💧 Water

Water Hardness Calculator

Convert water hardness between ppm, German degrees, French degrees, grains per gallon and more — or work it out from your calcium and magnesium readings. Press Calculate.

Written and reviewed by SM Arijul Islam · Last reviewed 29 October 2026 · Editorial standards · Report an error

What water hardness actually measures

Hardness is the total of the dissolved calcium and magnesium in water, reported as if it were all calcium carbonate. That "as CaCO₃" convention is the reason the same water can be quoted as 150 ppm, 8.4 German degrees and 8.8 grains per gallon without any of the numbers being wrong. They are the same measurement wearing different clothes.

Every factor below is exact chemistry: calcium has an equivalent weight of 20.04 g per equivalent and magnesium 12.15, while calcium carbonate is 50.04 g per equivalent. That is why 1 mg/L of calcium contributes 2.497 mg/L of hardness and 1 mg/L of magnesium contributes 4.118.

Hardness classification

Hardness ranges and what they mean in the usual units
Category	mg/L as CaCO₃	°dH	gpg	What to expect
Soft	0 – 60	0 – 3.4	0 – 3.5	Lathers easily, no scale, may be slightly corrosive
Moderately hard	60 – 120	3.4 – 6.7	3.5 – 7	Noticeable scale in kettles over time
Hard	120 – 180	6.7 – 10.1	7 – 10.5	Scale on taps, soap scum, dull laundry
Very hard	over 180	over 10.1	over 10.5	Rapid scale build-up, appliances suffer

Unit conversion factors

Each hardness unit expressed in mg/L as calcium carbonate
Unit	Equals (mg/L as CaCO₃)
1 ppm / mg/L as CaCO₃	1
1 °dH (German degree)	17.848
1 °fH (French degree)	10
1 °e (Clark degree)	14.3
1 gpg (grain per US gallon)	17.118
1 mmol/L	100.09
1 mEq/L	50.044
1 mg/L as CaO	1.7848
1 mg/L as Ca²⁺	2.497
1 mg/L as Mg²⁺	4.118

Worked examples

10 °dH is 10 × 17.848 = 178 mg/L as CaCO₃, which is hard water.
40 mg/L calcium plus 12 mg/L magnesium gives (40 × 2.497) + (12 × 4.118) = 149 mg/L, right in the hard band.
7 gpg, a common US utility figure, is 7 × 17.118 = 120 mg/L as CaCO₃.
2 mmol/L is 200 mg/L as CaCO₃, very hard — the sort of water that ruins a kettle in months.

Frequently asked questions

What is the difference between ppm and mg/L?

None in practice. One litre of water weighs essentially one kilogram, so one milligram per litre and one part per million land on the same number.

Why "as CaCO₃"?

Hardness is a mix of calcium and magnesium, so chemists agree to express the total in a single currency — the equivalent amount of calcium carbonate. Without that convention, every lab would report something different.

Do I need to treat hard water?

Hard water is not a health problem and some studies suggest the minerals are beneficial. The reasons to soften are practical: scale in appliances, soap that will not lather, and higher energy bills from scaled heating elements.

Is hardness the same as alkalinity?

No, and they are often confused. Hardness measures calcium and magnesium; alkalinity measures the water's ability to neutralise acid, mostly bicarbonates. They frequently rise and fall together, but they are separate tests.

Common mistakes to avoid

Treating °dH and °fH as interchangeable. One German degree is 17.85 mg/L; one French degree is 10 — a difference of nearly 80%.
Adding calcium and magnesium as raw mg/L without weighting them. Magnesium contributes 1.65 times as much hardness per milligram.
Reading a TDS meter as a hardness test. Total dissolved solids includes sodium, chloride and everything else.
Assuming soft water is always better for plumbing. Very soft water can be aggressive towards metal pipes.

Sources and standards

Hardness is expressed as an equivalent concentration of calcium carbonate even though the water contains calcium and magnesium salts in varying proportions. The WHO guidance on hardness in drinking water explains that convention and what it does and does not imply for health; the page keeps mg/L as CaCO3, mmol/L and the German, French and English degrees separate.

World Health Organization - Hardness in Drinking Water