Wine SO₂ Addition Calculator (Free Sulfite by pH)
Free SO₂ target from wine pH (0.5 or 0.8 mg/L molecular), the addition needed in mg/L, and the amount of potassium metabisulfite powder, Campden tablets or 10% solution for your volume.
How it works
Free SO₂ (mg/L) = molecular target × (1 + 10^(pH − 1.81)); KMS grams = mg/L × litres ÷ 0.57 ÷ 1000; 1 Campden tablet ≈ 0.44 g KMS ≈ 50 mg/L in 4.5 L
Sulfur dioxide protects wine from oxidation and spoilage microbes, but only its molecular form does the antimicrobial work, and how much of the free SO₂ is molecular depends steeply on pH: at pH 3.0 about 6% is molecular, at 3.8 barely 1%. That is why the same 30 mg/L that guards a crisp low-pH white does little for a soft high-pH red, and why winemakers target a molecular level — 0.5 mg/L for reds, 0.8 for whites and sweet wines — and back-calculate the free SO₂ needed from pH. Potassium metabisulfite yields about 57% of its weight as SO₂. New additions partly bind to sugars and aldehydes, so measure free SO₂ a week later and top up.
Worked example
A 20 L white at pH 3.5 with 10 mg/L free SO₂, target 0.8 molecular: needs 40 mg/L free, so add 30 mg/L — 0.60 g of 57% potassium metabisulfite, or about 1.4 Campden tablets.
Frequently asked questions
How much SO₂ at crush?
Typically 30–50 mg/L (about 1 Campden tablet per 4.5 L gives 50) for sound fruit, more for mouldy grapes; wait 24 h before pitching yeast.
Does SO₂ cause headaches?
Rarely — sulfite sensitivity affects a small minority, mostly asthmatics. Dried fruit carries far more sulfite than wine. Histamines and alcohol are the more common culprits.
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This calculator is for educational purposes. Double-check important results before acting on them.