Water Hardness Chart: Convert PPM to Grains per Gallon

1 grain per U.S. gallon (gpg) equals about 17.1 milligrams per liter (mg/L) of hardness as calcium carbonate. Divide mg/L or ppm by 17.1 to get gpg.

The U.S. Geological Survey classifies water from soft through very hard using these ranges. Hardness primarily reflects dissolved calcium and magnesium and is normally reported as an equivalent concentration of calcium carbonate (CaCO₃) (USGS).

Classification	Hardness, mg/L or ppm as CaCO₃	Hardness, gpg	What it commonly means at home
Soft	0–60	0–3.5	Little hardness scale
Moderately hard	61–120	3.6–7.0	Some spotting or scale may appear
Hard	121–180	7.1–10.5	Scale and increased soap use become more noticeable
Very hard	More than 180	More than 10.5	Greater scale potential; softening is more likely to be useful

The household effects are practical tendencies, not regulatory or health thresholds. USGS notes that hard water reacts with soap and can form calcium-carbonate deposits when heated. EPA’s list of federal primary drinking-water standards does not include hardness (EPA). A hardness result also does not establish whether water is otherwise safe; contaminants require their own tests.

PPM-to-GPG conversion chart

PPM or mg/L as CaCO₃	GPG
17.1	1.0
50	2.9
60	3.5
100	5.8
120	7.0
150	8.8
180	10.5
200	11.7
250	14.6
300	17.5
400	23.4

For values not shown:

• GPG = mg/L ÷ 17.1
• mg/L = GPG × 17.1

Example: 257 mg/L as CaCO₃ converts to 257 ÷ 17.1 = 15.0 gpg. Alameda County Water District uses the same conversion and gives 100 mg/L as 5.85 gpg (ACWD). For dilute household water reports, ppm and mg/L are treated as numerically equivalent when both values are expressed on the same CaCO₃ basis.

Which number goes into a water softener?

Use the measured total hardness in gpg, not the chart’s category name. If the control accepts only whole numbers, follow the model manual’s rounding instructions. Do not enter a TDS reading: total dissolved solids is not the same measurement as hardness.

If the water contains iron, do not apply a universal correction. The adjustment and the forms of iron a softener can treat depend on the exact system. For example, the current Whirlpool WHES30/WHES33 manual says to add 5 grains to the hardness setting for each 1 ppm of ferrous, or clear-water, iron. It also says those models do not remove ferric or bacterial iron and specifies model-dependent clear-water iron limits (Whirlpool manual). Follow the instructions and limits for your model. See the separate guide to when a softener can treat iron.

Hardness also determines daily softening demand:

Daily hardness load = hardness in gpg × softened water used per day in gallons

At 10 gpg and 240 gallons per day, the load is 2,400 grains per day. Compare that load with the unit’s usable capacity at the selected salt dose—not just the large nominal “grain capacity” in its name. Then verify service flow and installation space as described in the softener sizing guide.

Get a hardness number you can use

1. Public water: Check the utility’s current water-quality report or ask the utility for hardness in your service area. EPA requires community water systems to issue annual Consumer Confidence Reports, but the federally required contents focus on water sources and regulated contaminants, so hardness may not appear (EPA).
2. Private well: Use a laboratory hardness test from an untreated sample. USGS cautions that national and regional hardness maps are not suitable for decisions at an individual property.
3. Existing softener: Test an untreated tap and a treated cold-water tap using the same method. The comparison shows whether the unit is reducing hardness; follow the inlet-versus-outlet test procedure.

Sample before treatment when setting or sizing a softener. A test taken only from a softened faucet measures the softener’s output, not the hardness load entering the system.