Sulfite Addition Calculator (Wine SO2)
Calculate potassium metabisulfite (K-Meta) or Campden tablet dose to achieve target free SO2 in wine.
Accounts for wine pH for accurate protection.
Sulfite (SO2) addition is the most critical chemical intervention in winemaking. Sulfur dioxide protects wine from oxidation and microbial spoilage, preserving color, aroma, and flavor.
The Formula:
SO2 to add (mg/L = ppm) = Target free SO2 − Current free SO2
Potassium Metabisulfite (KMS) needed (grams) = (SO2 ppm × Volume in liters) / 570
The factor 570 accounts for the fact that KMS is approximately 57% effective SO2 by weight.
Molecular SO2 (Active Protection):
Only a sliver of the free SO2 in wine does the actual protecting. That sliver is molecular SO2, and the fraction of free SO2 present in that form collapses as pH rises, because sulfurous acid has a pKa of 1.81:
Free SO2 needed = Molecular target × (1 + 10^(pH − 1.81))
Two molecular targets are in common use: 0.5 mg/L for reds, where tannin and colour give some protection of their own and higher additions taste harsh, and 0.8 mg/L for whites and rosés, which have neither and oxidise faster.
| Wine pH | Free SO2 for 0.5 (red) | Free SO2 for 0.8 (white, rosé) |
|---|---|---|
| 3.0 | 8 mg/L | 13 mg/L |
| 3.2 | 13 mg/L | 20 mg/L |
| 3.4 | 20 mg/L | 32 mg/L |
| 3.6 | 31 mg/L | 50 mg/L |
| 3.8 | 49 mg/L | 79 mg/L |
Look at how steep that is. Every 0.2 of pH costs you roughly half as much again in free SO2. This is the single strongest argument for getting acid right before you worry about sulfite: dropping a wine from pH 3.8 to 3.5 cuts the sulfite requirement in half, and the wine tastes better for it.
Worked Example:
19-litre batch of Chardonnay at pH 3.4, so the 0.8 mg/L white target applies:
- Free SO2 needed at that pH: 32 mg/L
- Current free SO2: 8 mg/L
- Deficit: 24 mg/L
KMS needed = (24 × 19) / 570 = 456 / 570 = 0.80 grams
Dissolve in 50 mL of wine or water, stir gently into the batch.
Addition Timing:
| Stage | Free SO2 Target |
|---|---|
| Crush/pressing | 50–75 mg/L (inhibit wild yeast) |
| Post-fermentation | 25–35 mg/L |
| Pre-bottling | 30–40 mg/L |
| Bottled wine (for aging) | 35–45 mg/L |
Practical Tips:
- Test free SO2 regularly with the Ripper method or an aeration-oxidation kit. Guessing at free SO2 is guessing at the whole calculation.
- Never add SO2 during active fermentation. It kills the yeast, and it kills the malolactic bacteria too, so hold off until both are finished.
- Check your local limits. The European Union caps total SO2 at 150 mg/L for dry red and 200 mg/L for dry white and rosé, rising to 200 and 250 for wines above 5 g/L of residual sugar. The United States allows 350 mg/L. Free SO2 is only a fraction of total, but a free reading anywhere near those numbers means the total is already over.
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