Water Hardness Scale: GPG, mmol/L, PPM Chart

Nobody wants to have hard water in the house. Damage to internal piping, appliances, limescale build-up, and skin irritation are just a few problems that hard water causes. In order to figure out if our water is too hard, we have to consult the water hardness scale (preferably a hard water ppm chart we list below).

Example of white stains hard water causes. Limescale build-up in internal piping is extremely problematic.

According to the water hardness scale, more than 85% of US households have hard water in their piping. How to know if you have hard water? Simple:

The water hardness scale is a benchmark on how hard our water is.

Here is a quick hard water ppm chart that gives you a spectrum of water hardness in parts per million (ppm):

water hardness spectrum and scale

Scale Of Water Hardness (From Soft To Very Hard Water)

The internationally recognized water hardness scale recognizes 4 water hardness classes; from soft, moderately hard, hard, to very hard. By definition, water hardness is the total sum of molar concentrations of calcium ions (Ca2+) and magnesium ions (Mg2+).

We have there three main units with which we denote water hardness:

  • Parts per million or PPM is equivalent to 1 mg/L CaCO3 (calcium carbonate). This is the same as mg/L (even if the ions are not specified; could be calcium ions, magnesium ions, or both). PPM is also equivalent to American degrees.
  • Grains per gallon or GPG.
  • Millimoles of calcium carbonate (CaCO3) per liter or mmol/L.

We also have other less-known units, such as degrees of general hardness (dGH or German degrees), Clark degrees or English degrees, French degrees, and so on.

In the US, the unit of water hardness is usually parts per million (ppm) or grains per gallon (GPG).

Here is the full water hardness scale (including hard water ppm chart) according to the USGS:

Classification Hardness (PPM) Hardness (GPG) Hardness (mmol/L)
Soft 0–60 0–3.50 0–0.60
Moderately hard 61–120 3.56-7.01 0.61–1.20
Hard  121–180 7.06-10.51 1.21–1.80
Very hard  ≥ 181 ≥ 10.57 ≥ 1.81

mmol/L is a metric unit, usually in use outside of the US.

How to tell if you have hard water?

First, you have to measure the water hardness with a water hardness test. You can get it online for less than $20.

If you measure that your water hardness is below 60 ppm or 3.50 GPG, you don’t have to worry about hard water. By classification, you have soft water.

The reality is worse:

More than 90% of people find that their water is either moderately hard, hard, or very hard. Essentially, getting a 100+ ppm water hardness is not unusual; it’s actually very common.

What To Do If You Find Out You Have Hard Water?

The easiest thing is to just ignore the problem. That’s not recommended.

The immediate effects of using hard water might be apparent on your skin or on your dishes, and faucets. Hard water with a high concentration of calcium and magnesium ions causes white stains pretty much everywhere.

The more concerning are the long-term effects. Hard water will eventually lead to limescale build-up in your piping, washing machine, and so on, thereby limiting the lifespan of essential plumbing.

Here’s the deal:

You might not like it but fighting hard water is a smart long-term decision. Pretty much the only way that is very effective at softening water and eliminating all the problems that come with it is by installing a water softener. You can find a list of the best water softeners here.

These devices are designed to reduce the water hardness from 200, 300, or even 400 ppm to below 60 ppm. This process is simply called water softening.

10 thoughts on “Water Hardness Scale: GPG, mmol/L, PPM Chart”

  1. We are considering a move and I have been studying water quality lately.

    Neurosurgeon and Health Educator Dr. Jack Kruse favors hard water, high altitude spring waters and DDW Deuterium Depleted Water for health so while hard water may need to be avoided in pipes and household appliances drinking and water to cook with may matter a lot more that is commonly known for health.
    I’ve read that studies have shown hard water areas have less heart issues.

    This bears more research.
    Let me know what you know about this.

    • Hi Martha, thank you for the great insight. Well, basically, the water hardness scale is to determine the effect on pipes and water-using appliances such as washing machines. The effect on human health, however, is altogether a different story and there are many diverse opinions of the matter. Our solemn goal here is to protect the piping and appliances.

  2. My water ph is 7 with no traces of iron. The 7 number is in the range of bottled and municipal water. My well is not in a limestone formation. Do I have water that needs softening?

    • Hi John, in general, pH can give some information about hardness (but it’s not the complete picture). Again, generally speaking, soft water will have acidic properties (it will have a pH below 7.0). If you have a 7 pH water, you probably don’t need softening. If the pH would be 8, then you might consider it. Hope this helps.

  3. A few thoughts on water hardness. Heating water decreases solubility, a somewhat unique characteristic of this solvent. It is evident in a used tea kettle. If there is a lot of scale inside the kettle, the water is hard and deposits as the water is warmed.

    For healthy people, it is not a concern. Indeed dentists have said that well water from farm wells contributes to strong teeth. This varies by the geological region, the rock strata but is common.

    The tea kettle deposits are stone, a lousy heat transfer media so it takes more energy to force the heat from the flame through the insulator into the water. Engineers worry about this cost; most folks do not care. If you care, occasionally heat common vinegar in the kettle; this mild acid dissolves the scale.

    • Hi there, hard water is tough on pipes and appliances that use water. Health-wise, we don’t have a definitive answer; it might be bad, good, or it just doesn’t matter. Engineers worrying about the cost due to loss of heat exchange is a valid point as well.

  4. Martha,

    In the UK, it’s recommended that in hard water areas the use of a water softener will protect pipes, washing machines, etc., but that the feed to the kitchen cold tap be unsoftened because of the statistics to which you refer. This is the configuration we have been using for many years (~30 years) with no ill-effects on health.

    • Hi William, thank you for your input. You are correct; the primary reason to use water softeners is to protect pipes and appliances. There seems to be no known correlation between hard water and health. I’ve seen argumentation that hard water might even be good for health since it provides the body with minerals like calcium and magnesium. So, no comment about health, the whole point is to protect our pipes and appliances.


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