How To Calculate The PPM Concentration In DIY Supplements


New member

I have been thinking of using dry chemicals to dose my reef tanks for Alk, Ca and Mg. And I have been reading on the subject. Randy Holmes Farley's articles have a lot of information. However I am unable to understand how the PPM concentration of supplements as is mentioned in the articles is being calculated by him.

The article I refer to is -

In the article, under Recipe 1, Part 1: The calcium part, 500gm of Calcium Chloride Dihydrate is used. Using the atomic weights of Ca and the other elements, a 1M solution of 147 gm of the dry chemical would give approximately 40,000 ppm. 500gm of the chemical would mean roughly 136,000 ppm. However the article mentions that the solution created using the recipe has only 37,000ppm.

Similarly,Receipe 1, Part 3B, calculates Mg and Sulphates at 47,000 and 187,000 ppm, whereas using the calculations mentioned above I get much higher values.

I maybe incorrect or using a wrong formula.

Is there any article that explains clearly how the ppm can be calculated for a particular element using an amount of dry chemical and water. If any one knows the method Randy has used please do share. Or if Randy reads through this post, then his response will be much appreciated.


-RT * ln(k)
Calcium weighs 40 and CaCl2*2H2O weighs 147. If you divide 40/147 = 0.2721. So that's the fraction of calcium that is in calcium chloride dihydrate. You can go through the moles and back if you want, but in the end the math reduces to that.

500g * 0.2721 = 136.05g of calcium going into 1 gallon of water.

If I divide 136.05g / 3.8L I get 35.8g/L which would be 35,800ppm. Given the rounding, that's pretty darn close to what Randy got.

Are you remembering in your calculations to account for the water of hydration in those weights?


New member
Thanks David.

Yes, I did work it back and forth to get 35800. However I don't get it the same way when it comes to Magnesium.

Magnesium is 20% of MgSO4. The recipe states 8 cups/ 64 ounces, which is 1814 gms. 20% of that is 360 gm per gallon or around 94.7 gm per litre or 94750 ppm, unless and until I am taking an incorrect quantity. Similarly Sulphates works out to 378,000 ppm.


-RT * ln(k)
Which hydrate formula are you using for mag sulfate? The hexahydrate weighs in at 228.5 and the heptahydrate at 246.5. Mag atomic weight is 24, so it would only be ca. 10% by weight of mag sulfate.


New member
I think that's where I have been going wrong. I was not taking the hydrates into account. Makes sense now. Thanks a lot guys.

I am only getting anhydrous chemicals in my city. Can I use anhydrous salts instead of hydrated ones. Apart from having to calculate the concentration correctly is there anything else that I need to worry of when I do that? Also, I am getting lab grade salts. Is that a major advantage?

Randy Holmes-Farley

Reef Chemist
Premium Member
Using anhydrous is fine.

Whether any particular lab grade salts are better or worse (or, in fact, the exact same material), will depend on exactly what the impurities are. :)