What is This?

This calculator is based on an equation developed by Jonathan Gagné to handle mixed methods with both immersion and percolation stages. Although it is actually more than that, a grand unified equation that applies to pure immersion and pure percolation as well as mixed methods.

You can read about it on his post on the subject for the equation integrating the slurry and beverage TDS, and another post of his for accounting for absorbed water. The equation he came up with (adapted to use percents entered as 1.45% instead of .0145) is:

\boxed{Exty=100*(\frac{TDS\; Brew-TDS\; Slurry}{100-TDS\; Slurry}*\frac{Brew\; Mass}{Dose}+\frac{TDS\; Slurry}{100-TDS\; Slurry}*(\frac{Total\; Water}{Dose}-fabs)}

Bit unwieldy to enter by hand, and VST CoffeeTools doesn’t support it, so I’ve thrown together a calculator to help out.

TDS Slurry

Your slurry TDS is a measurement of the dissolved solids that can be applied to everything that didn’t make it to your cup. It’s rather difficult to get this directly from the leftover slurry, but you can get it quite easily by collecting the last few drops and measuring it separately from the main brew measurement. On a true immersion method we can safely take this to be equal to the TDS of the brew, though on a percolation or mixed method is needs to be measured.

It is often assumed to be 0% in percolation, in fact the standard equation only works if it’s assumed to be 0%, but that is rarely if ever reflective of reality.

Fraction of Absorbed Water (fabs)

What is fabs?

I’m only going to touch on the most basic level here, and you should check out Jonathan’s post that details it very nicely for more.

Not all water left in the slurry contributed to the brew. Only the water that was left between particles. The water that worked it’s way into the larger particles and stayed there has only contributed minimally, if at all. The purpose of fabs, the fraction of absorbed water, is to find what fraction of the water is stuck inside the particles and remove it from our calculations.

Right now, I’m unsure what these numbers should be other than some general guidelines like that it should be lower than the LRR, or liquid retained ratio. But once these are found, I intend to share them here. Most likely as a fraction of LRR.

fabs per Method

French Press\(fabs=?\)


This calculator gets you the equivalent of doing the math by hand. It does not include compensation for moisture or CO2 like VST CoffeeTools. That may or may not make it more or less accurate, but it does make it more comparable to other’s measurements.

This method is also not standard. It’s probably more accurate, but that doesn’t mean you can compare numbers you get from it to numbers from someone using the standard equations. I’ve added standard equation results to the calculator to help with that.