Sulfur Calculator (Wine)

Molecular SO₂ (also called active SO₂) is the undissociated form of sulfur dioxide that actually provides antimicrobial protection in wine. Only a small fraction of the free SO₂ exists as molecular SO₂ — and it's this fraction that inhibits yeast and bacteria. Total or free SO₂ numbers alone don't tell you if your wine is adequately protected; molecular SO₂ does.

pH is the single most important factor determining how much of your free SO₂ is in the active molecular form. Lower pH (more acidic) wines have a higher proportion of molecular SO₂. At pH 3.0, roughly 6% of free SO₂ is molecular, whereas at pH 3.8, only about 0.6% is molecular — meaning you need significantly more free SO₂ at higher pH to achieve the same protection level.

Most winemakers target 0.5 mg/L molecular SO₂ for dry red wines, 0.8 mg/L for dry white and rosé wines, and 1.0–1.5 mg/L for sweet or dessert wines. These levels effectively suppress microbial activity while keeping total SO₂ within legal and sensory limits.

Temperature influences the dissociation constant (Ka) of sulfurous acid, which shifts how free SO₂ distributes between molecular, bisulfite, and sulfite forms. Warmer wines have slightly different molecular SO₂ fractions than cold-stored wines, so it's important to measure or adjust at the actual wine temperature.

Yes, alcohol affects the activity of SO₂ in wine to a small but measurable degree. Higher alcohol wines have slightly different SO₂ equilibria, and some winemaking references adjust the effective pH slightly based on ABV when calculating molecular SO₂ targets.

Free SO₂ is the portion of sulfur dioxide not bound to other wine components (like acetaldehyde, sugars, or pigments). Total SO₂ is the sum of free and bound SO₂. Only free SO₂ contributes to molecular SO₂ and antioxidant/antimicrobial protection. Bound SO₂ is largely inactive. This calculator uses free SO₂ as the input.

Legal limits for total SO₂ vary by region and wine style. In the EU, typical limits are 150 mg/L for dry red wines, 200 mg/L for dry white and rosé, and up to 400 mg/L for some sweet wines. In the US, wines containing 10 ppm or more SO₂ must display 'Contains Sulfites' on the label. Always check current regulations for your specific market.

First, use this calculator to determine how much free SO₂ you need to hit your molecular SO₂ target. Then subtract your current free SO₂ to find the deficit. Common additions include potassium metabisulfite (KMS/SMS) powder or solution, or gaseous SO₂. A standard calculation is: grams of KMS needed = (volume in liters × SO₂ deficit in mg/L) ÷ 570.