PPM To mmol/L Water Hardness Calculator: Insert PPM, Get mmol/L CaCO3

We can express water hardness in PPMs as well as in mmol/L of CaCO3 (calcium carbonate). It is quite easy to convert PPM to mmol/L. We will show you exactly how you can calculate how many mmol/L is any amount of PPMs. On top of that, you will get these 2 free online tools that make this conversion much easier:

  1. PPM To mmol/L Calculator. Here you just insert PPMs, and the calculator will automatically convert it to mmol/L of CaCO3.
  2. PPM To mmol/L Chart. We have converted 0-1,000 PPM to mmol/L, and summarized all the results in a neat conversion chart. In the chart, we have also color-coded water hardness categorization, namely soft water (0-60 ppm) with this green color, moderately hard water (61-120 ppm) with this light blue color, hard water (121-180 ppm) with this blue color, and very hard water (181+ ppm) with this dark blue color.

Here is the key PPM to mmol/L of CaCO3 formula (Parts Per Million to millimol per Liter):

Water Hardness (mmol/L of CaCO3) = PPM / 100.09

Basically, to get mmol/L, we have to divide PPMs by 100.09. Roughly speaking, 1 PPM is equal to 0.01 mmol/L of CaCO3 (we make less than 0.1% accuracy error with this approximation).

Quick Example: How many mmol/L of CaCO3 is 100 PPMs? Let’s use the formula above:

Water Hardness (mmol/L of CaCO3) = 100 PPM / 100.09 = 0.999 mmol/L Of CaCO3

We almost always use 100 PPM = 1 mmol/L of CaCO3 approximation. In the following calculator and chart, you can see that we calculate the resulting mmol/L to 2 decimal digits. You can freely use both of these tools here:

PPM To mmol/L Of CaCO3 Calculator


Let’s say you want to convert 120 PPM to mmol/L of CaCO3. Use the PPM slider, slide it to ‘120’, and you get the result:

120 PPM is equal to 1.20 mmol/L of CaCO3.

Here is the whole conversion chart you can consult as well:

PPM To mmol/L Of CaCO3 Chart

PPM (Parts Per Million): mmol/L Of CaCO3 (Calcium Carbonate):
0 PPM 0 mmol/L Of CaCO3
1 PPM 0.01 mmol/L Of CaCO3
5 PPM 0.05 mmol/L Of CaCO3
10 PPM 0.10 mmol/L Of CaCO3
15 PPM 0.15 mmol/L Of CaCO3
20 PPM 0.20 mmol/L Of CaCO3
25 PPM 0.25 mmol/L Of CaCO3
30 PPM 0.30 mmol/L Of CaCO3
35 PPM 0.35 mmol/L Of CaCO3
40 PPM 0.40 mmol/L Of CaCO3
45 PPM 0.45 mmol/L Of CaCO3
50 PPM 0.50 mmol/L Of CaCO3
60 PPM 0.60 mmol/L Of CaCO3
70 PPM 0.70 mmol/L Of CaCO3
80 PPM 0.80 mmol/L Of CaCO3
90 PPM 0.90 mmol/L Of CaCO3
100 PPM 1.00 mmol/L Of CaCO3
110 PPM 1.10 mmol/L Of CaCO3
120 PPM 1.20 mmol/L Of CaCO3
130 PPM 1.30 mmol/L Of CaCO3
140 PPM 1.40 mmol/L Of CaCO3
150 PPM 1.50 mmol/L Of CaCO3
160 PPM 1.60 mmol/L Of CaCO3
170 PPM 1.70 mmol/L Of CaCO3
180 PPM 1.80 mmol/L Of CaCO3
190 PPM 1.90 mmol/L Of CaCO3
200 PPM 2.00 mmol/L Of CaCO3
250 PPM 2.50 mmol/L Of CaCO3
300 PPM 3.00 mmol/L Of CaCO3
350 PPM 3.50 mmol/L Of CaCO3
400 PPM 4.00 mmol/L Of CaCO3
450 PPM 4.50 mmol/L Of CaCO3
500 PPM 5.00 mmol/L Of CaCO3
550 PPM 5.50 mmol/L Of CaCO3
600 PPM 6.00 mmol/L Of CaCO3
650 PPM 6.50 mmol/L Of CaCO3
700 PPM 7.00 mmol/L Of CaCO3
750 PPM 7.50 mmol/L Of CaCO3
800 PPM 8.00 mmol/L Of CaCO3
850 PPM 8.50 mmol/L Of CaCO3
900 PPM 9.00 mmol/L Of CaCO3
950 PPM 9.50 mmol/L Of CaCO3
1000 PPM 10.00 mmol/L Of CaCO3

If you check the colors in this chart (or consult this water hardness PPM scale), you can see that:

  • Soft water contains from 0 to 0.60 mmol/L of CaCO3.
  • Moderately hard water contains from 0.61 to 1.20 mmol/L of CaCO3.
  • Hard water contains from 1.21 to 1.80 mmol/L of CaCO3.
  • Very hard water contains more than 1.81 mmol/L of CaCO3.

With the calculator and this chart, you will quite easily convert PPM to mmol/L of CaCO3. If you need some help, you can use the comment section below, give us some numbers, and we can help you make the conversion.

Here are some similar calculators that might be helpful:

2 thoughts on “PPM To mmol/L Water Hardness Calculator: Insert PPM, Get mmol/L CaCO3”

    • Hi Rebecca, you are probably referring to PPM to dH or dGH conversion (these are known as degrees of Hardness or degrees of General Hardness, also known as German degrees). We have a PPM to dGH calculator here; you can see that 138 PPM is equal to 7.73 degrees of Hardness (dH / dGH).

      138 PPM or 7.73 dH is categorized as “Hard” water. Namely, water with hardness from 121 to 180 PPM (or 6.73 to 10.09 dH) is considered “Hard” water. Above that (181+ PPM or 10.10 dH) is considered “Very Hard” water, while 61 PPM to 120 PPM is considered “Moderately Hard” water. Below that, you have “Soft” water. Hope this helps.


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