Free Calculator · Published October 2026
Calcium and Magnesium Hardness Calculator (as CaCO3)
Add the two ions. Get hardness as calcium carbonate, a USGS band, and grains from the 17 mg/L factor.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
Book a DemoA Calcium and Magnesium Hardness Calculator Adds the Ions
Use this page when the report lists the ions, not a line that already says hardness as CaCO3. Unit conversion is the water hardness converter. How to collect the sample is how to test water hardness.
The calcium ion, not hardness as CaCO3 and not grains.
Zero is allowed when calcium is above zero, and the reverse.
Important: All calculator results are ballpark estimates
All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
What the Calcium and Magnesium Hardness Calculator Measures
The ion is not the hardness line
Hardness as calcium carbonate is calcium times 2.497 plus magnesium times 4.118, when both ions are in milligrams per liter. Forty milligrams per liter of calcium is about 100 mg/L of hardness from calcium alone. Twelve milligrams per liter of magnesium is about 49 mg/L more. The sum is the total this page reports. If the laboratory already printed hardness as CaCO3, use that printed line. Rebuild only when you are checking the arithmetic or when the report gave the ions and left hardness blank.
USGS bands, not a pass or fail
The USGS Water Science School describes general hardness as 0 to 60 mg/L soft, 61 to 120 moderately hard, 121 to 180 hard, and more than 180 very hard, as calcium carbonate. This tool places a result through 60, above 60 through 120, above 120 through 180, and above 180 on those bands. A value of 120.2 lands in the hard band because it is above 120. The bands do not say to install a softener. They give a shared word for the number. See do I need a water softener for the decision, which also depends on iron, taste, and what the customer is willing to maintain.
Worked examples
Same factors as the form. Grains are total hardness divided by 17.
| Case | As CaCO3 | Grains | Band |
|---|---|---|---|
| 40 mg/L calcium, 12 mg/L magnesium | 149.3 mg/L | 8.78 | Hard (above 120 through 180) |
| 20 mg/L calcium, magnesium left at zero | 49.9 mg/L | 2.94 | Soft (through 60 mg/L) |
| Calcium left at zero, 20 mg/L magnesium | 82.4 mg/L | 4.84 | Moderately hard (above 60 through 120) |
Alkalinity and pH are other lines
Alkalinity is the buffer, mostly bicarbonate, and it is not this sum. The alkalinity in well water page keeps that distinction. pH is a separate reading. The Langelier saturation index calculator asks for calcium hardness, alkalinity, pH, temperature, and TDS because those are different inputs. Do not paste alkalinity into the magnesium field. Do not paste pH into calcium.
Grains use 17, not 17.1
NDSU Extension WQ1031 prints 17 mg/L as one grain per gallon. This page divides total hardness by 17. The hardness converter uses 17.1. On a 150 mg/L result the two grains differ in the first decimal. Pick one factor for the valve setting and write it down. Iron and manganese are not in this sum. If the softener uses compensated hardness, add that compensation after this page, from the method the valve maker prints, and do not pretend this calculator did it.
What to write on the job ticket
Calcium ion, magnesium ion, the hardness as CaCO3 you calculated, the band, and the grains with the factor 17. If the lab already printed hardness, write both numbers and say they were a check, not a second test. A water treatment dealer who saves that note can explain a softener setting without retyping the ions. SubcontractorHub keeps the note on the customer record with the lab page.
Frequently Asked Questions
How do calcium and magnesium ions become hardness?
It turns a calcium result and a magnesium result, both in milligrams per liter of the ion, into hardness as calcium carbonate. Calcium is multiplied by 2.497 and magnesium by 4.118, then the two products are added. Those factors are the molecular-weight ratios used in the Standard Methods 2340 B style of calculation. Confirm them against the current edition. One ion may be zero if the other is above zero.
Is calcium the same number as hardness?
No. A lab line that says calcium in milligrams per liter is the ion, not hardness as calcium carbonate. Hardness from calcium alone is about 2.5 times the calcium ion. Magnesium adds more hardness per milligram than calcium does. Typing the calcium ion into a softener valve as if it were grains or as if it were hardness as CaCO3 is a different mistake from this page.
What do the USGS hardness bands mean?
The U.S. Geological Survey Water Science School classifies hardness as calcium carbonate: 0 to 60 milligrams per liter soft, 61 to 120 moderately hard, 121 to 180 hard, and more than 180 very hard. This tool maps a result through 60, above 60 through 120, above 120 through 180, and above 180 onto those bands. The bands are a description, not a treatment requirement and not a private-well statute.
Why is grains per gallon divided by 17 here?
North Dakota State University Extension WQ1031 prints 17 milligrams per liter as one grain per gallon. This page uses that factor. The water hardness converter on this site uses 17.1. The two pages can disagree by a fraction of a grain. Write down which factor you used.
Does this replace a hardness test?
No. It only adds the two ions you type. If the report already prints hardness as calcium carbonate, use that line and do not rebuild it unless you are checking the lab's arithmetic. Alkalinity is not hardness. pH is not hardness. A softener decision still needs the test method, iron, and the flow the valve will see.
Save the Hardness Note With the Lab Page
The ions, the sum as CaCO3, and the factor you used for grains belong on the job.
Get a Free DemoAll calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements. It does not size a softener and it does not replace a laboratory hardness line.