Enter your calcium and magnesium results — or a single reading from a test kit — to get total hardness in mg/L, GPG, °dH and °fH, the USGS hardness class, and the softener capacity you need.
Enter calcium and magnesium, or a single hardness reading from your test kit
Ion concentrations (most accurate)
Calcium (Ca²⁺)
mg/L
Magnesium (Mg²⁺)
mg/L
— or use a single hardness reading —
Total Hardness (direct reading)
Softener sizing (optional)
75 gallons per person per day is a common sizing assumption. Change it if you know your actual use.
⚠ Please enter at least one valid measurement — either Calcium + Magnesium concentrations, or a direct hardness reading greater than zero.
Your Water Hardness Results
Total Hardness as CaCO₃
—
mg/L (ppm)
—
Grains per Gallon
— GPG
Parts per Million
— PPM
German Degrees
— °dH
French Degrees
— °fH
Calcium Hardness
— mg/L
Magnesium Hardness
— mg/L
Hardness Scale
📖 How To Use
How to Use This Water Hardness Calculator
Option A: calcium and magnesiumIf a lab report lists calcium (Ca) and magnesium (Mg) in mg/L, enter both. The calculator converts each to its calcium carbonate equivalent and adds them. If you enter these, they're used instead of a single reading.
Option B: a single readingIf your test strip or drop kit gives one hardness number, enter it and choose the unit — mg/L (ppm), grains per gallon, German or French degrees.
Softener sizing (optional)Enter the number of people, daily use per person and how often the softener should regenerate to get the capacity you need in grains.
Read the resultsYou'll get total hardness in every common unit, the USGS hardness class, and the softener capacity.
Tip: Hardness can change with the season and with your utility's source, and a private well can change too. If a result looks wrong, re-test — and ask your water utility for its latest water quality report, which usually lists hardness.
📐 The Formula
Water Hardness Formula Explained
Total hardness is the calcium and magnesium in the water, each expressed as the amount of calcium carbonate (CaCO₃) it's equivalent to:
Total hardness (mg/L as CaCO₃) = Ca × 2.497 + Mg × 4.118
GPG = mg/L ÷ 17.118 · °dH = mg/L ÷ 17.848 · °fH = mg/L ÷ 10
The factors come from the atomic weights: CaCO₃ (100.09) divided by calcium (40.08) gives 2.497, and divided by magnesium (24.31) gives 4.118. For water, mg/L and ppm are the same.
USGS hardness classes
Class
mg/L as CaCO₃
GPG
°dH
What you'll notice
Soft
0 – 60
0 – 3.5
0 – 3.4
Soap lathers easily; little or no scale
Moderately hard
61 – 120
3.6 – 7.0
3.4 – 6.7
Some scale in kettles and on fixtures
Hard
121 – 180
7.1 – 10.5
6.8 – 10.1
Noticeable scale, spotting, more soap needed
Very hard
Over 180
Over 10.5
Over 10.1
Heavy scale in heaters, pipes and appliances
Unit conversions
From
To mg/L
To GPG
To °dH
To °fH
1 mg/L (ppm)
1
÷ 17.118
÷ 17.848
÷ 10
1 GPG
× 17.118
1
× 0.959
× 1.712
1 °dH
× 17.848
× 1.043
1
× 1.785
1 °fH
× 10
× 0.584
× 0.560
1
Softener sizing formula
Capacity (grains) = People × Gallons per person per day × Hardness (GPG) × Days between regenerations
Example: 4 people × 75 gal × 10 GPG × 7 days = 21,000 grains
A softener's rated capacity depends on how much salt it uses per regeneration, so compare against the capacity at the salt setting you'll actually use. If your water also contains iron, softener makers add extra capacity for it — check the manufacturer's sizing guide.
💡 Use Cases
When Hardness Matters
Plumbing and appliances: Hard water leaves limescale in water heaters, kettles, dishwashers and pipes, and on showerheads and taps. Scale insulates heating elements and narrows pipes, which is the main reason people fit softeners.
Storage tanks: Hard water leaves scale and sediment on tank walls and float valves — scaled valve seats are a common reason tanks overflow. Clean the tank periodically, and see our guide to tank care.
Chlorine disinfection: Hardness itself doesn't use up chlorine, but hard water often has a higher pH, and chlorine disinfects less effectively as pH rises. Check pH with our pH adjustment calculator.
UV and filters: Hardness scale coats the quartz sleeve of UV units and reduces their dose — check the hardness limit on your UV system's spec sheet.
Irrigation: Hard water can leave deposits that clog drip emitters over time; filters and periodic flushing help.
❓ FAQ
Frequently Asked Questions
What is water hardness?
The amount of dissolved calcium and magnesium in water, usually given as mg/L of calcium carbonate (CaCO₃) or as grains per gallon. It's what causes limescale and makes soap harder to lather.
How do I calculate hardness from calcium and magnesium?
Multiply calcium (mg/L) by 2.497 and magnesium (mg/L) by 4.118, then add them. For 40 mg/L calcium and 10 mg/L magnesium: 40 × 2.497 + 10 × 4.118 = 141.1 mg/L as CaCO₃, or 8.2 grains per gallon — hard water on the USGS scale.
What is the difference between GPG and ppm?
Grains per gallon (GPG) is the US softener industry unit; ppm (the same as mg/L) is used on lab reports. 1 GPG = 17.1 ppm. German degrees (°dH) are common in Europe: 1 °dH = 17.8 ppm.
Is hard water safe to drink?
Yes. WHO hasn't set a health-based guideline for hardness, and the calcium and magnesium in hard water are minerals people need. Hard water is mainly a nuisance — scale, spotting and extra soap use. Very hard water can taste mineral, and very soft water can be more corrosive to pipes.
What size water softener do I need?
Multiply people × gallons per person per day × hardness in GPG × days between regenerations. For 4 people using 75 gallons each, with 10 GPG water and a weekly regeneration: 4 × 75 × 10 × 7 = 21,000 grains. Choose a softener rated at or above that at the salt setting you'll use; add capacity for iron following the manufacturer's guide.
Does hard water affect chlorine disinfection?
Not directly — calcium and magnesium don't use up chlorine. But hard water often has a higher pH, and chlorine works less well as pH rises: hypochlorous acid, the strong disinfecting form, drops from about half of free chlorine at pH 7.5 to roughly a quarter at pH 8. If your pH is high, allow longer contact time or adjust pH.
How do I test water hardness at home?
Use hardness test strips or a drop-count titration kit — the drop kits are more precise. A TDS meter doesn't measure hardness: it includes sodium and other salts, so softened water can read high TDS with almost no hardness. For calcium and magnesium separately, use a lab test or check your utility's water quality report.
Does a water softener lower TDS?
No. A softener swaps calcium and magnesium for sodium (or potassium), so the water stops forming scale but its total dissolved solids stay about the same. To lower TDS, use reverse osmosis — see our TDS calculator.
✅ Sources & Review
Sources and review
The classes and conversions on this page come from these references:
US Geological Survey (USGS), Hardness of Water — the soft / moderately hard / hard / very hard classes.
Calcium carbonate equivalents from standard atomic weights (Ca 40.08, Mg 24.31, CaCO₃ 100.09).
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
