All 9 Water Hardness Units Of Measure Explained (+ Conversions)

We express water hardness in a lot of different units. These include PPM, GPG, °dH/dGH, mg/L, mmol/L,°fH/fGH, Clark degrees (°e or °Clark), TDS, and EC (microsiemens/cm). Understandably, with so many water hardness units, it’s easy to feel a bit confused.

That’s why we are going to take a structured approach here, covering all these 9 water hardness measurement units one-by-one to get the complete overview.

On top of that, we also present conversions between them (Example: Here is a PPM to GPG conversion), and give you an insight into water hardness unit ranges for water hardness categorization (“Soft” vs. “Moderately Hard” vs. “Hard” vs. “Very Hard” water). Example: 110 PPM means you have “Moderately Hard” water because it falls within the 61-120 PPM range.

Namely, total water hardness is caused by the presence of both calcium ions (Ca2+) and magnesium ions (Mg2). Water hardness units express, in most basic terms, the sum concentration of these 2 water hardness ions (Water Hardness = [Ca2+] + [Mg2+]).

First of all, let’s quickly look at what all these units mean and quick definitions (we’ll delve into each of them in turn further on):

  1. PPM stands for Parts Per Million. 1 PPM is defined as 1 mg/L of CaCO3 (same as 0.4 mg/L Ca2+).
  2. GPG stands for Grains Per Gallon. 1 GPG is defined as 1 grain of CaCO3 (64.8 mg) dissolved in 1 US gallon (3.79 liters).
  3. °dH/dGH stands for degrees of General Hardness (or German degrees – deutsche Härte). 1°dH is defined as 10 mg/L of CaO.
  4. mg/L are milligrams per liter. We can express water hardness in terms of mg/L CaCO3, mg/L CaO, and mg/L Ca2+ (3 different units).
  5. mmol/L are millimols per liter. We usually use mmol/L CaCO3 (equal to 100.08 mg/L CaCO3) or mmol/L Ca2+ (equal to 40.08 mg/L Ca2+).
  6. °fH/fGH stands for French degrees of General Hardness. 1 French degree (°fH) is defined as 10 mg/L of CaCO3.
  7. Clark degrees (°Clark) are the same as English degrees (°e). 1 Clark degree or English degree is defined as 1 grain of CaCO3 (64.8 mg) per 1 Imperial gallon (4.55 liters). This is basically the European version of GPG (Grains Per Gallon) but we use Imperial gallons here, instead of US gallons (4.55 liters vs 3.79 liters).
  8. TDS stands for Total Dissolved Solids. We get this result when we use a TDS meter to measure water hardness and commonly refer to the result as, for example, 100 TDS. 1 TDS is equal to 1 PPM (basically the same unit).
  9. EC stands for Electrical Conductivity and is expressed in microsiemens per centimeter (µS/cm). We convert this EC measurement to PPMs (result depends on the EC meter origin country). If you use an American-made EC meter, 1 EC is equal to 0.5 PPM.

Let’s break these down, starting with the PPMs:

1. PPM Or Parts Per Million

PPM is the most commonly used water hardness unit. It is based on the mg/L of CaCO3 (calcium carbonate) scale. Namely, 1 PPM is equal to 1 mg/L CaCO3.

It is important to note that water hardness PPMs are not the same as conventional PPMs (where 1 PPM means 1 part per million or 0.0001% percentage). Rather, we that that 1 mg/L CaCO3 as 1-part.

World Health Organization (WHO) specified here that drinking water has between 10 PPM and 500 PPM water hardness (this ranges from “Soft” water to “Very Hard” water).

Here are the PPM water hardness ranges for water hardness categorization:

Water Hardness Categorization: PPM Range:
“Soft” Water 0 – 60 PPM
“Moderately Hard” Water 61 – 120 PPM
“Hard” Water 121 – 180 PPM
“Very Hard” Water Above 180 PPM

Most households in the US (85%, according to the US Geological Survey) have either “Hard” water or “Very Hard” water. That means that most of us have above 121 PPM water hardness. The whole point of water softeners, for example, is to get these PPMs to below 60 PPM (“Soft” water).

results of very hard water
If you have “moderately hard” or “hard” water, you will notice these water stains.

You can convert water hardness PPM unit to other water hardness measurement units with these converters:

2. GPG Or Grains Per Gallon

In the US, we like to express water hardness in GPG units. Specifically, when you want to size a water softener (more about that here), we have to use water hardness in terms of grains per gallon (GPG). This is because water softener capacity is also expressed in GPG (Example: 24,000-grain, 30,000-grain, 36,000-grain water softeners).

GPG water hardness unit denotes how many grains of CaCO3 are dissolved in 1 US gallon. 1 grain of CaCO3 is equal to 64.8 mg of CaCO3, and the US gallon is equal to 3.79 liters.

grain of calcium carbonate per gallon explained
Here is a test tube with 5g of CaCO3 from our lab. These 5 grams are equal to 77 grains of CaCO3.

