Sulfur Calculator (Wine)

Enter your wine's free SO₂ (ppm) and pH to calculate the molecular SO₂ — the active antimicrobial form that actually protects your wine. You can also input temperature and alcohol by volume to refine the result. The calculator returns molecular SO₂ (mg/L), the percent active SO₂, and whether your wine is in a safe protection range. Also try the Smoothie Ratio.

mg/L (ppm)

The free SO₂ measured in your wine (typically via titration)

The measured pH of your wine. Typical range: 3.0–3.8

°C

Temperature at which the wine is being stored or measured

%

Alcohol content of your wine

Used to show the recommended molecular SO₂ target for your wine style

mg/L

Typical targets: 0.5 mg/L (red), 0.8 mg/L (white/rosé), 1.0–1.5 mg/L (sweet wines)

Results

Molecular SO₂

--

% of Free SO₂ that is Molecular

--

Protection Status

--

Free SO₂ Needed for Target Molecular SO₂

--

SO₂ Adjustment Needed

--

Frequently Asked Questions

What is molecular SO₂ and why does it matter for 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. See also our Priming Sugar Calculator.

How does pH affect molecular SO₂ in wine?

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.

What is the recommended level of molecular SO₂ for wine?

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.

How does temperature affect the molecular SO₂ calculation?

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. You might also find our find Servings From Keg with Keg Volume Calculator useful.

Does alcohol content (ABV) affect molecular SO₂?

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.

What is the difference between free SO₂ and total SO₂?

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.

What are the legal SO₂ limits in wine?

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.

How do I add SO₂ to reach a target molecular SO₂ level?

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.