Molecular SO2 Calculator

Enter your wine's free SO2 and pH to find the molecular SO2 (the active antimicrobial form), plus the free SO2 needed to hit your target.

Quick Facts

Formula
Molecular SO2 = Free SO2 / (1 + 10^(pH - 1.81))
Uses pKa1 = 1.81 for sulfurous acid. Most wines target 0.5-0.8 mg/L molecular SO2.

Your Results

Calculated
Molecular SO2
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Active antimicrobial form (mg/L)
Molecular fraction
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Share of free SO2 that is molecular
Free SO2 for target
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Free SO2 needed at this pH (mg/L)
Adjustment
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Change vs. current free SO2

Ready

Enter free SO2, pH, and a target, then calculate.

Molecular SO2 in wine: what this calculator does

When you add sulfur dioxide (SO2) to wine, only part of the "free" SO2 you measure is actually able to protect the wine. Free SO2 exists in equilibrium between three species: molecular SO2 (dissolved SO2 gas, the active form), bisulfite ion (HSO3-), and a very small amount of sulfite ion (SO3 2-). Of these, only molecular SO2 is uncharged and small enough to diffuse across microbial cell membranes, so it is the fraction that inhibits spoilage yeast (such as Brettanomyces) and bacteria. This calculator converts your measured free SO2 and wine pH into the molecular SO2 concentration, and tells you how much free SO2 you would need to reach a chosen molecular target.

The formula

The molecular fraction of free SO2 is governed by the first dissociation of sulfurous acid, with pKa1 = 1.81 (at typical wine temperatures near 20 °C):

Molecular SO2 (mg/L) = Free SO2 (mg/L) ÷ (1 + 10^(pH − 1.81))

To find the free SO2 required to hit a target molecular level, rearrange:

Free SO2 needed = Target molecular SO2 × (1 + 10^(pH − 1.81))

The key insight is that pH does most of the work. As pH rises, the term 10^(pH−1.81) grows, the molecular fraction shrinks, and you need dramatically more free SO2 to keep the same protection.

Why pH matters so much

The relationship is exponential in pH, not linear. Each 0.1 increase in pH cuts the molecular fraction by roughly 20%. That is why a high-pH wine can be microbially fragile even at a free SO2 reading that would be perfectly safe in a low-pH wine.

Reference points at 0.8 mg/L target molecular SO2

Using the formula above, the free SO2 needed for the common 0.8 mg/L molecular target is:

  • pH 3.0: about 13 mg/L free SO2 (molecular fraction ~6.1%)
  • pH 3.3: about 25 mg/L free SO2 (molecular fraction ~3.2%)
  • pH 3.5: about 40 mg/L free SO2 (molecular fraction ~2.0%)
  • pH 3.7: about 63 mg/L free SO2 (molecular fraction ~1.3%)
  • pH 4.0: about 125 mg/L free SO2 (molecular fraction ~0.6%)

Because legal total SO2 limits (for example, 350 mg/L in the US for table wine) and sensory thresholds cap how much SO2 you can add, protecting a wine above pH ~3.8 by SO2 alone becomes impractical — winemakers usually lower pH with tartaric acid first.

Frequently Asked Questions

What molecular SO2 level should I target?
The widely cited target for microbial stability is 0.8 mg/L molecular SO2, which is considered sufficient to inhibit most spoilage organisms including Brettanomyces. A minimum of about 0.5 mg/L is often used for wines that will be consumed soon or are otherwise low-risk. Below ~0.5 mg/L, free SO2 provides little antimicrobial protection regardless of the total number on the label.
Why does the same free SO2 protect one wine but not another?
Because pH sets the molecular fraction. At pH 3.3, 25 mg/L free SO2 gives ~0.8 mg/L molecular; at pH 3.7, that same 25 mg/L free SO2 gives only ~0.32 mg/L molecular — below the protective range. Two wines with identical free SO2 readings can be worlds apart in real protection if their pH differs.
Which pKa value does this calculator use, and does temperature matter?
It uses pKa1 = 1.81, the standard value for sulfurous acid near 20 °C used throughout enology references. The equilibrium is mildly temperature-dependent (molecular SO2 rises a little in colder wine), but the fixed-pKa formula is the accepted industry approximation and is accurate enough for cellar decisions. Measure free SO2 by aeration-oxidation or Ripper titration for the most reliable input.

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