All water with above 3.51 GPG water hardness is considered non-soft water. Here is the full GPG water hardness chart:

Water Hardness Categorization: GPG Range:
“Soft” Water 0 – 3.50 GPG
“Moderately Hard” Water 3.51 – 7.01 GPG
“Hard” Water 7.02 – 10.52 GPG
“Very Hard” Water Above 10.52 GPG

From this chart we see, for example, that if you have 10 GPG water hardness, this is already considered “Hard” water. Once you go above 10.52 GPG, you are in the range of “Very Hard” water.

Here are converters from GPG to other water hardness measurement units we built for you guys at Waternity Lab:

3. °dH / dGH Or Degrees Of General Hardness

°dH are degrees of hardness, also referred to as German degrees (deutsche Härte in the  German language). This water hardness unit is based on the mg/L CaO scale (calcium oxide). Specifically, Germans decided that 1°dH should be equal to 10 mg/L CaO, and it kind of stuck.

Here is a bit of a comparison:

  • In the US, we use PPM or 1 mg/L CaCO3 (seldom also referred as American degrees of water hardness)
  • In Germany, they have this °dH (German degrees) that are based on 10 mg/L CaO equivalent.

Here is the water hardness °dH scale:

Water Hardness Categorization: °dH Range:
“Soft” Water 0 – 3.37 °dH
“Moderately Hard” Water 3.38 – 6.74 °dH
“Hard” Water 6.75 – 10.11 °dH
“Very Hard” Water Above 10.12 °dH

You can see that, quantitatively, these degrees of General hardness (°dH) are very similar to grains per gallon (GPG). Namely, 1 GPG is equal to 1.043°dH (only 4.3% difference).

In order to convert °dH / dGH to other water hardness units, you can use these converters:

4. mg/L Or milligrams per Liter Of CaCO3, CaO, Calcium Ions

This is viewed as the quintessential water hardness unit. If water hardness had a base unit (which it doesn’t because all of them are derived), it would be mg/L. All other units such as PPM, GPG, dGH, etc, are usually based on the mg/L water hardness unit.

Now, mg/L stands for milligrams per liter but of what chemical? For water hardness, we commonly use these 3 different mg/L units:

  1. mg/L CaCO3 (calcium carbonate). 1 PPM, for example, is defined as 1 mg/L CaCO3 equivalent. We will also see that French degrees of water hardness are defined as 10 mg/L CaCO3 equivalent.
  2. mg/L CaO (calcium oxide). Degrees of general hardness (°dH) are defined at 10 mg/L CaO2 equivalent.
  3. mg/L Ca2+ (calcium ions). mmol/L CaCO3 (water hardness unit we will explore in the next chapter) can be defined as 40.08 mg/L Ca2+.

Let’s have a look at the mg/L water hardness scale. Now, since we have 3 different chemicals for mg/L water hardness units, we have 3 separate columns here:

Water Hardness Categorization: mg/L CaCO3 Range: mg/L CaO Range: mg/L Ca2+ Range:
“Soft” Water 0 – 60 mg/L CaCO3 0 – 33.62 mg/L CaO 0 – 24.03 mg/L Ca2+
“Moderately Hard” Water 61 – 120 mg/L CaCO3 33.63 – 67.23 mg/L CaO 24.04 – 48.05 mg/L Ca2+
“Hard” Water 121 – 180 mg/L CaCO3 67.24 – 100.85 mg/L CaO 48.06 – 72.08 mg/L Ca2+
“Very Hard” Water Above 181 mg/L CaCO3 Above 100.85 mg/L CaO Above 72.08 mg/L Ca2+

From these 3 mg/L water hardness units, we can see that mg/L Ca2+ are the bigger unit. You can see that you have very hard water if you have above 181 mg/L CaCO3, above 100.85 mg/L CaO, or above 72.08 mg/L Ca2+. On the other hand, mg/L CaCO3 are the smallest of these 3 mg/L units.

You can convert mg/L of CaCO3, CaO, or calcium ions to other water hardness units using these converters:

5. mmol/L Of CaCO3, CaO, Or Calcium Ions

mmol/L is quite similar to mg/L water hardness units. While before we used the weight of water hardness chemicals (mg), we are now using molarity (mmol, to be exact). Thus we have 3 different mmol/L units, namely:

  • mmol/L CaCO3.
  • mmol/L CaO.
  • mmol/L Ca2+.

This mmol/L is especially large water hardnes unit. If we take the smallest of the 3 – mmol/L CaCO – we can see that 1 mmol/L CaCO3 is equal to 100.08 PPM. So, it’s a more than 100 times larger unit than a familiar PPM unit.

