01
The purpose
What does this calculator estimate?
Convert water hardness between ppm, grains per gallon, German °dH, French °fH, and more — instantly.
Water Chemistry · B16
Water Hardness Unit Converter
Convert water hardness between ppm, grains per gallon, German °dH, French °fH, and more — instantly.
Measure the inputsPool test kits usually report hardness as ppm CaCO₃ — the base unit here.
Read the full resultUS water softeners and well reports often use grains per gallon (gpg); 1 gpg ≈ 17 ppm.
Check the boundaryEuropean equipment may show German °dH or French °fH — convert before comparing to pool targets.
Useful forWater ChemistryPlanningComparisonVerification
Live calculation
Hardness converter
✓ Checked May 2026
Enter a value and its unit; we'll show all the others.
Converted value
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Enter a hardness value and choose its source unit.
Enter the known values. Let the engine handle the calculation.
✓ Formula checked↻ Live result○ No account needed
Planning estimate only. Confirm product labels, equipment specifications, qualified-professional advice and local requirements before acting.
The result, explained
Use the Water Hardness Unit Converter result with context.
Convert water hardness between ppm, grains per gallon, German °dH, French °fH, and more — instantly.
02
The tool
What information does it use?
- Hardness value: enter a measured or verified value
- Input unit: ppm CaCO3, gpg, mg/L Ca, °dH, °fH, or mmol/L.
03
The method
How to use this calculator
- Hardness valueEnter the value you can verify.
- Input unitppm CaCO3, gpg, mg/L Ca, °dH, °fH, or mmol/L.
04
The relationship
The formula in plain English
All units convert through a common base: ppm as CaCO₃
1 gpg = 17.12 ppm · 1 °dH = 17.85 ppm · 1 °fH = 10.0 ppm · 1 mmol/L = 100.09 ppm.05
Worked through
A worked example: convert 12 grains per gallon to pool units
- 12 gpg × 17.12 = 205 ppm CaCO₃
- 205 ÷ 17.85 = 11.5 °dH
- 205 ÷ 10.0 = 20.5 °fH
| ppm CaCO₃ | Grains/gal | German °dH | French °fH | mmol/L |
|---|---|---|---|---|
| 50 ppm | 2.9 gpg | 2.8 °dH | 5.0 °fH | 0.50 |
| 100 ppm | 5.8 gpg | 5.6 °dH | 10.0 °fH | 1.00 |
| 150 ppm | 8.8 gpg | 8.4 °dH | 15.0 °fH | 1.50 |
| 200 ppm | 11.7 gpg | 11.2 °dH | 20.0 °fH | 2.00 |
| 300 ppm | 17.5 gpg | 16.8 °dH | 30.0 °fH | 3.00 |
| 400 ppm | 23.4 gpg | 22.4 °dH | 40.0 °fH | 4.00 |
06
Why it matters
How the result supports the next decision
Pool test kits usually report hardness as ppm CaCO₃ — the base unit here.
US water softeners and well reports often use grains per gallon (gpg); 1 gpg ≈ 17 ppm.
07
Good inputs
Improve the estimate before relying on it
- Pool test kits usually report hardness as ppm CaCO₃ — the base unit here.
- US water softeners and well reports often use grains per gallon (gpg); 1 gpg ≈ 17 ppm.
- European equipment may show German °dH or French °fH — convert before comparing to pool targets.
- Calcium hardness (pool balance) is the calcium portion; total hardness also includes magnesium.
- When a fill-water report lists 'hardness,' confirm the unit before dosing calcium chloride.
08
The boundary
How to interpret the result
Follow the product label, add chemicals separately and retest before another dose.
The engine reports invalid inputs and safety warnings beside the result. Resolve those messages before using the estimate.
09
Before acting
What should override this estimate?
- Measured site conditions and current test results
- Product labels and manufacturer specifications
- Qualified-professional advice and applicable local codes
10
Common questions
Frequently asked questions
What unit is pool calcium hardness measured in?
Pool test kits report calcium hardness as ppm (parts per million) of calcium carbonate, written ppm CaCO₃. That's the base unit pool chemistry targets use — for example, 200–400 ppm for plaster pools. Other scales like grains per gallon need converting before you compare them.
How do I convert grains per gallon to ppm?
Multiply grains per gallon by 17.12 to get ppm as CaCO₃. So 10 gpg equals about 171 ppm. Water softener specs and many US well-water reports use grains, while pool chemistry uses ppm, so this conversion comes up often.
What's the difference between °dH and °fH?
They're European hardness scales: German degrees (°dH) where 1 °dH ≈ 17.85 ppm CaCO₃, and French degrees (°fH) where 1 °fH = 10.0 ppm CaCO₃. Imported equipment and overseas water reports use these, so converting to ppm lets you apply standard pool targets.
Is total hardness the same as calcium hardness?
Not quite. Total hardness includes both calcium and magnesium, while pool balance specifically targets calcium hardness. Some test strips report total hardness, which reads higher than calcium alone. Use a calcium-specific test for pool balance decisions.
Why does hardness unit conversion matter for pools?
If your fill-water report or softener uses a different unit than your pool kit, you could badly misjudge your calcium level and over- or under-dose calcium chloride. Converting everything to ppm CaCO₃ first ensures your dosing math lines up with standard pool targets.
How many ppm is 1 grain of hardness?
One grain per gallon equals about 17.12 ppm as calcium carbonate.
What is considered hard water?
Generally, water above about 120 ppm (7 gpg) is considered hard; above 180 ppm (10.5 gpg) is very hard.
Do I use total or calcium hardness for my pool?
Use calcium hardness for pool balance decisions; total hardness includes magnesium and reads higher.
Primary sources
Use the result with current guidance.
These official references support safe planning context. Product labels, local requirements and qualified-professional advice take priority over a general calculator estimate.