Molecular SO2 Calculator
Only the molecular form is antimicrobial, and at wine pH almost all of it is not.
The formula
molecular SO2 = free SO2 / (1 + 10^(pH - 1.81))
Free sulfur dioxide tells you almost nothing
Only the molecular form of SO2 is antimicrobial, and how much of the free SO2 is in that form depends entirely on pH. Sulfur dioxide is a weak acid with a pKa of 1.81, so at wine pH the great majority is bisulfite and inert. A wine at pH 3.2 has more than twice the molecular SO2 of one at pH 3.6 with identical free SO2.
The target is 0.5 to 0.8 mg/L
Below about 0.5 mg/L molecular there is not enough to inhibit spoilage organisms; above roughly 0.8 it starts to be perceptible on the nose. That narrow window is what a winemaker is actually aiming at, and hitting it means measuring pH and free SO2 together rather than dosing to a number.
High-pH wine needs far more
Because the relationship is logarithmic, a wine at pH 3.8 needs roughly four times the free SO2 of one at pH 3.2 for the same protection — enough that it becomes sensorially obvious and legally constrained. This is a large part of why acidification is done at all: it is cheaper to lower the pH than to sulfur past the point of taste.
Recipes are starting points
Ovens run hot or cold by 20 degrees, flour absorbs water differently by season and brand, and the same joint of meat cooks at a different rate depending on its shape rather than its weight. Everything here is a planning figure that gets you close enough to start, and the thermometer, the scales and your own notes from last time are what finish the job.