Calculate free and molecular SO₂ levels in wine. Adjusts for pH, temperature, and alcohol to ensure proper antimicrobial protection during winemaking.
Sulfur dioxide (SO₂) is one of the main preservatives used in winemaking. It helps limit oxidation and spoilage, but the amount that actually protects the wine depends heavily on pH.
This calculator converts between molecular SO₂ targets, free SO₂ targets, and the amount of potassium metabisulfite or Campden tablets needed to reach them. At lower pH, a smaller amount of free SO₂ produces the same protective effect; at higher pH, more is required.
That makes the page useful when you want to check whether a wine is adequately protected instead of relying on a rough rule of thumb.
SO₂ targets are easy to misjudge because the same free SO₂ reading has a different effect at different pH levels. This calculator helps translate that chemistry into an addition amount you can measure and apply.
Molecular SO₂ = Free SO₂ / (1 + 10^(pH - 1.81)). Required Free SO₂ = Target Molecular × (1 + 10^(pH - 1.81)). K₂S₂O₅ addition: mg = (target ppm - current ppm) × volume(L) / 0.57.
Result: Need 29 ppm free SO₂ → add 0.57g potassium metabisulfite
At pH 3.5: Free SO₂ needed = 0.6 × (1 + 10^(3.5 - 1.81)) = 29.3 ppm. Currently at 15 ppm, need 14.3 ppm more. Volume = 22.7L. Addition = 14.3 × 22.7 / 0.57 = 569 mg ≈ 0.57g K₂S₂O₅.
The molecular SO₂ equation involves a negative logarithmic relationship: molecular SO₂ = free SO₂ ÷ (1 + 10^(pH−1.81)). This means every 0.1 unit increase in pH roughly halves the proportion of molecular SO₂. At pH 3.0, 5.9% of free SO₂ is molecular. At pH 3.5, 1.5%. At pH 4.0, just 0.4%.
Potassium metabisulfite (K₂S₂O₅) is 57% SO₂ by weight. For precise additions, weigh the powder on a 0.01g scale. Campden tablets are pre-measured at ~0.44g per tablet (delivers ~67 ppm/gallon). Tablets are convenient for small batches but imprecise for large volumes.
At crush: 25–50 ppm to inhibit wild yeast (skip for natural fermentation). Post-fermentation: adjust to 0.5–0.8 ppm molecular. At racking: test and adjust. Before bottling: final adjustment. Free SO₂ declines over time as it binds with wine compounds — plan to test and replenish every 4–8 weeks during aging.
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Total SO₂ = Free + Bound. Free SO₂ is the portion available for protection. Molecular SO₂ is the fraction of free SO₂ that's actually antimicrobial — it's determined by pH. Bound SO₂ is tied to compounds and does nothing.
At lower pH (more acidic), a higher proportion of free SO₂ exists in molecular form. At pH 3.0, about 6% of free SO₂ is molecular. At pH 3.5, only 1.5%. At pH 4.0, just 0.4%. This is why high-pH wines need much more SO₂.
0.5 ppm is the minimum for protection against bacteria. 0.8 ppm provides a safety margin. For sweet wines or high-pH wines, target 0.8 ppm. Dry wines at pH ≤3.4 can target 0.5 ppm.
One standard Campden tablet (potassium metabisulfite) adds approximately 67 ppm to 1 gallon. For larger volumes, it's easier to weigh K₂S₂O₅ powder.
Yes. Excessive SO₂ causes a burning sulfur smell and taste. Keep free SO₂ below 50 ppm for most wines. Legal limits: 350 ppm total SO₂ in the US, 200 ppm in the EU (red).
At crush (to inhibit wild yeast), after fermentation, before bottling, and at every racking. Always test free SO₂ before adding — don't guess.