SO2 Wine Calculator
Enter your wine pH and current free SO2 to see the molecular (active) SO2 level, compare it with the recommended target for your wine type, and get the exact amount of potassium metabisulfite or Campden tablets to add. All results update as you type.
Why molecular SO2 matters more than free SO2
Wine producers typically measure free SO2 (the total unbound sulfur dioxide dissolved in wine), but the part that actually fights bacteria and oxidation is a tiny fraction called molecular SO2 (sometimes written as SO2 molecular or active SO2). Free SO2 exists in three forms in wine: molecular SO2, bisulfite (HSO3-), and sulfite (SO32-). Only the molecular form is volatile and antimicrobial. Because the distribution between these forms depends almost entirely on pH, a wine at pH 3.2 needs far less free SO2 than the same wine at pH 3.8 to maintain the same protective level. This is why pH management is so central to SO2 efficacy.
How to add SO2 correctly
The three most common SO2 sources for home and professional winemakers are potassium metabisulfite (KMS) powder, Campden tablets, and a 10% KMS stock solution. KMS powder dissolves to release SO2, but only about 57% of its weight becomes free SO2, and the rest bonds immediately. Campden tablets each contain 440 mg of KMS, releasing roughly 250 mg of free SO2. A 10% stock solution is 10 g of KMS dissolved in 90 mL of water, giving roughly 57 mg of free SO2 per mL of solution (adjusted for efficiency). Always dissolve KMS in a small amount of the wine being treated before stirring it gently through the bulk. Never add dry powder directly to a barrel or tank without pre-dissolving, as it can create localised hot spots and off-aromas. After any addition, wait at least 24 hours before re-testing free SO2.
When to test and add SO2
Free SO2 should be tested and, if necessary, adjusted before each major cellar operation: racking, filtering, fining, blending, and before bottling. Between operations, test every 4-8 weeks for wines in barrel or unlined tanks, which allow more SO2 to escape. Wines in sealed glass or lined stainless steel lose SO2 more slowly. The timing also matters for the wine stage: young wines with high total SO2 may not need additions as quickly, while older wines and those at high pH will deplete their free SO2 faster. Always target the correct molecular SO2 level for your wine type and the current pH of the wine, not a fixed free SO2 number.
Limitations and special cases
This calculator uses a pKa of 1.81 for the first dissociation of SO2 in wine, the value most widely accepted in enological practice (derived from Boulton et al., Principles and Practices of Winemaking). Some references use slightly different values (1.75-1.85), which will shift results by a few mg/L. For red wines, the presence of anthocyanins (colour pigments) that bind SO2 means laboratory-measured free SO2 can overstate the truly available fraction; additional acetaldehyde-bound SO2 can also mask the true free pool. Some red winemakers therefore aim for slightly higher free SO2 targets at the same pH. Wines high in sugar (above about 40 g/L residual) will also bind more SO2. This calculator does not include corrections for these binding effects; use it as a starting guide and adjust based on your own titration results and winemaking experience.
Recommended free SO2 by pH and wine type
| pH | Red wine (mg/L) | White / Rose (mg/L) | Dessert wine (mg/L) |
|---|---|---|---|
| 3 | 16 | 25 | 47 |
| 3.1 | 19 | 31 | 58 |
| 3.2 | 24 | 38 | 72 |
| 3.3 | 29 | 47 | 88 |
| 3.4 | 36 | 58 | 108 |
| 3.5 | 45 | 72 | 135 |
| 3.6 | 56 | 90 | 168 |
| 3.7 | 70 | 111 | 208 |
| 3.8 | 87 | 139 | 261 |
| 3.9 | 110 | 175 | 328 |
| 4 | 138 | 220 | 413 |
Target free SO2 values (mg/L) to maintain the recommended molecular SO2 level. Red wines: 0.5 mg/L molecular; whites: 0.8 mg/L molecular; dessert: 1.5 mg/L molecular.
Frequently asked questions
What is molecular SO2 and why is it important?
Molecular SO2 (also called active SO2 or free molecular sulfur dioxide) is the form of sulfur dioxide dissolved in wine as individual SO2 molecules rather than bisulfite or sulfite ions. It is the only form that is volatile, antifungal, and antibacterial. Research shows that 0.5 mg/L is the minimum effective level to inhibit most spoilage organisms in red wines, and 0.8 mg/L is recommended for whites. Dessert wines may need up to 1.5 mg/L because of higher residual sugar. Measuring free SO2 alone is not enough because the split between molecular SO2 and bisulfite depends entirely on the wine pH.
What is the formula for calculating molecular SO2?
The standard formula is: Molecular SO2 = Free SO2 / (1 + 10^(pH - 1.81)). The constant 1.81 is the pKa for the first dissociation of sulfurous acid in wine. This means that at pH 3.4, for every 58 mg/L of free SO2, approximately 1 mg/L is in the antimicrobial molecular form. The rest is bisulfite. As pH rises, a far larger fraction of free SO2 is tied up as bisulfite and you must add much more free SO2 to maintain the same molecular level.
How much potassium metabisulfite do I add per litre?
To raise free SO2 by 1 mg/L in 1 litre of wine, you need approximately (1/1000) x (190.32/128.13) / 0.57 grams of KMS, which works out to about 2.6 mg per litre per 1 mg/L increase in free SO2. For a practical example: to raise free SO2 by 20 mg/L in a 20-litre batch, you would need roughly 20 x 20 x 0.0026 = 1.04 g of KMS powder. This calculator works out the exact figure based on your inputs.
Can I use Campden tablets instead of KMS powder?
Yes. Standard Campden tablets weigh 440 mg each and contain potassium metabisulfite. When dissolved in wine they release approximately 250 mg of free SO2 (57% of 440 mg). To use this calculator with Campden tablets, select that option in the addition method selector and the result will tell you how many tablets you need, to the nearest quarter tablet. For small home batches, Campden tablets are very convenient; for larger volumes, KMS powder measured on a scale is more precise.
Why does pH affect how much SO2 I need?
Wine pH determines what fraction of free SO2 stays as active molecular SO2. The relationship follows an acid-base equilibrium. At pH 3.0, about 3% of free SO2 is molecular. At pH 3.5, it drops to about 1.1%, and at pH 4.0 it is only about 0.36%. This means a wine at pH 4.0 needs roughly eight times as much free SO2 to achieve the same protection as a wine at pH 3.0. It is one of the main reasons winemakers try to keep pH below about 3.6, since above that level the free SO2 additions needed become impractically large.
How do I measure free SO2 in wine?
The two most common home and small-winery methods are the Ripper titration (using iodine solution and a starch indicator) and the Aeration-Oxidation (AO) method. Ripper is quick and cheap but can overread slightly in red wines because anthocyanins react with the iodine. The AO method is more accurate, especially for reds, but requires more equipment. Commercial testing labs use a modified Ripper or an enzymatic method for high accuracy. For best results, test within 30 minutes of sampling to prevent SO2 loss.
What are the signs that my wine has too little SO2?
Early signs of SO2 deficiency include browning of white wines (oxidation), loss of fresh fruit aromas, and the development of a sherry-like or cooked character. In more severe cases, bacterial spoilage can produce off-aromas like volatile acidity (vinegar), mousiness, or geranium notes from Brettanomyces activity. Any unexplained haze after filtration can also suggest microbial activity related to insufficient SO2.