What is the SO₂ Wine Calculator?
Sulfur dioxide (SO₂) protects wine from oxidation and microbial spoilage. Winemakers commonly add it as potassium metabisulfite (KMS), a stable powder that releases roughly 57% of its weight as SO₂. This calculator tells you exactly how many grams of KMS to dissolve and add to reach your desired free SO₂ level.
How to use it
Enter your target free SO₂ in ppm (mg/L), your current free SO₂ measured by titration or test kit, and the volume of wine in liters. The calculator returns the grams of KMS to add. If your current level already meets or exceeds the target, no addition is required.
The formula explained
The dose equals the SO₂ increase needed (target minus current, in mg/L) multiplied by the volume in liters, which gives milligrams of pure SO₂. Dividing by 1000 converts to grams, and dividing by 0.5697 accounts for KMS being only about 57% SO₂ by weight:
$$\text{KMS (g)} = \frac{\left(\text{Target SO}_2 - \text{Current SO}_2\right) \times \text{Volume (L)}}{1000 \times 0.5697}$$
Worked example
Suppose you have 20 L of wine at 10 ppm free SO₂ and want to raise it to 35 ppm. The increase is 25 ppm. So $$\text{KMS} = \frac{25 \times 20}{1000 \times 0.5697} = \frac{0.5}{0.5697} \approx 0.878 \text{ grams}$$ of potassium metabisulfite.
FAQ
What is free SO₂ vs total SO₂? Free SO₂ is the active, protective portion; total includes SO₂ that has bound to compounds in the wine. Always dose against the free SO₂ reading.
Why 0.5697? Potassium metabisulfite (K₂S₂O₅) yields about 57% of its mass as SO₂ — this calculator uses 0.5697 (≈57%). Some suppliers cite 50–57%; check your product label and adjust.
How should I add the KMS? Dissolve the measured powder in a small amount of wine or water, then stir gently into the full batch. Re-test free SO₂ after mixing to confirm.
