Water Hardness Calculator
Use this water hardness calculator to turn routine lab numbers for calcium and magnesium into a clear hardness value you can act on. It works with mg/L, ppm, grains per gallon, German degrees, French degrees, English degrees, and mmol/L. You’ll also see a plain-English classification from “soft” to “very hard” so you know exactly what it means for pipes, kettles, and appliances.
How the calculator works
Hardness is the concentration of calcium and magnesium expressed as an equivalent amount of calcium carbonate (CaCO3). Converting to a CaCO3 basis lets you compare different waters with a single scale.
The calculator uses the standard relationship:
Hardness (mg/L as CaCO3) = 2.497 × Ca (mg/L) + 4.118 × Mg (mg/L)
Those factors come straight from chemistry. They are the molar mass of CaCO3 divided by the molar masses of Ca2+ and Mg2+. You get the CaCO3 “equivalent” for each ion and then add them. See University of Florida IFAS Equation 9 and the worked derivation at LSBU.
Quick start: three easy steps
- Enter calcium and magnesium concentrations from your test report. Choose units for each input.
- Pick an output unit for hardness (mg/L as CaCO3 by default). The calculator converts instantly.
- Read the classification badge to see if your water is soft, moderately hard, hard, or very hard.
Keep inputs empty until you’re ready. Use the Reset button to clear everything with one tap on mobile.
What counts as hard water?
The widely used U.S. Geological Survey (USGS) guideline defines these ranges for total hardness as CaCO3:
| Classification | Hardness (mg/L as CaCO3) | Typical gpg |
|---|---|---|
| Soft | 0–60 | 0–3.5 |
| Moderately hard | 61–120 | 3.6–7.0 |
| Hard | 121–180 | 7.1–10.5 |
| Very hard | >180 | >10.5 |
Supported units and conversions
Water labs and treatment guides use many units. You can stick with mg/L or ppm or you can switch to industry favorites like grains per gallon.
| Unit | What it means | Convert from mg/L as CaCO3 | Convert to mg/L as CaCO3 |
|---|---|---|---|
| mg/L (as CaCO3) | Milligrams of CaCO3 equivalent per liter | — | — |
| ppm (as CaCO3) | Parts per million | Same as mg/L in dilute water | Same as mg/L |
| gpg | Grains per US gallon | value ÷ 17.118 | value × 17.118 |
| °dH | German degrees | value ÷ 17.848 | value × 17.848 |
| °fH | French degrees | value ÷ 10 | value × 10 |
| °e (Clark) | English degrees | value ÷ 14.254 | value × 14.254 |
| mmol/L (as CaCO3) | Millimoles per liter of CaCO3 equivalent | value ÷ 100.09 | value × 100.09 |
Example calculations
Example 1 — municipal groundwater
You receive a report that lists calcium at 40 mg/L and magnesium at 12 mg/L.
- Hardness (mg/L as CaCO3) = 2.497×40 + 4.118×12 = 149.296 mg/L.
- Classification: Hard (falls in 121–180 mg/L).
- In grains per gallon: 149.296 ÷ 17.118 = 8.722 gpg.
- In German degrees: 149.296 ÷ 17.848 = 8.365 °dH.
Example 2 — surface water blend
A mixed source shows calcium 18 mg/L and magnesium 5 mg/L.
- Hardness = 2.497×18 + 4.118×5 = 74.186 mg/L.
- Classification: Moderately hard.
- In gpg: 74.186 ÷ 17.118 = 4.335 gpg.
Example 3 — very hard well
A homeowner test lists calcium 115 mg/L and magnesium 45 mg/L.
- Hardness = 2.497×115 + 4.118×45 = 467.665 mg/L.
- Classification: Very hard.
- In gpg: 467.665 ÷ 17.118 = 27.33 gpg.
Why hardness matters
- Scale fouls hot surfaces. Minerals precipitate on heating elements and inside kettles which raises energy use and shortens equipment life. The higher the hardness the faster the buildup.
- Soap behaves differently. Hardness ties up soaps and some detergents. You need more product for the same cleaning result.
- Water taste can shift. Many people describe very hard water as chalky. Taste is subjective yet scale formation is not.
- Softening has trade-offs. Ion-exchange softeners remove Ca and Mg and replace them with sodium or potassium. That improves lather and reduces scale and it can add sodium to the water. Always check manufacturer guidance for sizing using hardness in gpg.
For the background and the classification ranges see the USGS explainer.
How to measure water hardness
You have several practical options. Pick the one that fits the decision you need to make.
- Laboratory analysis. A certified lab measures calcium and magnesium accurately. Use this when you need defensible results for design or compliance.
- Titration kits. Field EDTA titration kits are common in plants and service trucks. They are quick and repeatable when used as directed.
- Test strips. Strips offer a rapid indication for households. They trade precision for speed which is fine for screening.
However you measure, convert your findings to mg/L as CaCO3. That value travels well across manuals and sizing charts.
FAQs
Is ppm the same as mg/L for hardness?
In typical drinking water you can treat 1 ppm of hardness as 1 mg/L of CaCO3 equivalent. That convention is used by USGS and many utilities.
Why does the formula weigh magnesium more than calcium?
Magnesium has a lower atomic mass than calcium. When you express both as CaCO3 equivalents the multiplier for Mg becomes larger. The factors 2.497 and 4.118 come from molar mass ratios documented by university texts and state extension notes such as IFAS SS540.
What unit should I use for sizing a softener?
Most residential softener literature uses grains per gallon. Convert mg/L to gpg by dividing by 17.118. The factor follows from the definition of 1 grain of CaCO3 per US gallon. See Grain per gallon.
Is hardness the same as alkalinity?
No. Hardness mainly reflects calcium and magnesium. Alkalinity reflects bicarbonate and carbonate buffering. The two can correlate in some waters yet they answer different questions. For a clear distinction see the alkalinity and hardness note from the University of Florida SS540.
Why do some guides list German degrees while others use French degrees?
Different countries adopted different reference units long before mg/L became common. The calculator covers both. Convert 1 °dH to mg/L as CaCO3 using 17.848. Convert 1 °fH with 10.
Accuracy notes and limitations
- Formula scope. The hardness equation accounts for Ca and Mg which dominate in natural waters. If your water has significant strontium then true hardness as CaCO3 is slightly higher. LSBU shows the Sr factor of 1.142. Most potable supplies have very little Sr so the difference is often negligible. See the LSBU discussion.
- Classification ranges. The “soft/moderately hard/hard/very hard” labels follow USGS guidance. Utilities elsewhere may publish slightly different bands for program purposes. The calculator keeps the USGS set for consistency.
- Significant figures. Results display to three decimals by default. Your lab report may not justify that many digits. Round to match your source if you are preparing official documents.
