Hi all!

I'm hoping an expert here can take a look at my water formula calculations and check for accuracy. I've adopted a spreadsheet intended to make concentrates that was originally based on some slightly incorrect assumptions from Barista Hustles numbers and tweaked it so I don't have to fiddle with concentrates and can just put everything directly into a gallon of distilled water.

As an example, let's say I'm using baking soda and Epsom salt and am aiming for 85 mg/l GH (85.173 ppm at 70 degrees f) and 57 mg/l of KH (57.116 ppm at 70 degrees f)

Water weight: 3851 g
Baking soda: 0.3695 g
Epsom salt: 0.8084 g

Baking soda calculations: 0.00168014 [2 x molecular weight baking soda / 100,000] * 57.11616084 [target KH as ppm] * (3851 g water/1000)

Epsom salt calculations: 0.0024647296 [molecular weight of MgSO4.7H2O / 100,000] * 85.1732223 [target GH as ppm] * (3851 g water/1000)

I'm not at all a chemistry guy, but for a better cup of coffee I've tried to dig in. Depending on what source I use, I get slight variations in the actual weight of Epsom salt but I think I've gotten close? Hopefully? So basically I plug in my desired GH and KH, weigh out the water, and I get my recipe.

Anyway, I'd be greatly appreciative if anyone here would be willing to take a look at my logic and see if it all ties out.
Thanks in advance.

Your results are essentially correct, although I see issues:

1. Doing these calculations is tedious enough without trying to make adjustments for water density at different temps. And it's more complicated than that anyway when you consider the effect of added minerals on water density. It's best to just say that a mass fraction of 1 ppm = 1 mg/L. And no need to get carried away with more than 3 or 4 significant digits.

2. 1 gallon = 3851 gram? I (and I think most everyone) just assumes that 1 US gallon = 3785 ml = 3785 gram in these sorts of calcs.

3. Your calculations use canned factors from Barista Hustle with no units. Most people reading this would be befuddled about where those magic number factors (e.g., "2 x molecular weight baking soda / 100,000") come from.

Here's how I would calculate it:
Given:
1 US gallon = 3.79 L
Molar mass MgSO4.7H2O = 246 mg/mmol
Molar mass NaHCO3 = 84.0 mg/mmol
Molar mass CaCO3 = 100 mg/mmol

Calculate molarity of 808 mg MgSO4.7H2O per US gallon:
808 mg / ( 246 mg/mmol * 3.79 L ) = 0.867 mmol/L

Calculate molarity of 370 mg NaHCO3 per US gallon:
370 mg / ( 84.0 mg/mmol * 3.79 L ) = 1.16 mmol/L

(notice that all the units cancel out properly to give you those mmol/L value.)

Now it's easy to convert mmol/L to the conventional CaCO3 equivalent units. For the divalent (Mg++) ion you multiply by 100 (the molar mass of CaCO3) and for the univalent (HCO3-) ion you multiply by 50 (half the molar mass of CaCO3):

0.867 mmol/L * 100 = 86.7 mg/L CaCO3 equivalent magnesium hardness
1.16 mmol/L * 50 = 58 mg/L CaCO3 equivalent bicarbonate alkalinity

A little tedious, but doing it this way makes it clear where the factors come from and that you didn't divide where you needed to multiply (because the units cancel out right.)

More info on these sorts of calculations can be found in this and following posts: Water Gurus, please chime in

Awesome, I should've clarified more on some of the aspects, for instance, a gallon of water here is almost always overfilled, that number was just from my latest measurement of a distilled gallon. And especially for the magic numbers as you put it

But seriously, thank you, and it's actually a bit of an honor to have to reply as I've been reading this forum for a while trying to grasp as much as I can (and specifically looking for your replies) and it has never ceased to amaze me that you reply so often with such in depth knowledge...and all for free too! It brightens my day that there are people like you so willing to spread information and your expertise.

I'll take your suggestions and make my updates to my spreadsheet. Thanks again!