Water Hardness Converter
Convert a water test result between grains per gallon, ppm (mg/L as CaCO3), German, French and English degrees, mmol/L and meq/L. The scale shows where your water sits from soft to very hard, and an optional step sizes a water softener.
Last updated:
US average is about 60–90 gallons per person per day.
Each ppm of iron adds about 4 gpg to the load.
How to use the Water Hardness Converter
- Type the hardness value from your test strip, lab report or water utility.
- Choose its unit: gpg, ppm (mg/L), °dH, °fH, °e, mmol/L or meq/L.
- Read the converted values and the soft-to-very-hard rating on the color scale.
- Optionally tick softener sizing and enter household size, water use and iron.
- Copy the results or download the hardness class chart.
Water hardness conversion factors
Every unit below measures the same thing: the total calcium and magnesium in the water, expressed as an equivalent amount of calcium carbonate (CaCO3). Convert to mg/L first, then to any other unit.
| Unit | Definition | = mg/L CaCO3 | = gpg | = mmol/L |
|---|---|---|---|---|
| 1 mg/L (ppm) | 1 mg CaCO3 per liter | 1 | 0.0584 | 0.00999 |
| 1 gpg | 1 grain (64.8 mg) CaCO3 per US gallon | 17.118 | 1 | 0.171 |
| 1 °dH (German) | 10 mg CaO per liter | 17.848 | 1.043 | 0.178 |
| 1 °e (English / Clark) | 1 grain CaCO3 per imperial gallon | 14.254 | 0.833 | 0.142 |
| 1 °fH (French) | 10 mg CaCO3 per liter | 10 | 0.584 | 0.1 |
| 1 mmol/L | 1 millimole of Ca²⁺ + Mg²⁺ per liter | 100.09 | 5.847 | 1 |
| 1 meq/L | 1 milliequivalent per liter (half a mmol of divalent ions) | 50.04 | 2.923 | 0.5 |
Reading a hardness test
- Test strips usually show ppm (mg/L) or gpg, often both. Strip readings are steps, such as 0, 25, 50, 120, 250 and 425 ppm, so treat them as approximate.
- Utility water reports in the US list "total hardness as CaCO3" in mg/L. Some list calcium and magnesium separately: hardness = Ca (mg/L) × 2.497 + Mg (mg/L) × 4.118.
- European appliances (dishwashers, espresso machines) are set in °dH or °fH; enter your US reading here to find the right setting.
- Aquarium kits often report GH in °dH; KH (carbonate hardness) uses the same °dH unit but measures alkalinity, not calcium and magnesium.
Tips for living with hard water
Scale builds up fastest in hot water, so water heaters, kettles and coffee makers suffer first. Above about 7 gpg (120 mg/L) a water softener usually pays off in less scale and less soap use. Below 3.5 gpg, softening is rarely worth it. Descale appliances with citric acid or vinegar, and set dishwashers to your measured hardness so they use the right amount of regeneration salt.
Frequently asked questions
How do I convert grains per gallon to ppm?
Multiply by 17.118. One grain per US gallon equals 17.1 mg/L (ppm) of hardness expressed as calcium carbonate, so 10 gpg is about 171 ppm. To go from ppm to gpg, divide by 17.118.
What is °dH and how does it convert?
°dH is the German degree of hardness: 10 mg of calcium oxide per liter, which equals 17.848 mg/L as CaCO3. So 1 °dH = 1.043 gpg = 0.178 mmol/L. Many dishwashers and coffee machines in Europe use °dH for their hardness settings.
What is the French degree (°fH)?
One French degree is 10 mg/L of CaCO3, which makes it the easiest to convert: multiply °fH by 10 to get ppm. 1 °fH equals 0.56 °dH and 0.1 mmol/L.
What hardness level counts as hard water?
The Water Quality Association scale rates water under 1 gpg (17.1 ppm) as soft, 1 to 3.5 gpg slightly hard, 3.5 to 7 gpg moderately hard, 7 to 10.5 gpg hard and above 10.5 gpg (180 ppm) very hard. The USGS uses 0–60, 61–120, 121–180 and over 180 mg/L.
Is ppm the same as mg/L for water hardness?
For dilute water, yes: 1 mg/L is effectively 1 part per million by mass. Hardness test results are usually expressed 'as CaCO3', meaning calcium and magnesium are both converted to the equivalent amount of calcium carbonate.
What size water softener do I need?
Multiply hardness in gpg (plus about 4 gpg for every ppm of iron) by the number of people and the gallons each uses per day. That is the daily grain load; multiply by 7 for a weekly regeneration and pick a softener rated at least that large.
Is hard water safe to drink?
Yes. Hardness comes mostly from calcium and magnesium and is not a health concern; it mainly causes scale in kettles, water heaters and pipes and makes soap lather less. There is no federal drinking water limit for hardness.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.