Sulfite Addition Calculator

Free SO₂ ppm alone doesn't tell you how much of it is actually doing antimicrobial work — that depends on molecular SO₂, which the equilibrium formula molecular SO₂ = free SO₂ / (1 + 10^(pH − 1.81)) shows is strongly pH-dependent. At a lower pH like 3.0, a larger share of the free SO₂ converts to the protective molecular form, so you need less free SO₂ ppm to hit the same protection level than you would at a higher pH like 3.6.

The calculator flags protection as adequate once your entered target free SO₂, combined with your wine's pH, computes a molecular SO₂ value of at least 0.5 ppm — the commonly cited minimum threshold for meaningful antimicrobial activity against spoilage organisms. If it shows inadequate, either raise your target free SO₂ or recognize that your wine's higher pH is working against you and will need a larger free SO₂ buffer to reach the same protection.

Two conversions sit between the ppm target and the grams you weigh out: first the ppm gap is converted to milligrams of pure SO₂ using your wine's volume in liters, then that SO₂ mass is divided by 0.576 because potassium metabisulfite powder is only about 57.6% SO₂ by weight — the rest is the potassium and metabisulfite structure carrying the SO₂. So the grams-of-KMS figure is always larger than the raw milligrams of SO₂ needed.

Re-test. The calculation treats your entered current free SO₂ as accurate and assumes the full calculated dose ends up as free SO₂ with no losses, but real additions lose some SO₂ to oxidation during mixing and some binds immediately with acetaldehyde and other wine compounds rather than staying free. The explainer recommends re-testing free SO₂ a few days after addition rather than assuming the target was hit exactly.