120 | 61 – 120 | 3.6 – 7.0 | Moderately hard |
| 121 – 180 | 121 – 180 | 7.1 – 10.5 | Hard |
| > 180 | > 180 | > 10.5 | Very hard |
Band summary (mg/L): soft 0–60; moderately hard 61–120; hard 121–180; very hard >1802.
Alternate terminology. Band boundaries are not perfectly uniform across publications. Some authorities use slightly different wording at the low end: WHO places water below 60 mg/L as soft and starts the next band at 60 mg/L1, and some sources add a “slightly hard” tier at 17–60 mg/L and use a rounded factor of 173. This page uses the MGS four-band table so the classifications stay consistent. If you see a report or product that calls 17–60 mg/L “slightly hard,” that is an alternate convention, not a correction to this table.
Understanding your reading
Interpretation is the purpose of this reference2. A hardness number by itself:
- tells you a general classification band (soft → very hard) used for context and discussion2; and
- does not, by itself, establish that treatment is required — hard water is widely regarded as a nuisance/aesthetic characteristic rather than a health hazard1.
How you act on the reading depends on intent:
- If you do not yet have a reliable reading, the hardness testing page covers how to obtain one.
- If you want the condition-level context—causes, effects and treatment landscape—use the hard-water guide.
- If you are planning a softener, use the water-softener sizing guide to combine hardness with household demand and exact equipment specifications. This page provides conversion formulas and static examples only.