Here is the mmol/L water hardness scale for all 3 species:

Water Hardness Categorization: mmol/L CaCO3 Range: mmol/L CaO Range: mmol/L Ca2+ Range:
“Soft” Water 0 – 0.60 mmol/L CaCO3 0 – 0.33 mmol/L CaO 0 – 0.24 mmol/L Ca2+
“Moderately Hard” Water 0.61 – 1.20 mmol/L CaCO3 0.34 – 0.67 mmol/L CaO 0.25 – 0.48 mmol/L Ca2+
“Hard” Water 1.21 – 1.80 mmol/L CaCO3 0.68 – 1.00 mmol/L CaO 0.49 – 0.72 mmol/L Ca2+
“Very Hard” Water Above 1.81 mmol/L CaCO3 Above 1.00 mmol/LCaO Above 0.72 mmol/L Ca2+

As we can see, all these 3 mmol/L units are very big. For example, you need above 180 PPM, for the water hardness to be categorized as “Very Hard” water. We can see that you only need above 1.81 mmol/L CaCO3 measurement for the water to be considered “Very Hard”.

6. °fH / fGH Or French Degrees Of General Hardness

The French historically used their own water hardness unit. This unit is known as French degrees (°fH) or sometimes abbreviated as °fGH because these are degrees of general hardness (that G is from ‘General’).

French degrees are quite straightforward:

1 French degree of water hardness is equal to 10 mg/L of CaCO3 or 10 PPM. French degrees are basically 10x larger units than PPMs.

Here is a water hardness scale for French degrees:

Water Hardness Categorization: French Degree (°fH) Range:
“Soft” Water 0 – 6.00 °fH
“Moderately Hard” Water 6.01 – 12.00 °fH
“Hard” Water 12.01– 18.00 °fH
“Very Hard” Water Above 18 °fH

7. Clark degrees (°Clark) Or English Degrees

The English (UK) developed their own water hardness unit. Specifically, it was a Scottish chemist, Thomas Clark, who developed it and after whom they are named. Clark degrees (°Clark) are sometimes referred to as English degrees (°e).

This is another grain-based water softener unit. Namely, 1 Clark degree water hardness is defined at 1 grain of CaCO3 (64.8 mg) dissolved in 1 Imperial gallon (4.55 liters). Basically, it’s kind of the same as GPG (grains per gallon), but here we use bigger Imperial gallons (US gallons contain 3.79 liters of water, while Imperial gallon contains 4.55 liters).

That’s why GPG and Clark degrees are quantitatively quite similar units. 1 GPG is equal to 1.20 Clark degrees. Since the US gallon is smaller, we get a more concentrated CaCO3. Hence, GPG unit is about 20% larger than Clark degrees.

It is quite useful to know Clark degrees to PPM conversion; 1 Clark degree is equal to 14.254 PPM.

Here is the Clark degree water hardness scale:

Water Hardness Categorization: Clark Degree (°Clark) Range:
“Soft” Water 0 – 4.20 Clark Degrees
“Moderately Hard” Water 4.21 – 8.41 Clark Degrees
“Hard” Water 8.42 – 12.62 Clark Degrees
“Very Hard” Water Above 12.62 Clark Degrees

8. TDS Or Total Dissolved Solids

TDS is not in itself a water hardness unit. However, since we use TDS meters to measure water hardness, we usually refer to water hardness in terms of TDS. Here is an example:

“Is 25 TDS safe for drinking water?”

Yes. 25 TDS is the same as 25 PPM. This is even considered “Soft” water.

Basically, TDS is the same as PPM. Why? Well, because TDS measurement is actually expressed in PPMs.

Example: Let’s say we measure 100 PPM water hardness with a TDS meter. We can call that ‘100 PPM water hardness’, but you will also hear ‘100 TDS water hardness’).

That also means that the TDS water hardness scale is the same as the PPM scale (you can find it in the 1st chapter).

9. EC Or Electrical Conductivity (Microsiemens Per Centimeter)

EC or Electrical Conductivity is similar to TDS measurement. With an EC meter, we measure conductivity in terms of microsiemens per centimeter (µS/cm). We can then convert the measured µS/cm to PPM using an appropriate PPM scale.

Namely, we have 3 different PPM scales, and we need to be aware of those when converting EC measurement to PPM or other water hardness units. Here are the 3 PPM scales:

  1. US uses 500 PPM scale. That means that 1 EC (µS/cm) is equal to 0.5 PPM.
  2. In Europe and internationally, we use the 640 PPM scale. Using these EC meters, 1 EC (µS/cm) is equal to 0.64 PPM.
  3. Australia has its own thing going with a 700 PPM scale. Down-under 1 EC (µS/cm) is thus equal to 0.7 PPM.

We hope that, all in all, you now have a good overview of all water hardness units. You might use 1-3 of them in practice, but we feel the need to cover all of them, hope you liked it.

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