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Discussion Starter · #1 ·
Please help me calculate my GH given these water parameters:

I’m embarrassed to say that after so many years in this hobby, I still can’t figure out how to calculate my GH.

Here are my water parameters:
• Calcium 10ppm
• Magnesium 4ppm
• Nitrate 9ppm
• Potassium 16ppm

I know GH only factors in Calcium and Magnesium level. But, if that’s the case isn’t that a poor indicator of true water quality? What if my Potassium level and Sulfate level are off the roof, like over 200ppm of each? Should GH and TDS reading be used in conjunction to get a true picture of your water parameter? I feel that GH is often over emphasized while TDS is neglected. BTW, my TDS meter reads 140 – does that sound about right given what I have above and using mostly RO/DI water or do I need to get it recalibrated?

I know KH is a function of GH and that the two are interconnected. So, If I have 2 degree of KH, does that mean I need that add that to my GH – I’m confused?

Lastly, can someone please confirm that 1 teaspoon of Calcium Carbonate raises approx. 1 degree of GH and 1 degree of KH to 25 gallon of water?


I ♥ BBA!
12,768 Posts
Here is the formula for Mg:

GH(ppm*) - 2.5 x Calcium(ppm*) = Mg(ppm*)

Since you know your Ca and Mg, levels using the above formula, you can extract GH to have the following formula:

GH (ppm) = 4.1 x Mg (ppm) + 2.5 x Ca (ppm)

GH = 4.1 x 4 (ppm) + 2.5 x 10 (ppm)

GH = 16.4 + 25

GH = 41.4 (ppm)

GH = 2.3 (dGH)

*If you have the figures in degress, multiply by 17.86


The answers to your other questions are probably within the articles I sourced, but honestly I don't remember without reading all the details. Hope the calculation at least helps.
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