How much sulfite does your wine need?
Sulfite (SO2) protects wine, mead and cider from oxidation and spoilage microbes. But only part of the free SO2 you add is in the protective molecular form, and that share depends heavily on pH. The lower the pH, the less sulfite you need.
The calculator finds the free SO2 that gives your target molecular level:
Free SO2 = molecular SO2 × (1 + 10^(pH − 1.81))
It then subtracts what's already in the wine and converts the difference into potassium metabisulfite (K-meta), which is about 57.6% SO2 by weight:
K-meta (g) = ppm to add × liters ÷ 0.576 ÷ 1000
| pH |
Free SO2 for 0.8 ppm molecular |
Free SO2 for 0.5 ppm molecular |
| 3.0 |
13 ppm |
8 ppm |
| 3.2 |
20 ppm |
13 ppm |
| 3.4 |
32 ppm |
20 ppm |
| 3.6 |
50 ppm |
31 ppm |
| 3.8 |
79 ppm |
49 ppm |
Practical tips
- Measure pH with a calibrated meter. Test strips are too coarse for this.
- Dissolve K-meta in a little cool water before adding it, and stir gently.
- Free SO2 drops over time as it binds to the wine. Recheck every couple of months, and before bottling.
- Above pH 3.8, the sulfite needed starts to be noticeable in aroma. Adjusting acidity is usually a better fix than more sulfite.
- If you plan to back-sweeten, add potassium sorbate together with the sulfite to stop refermentation. See the back-sweetening calculator.
Method and sources
- Free SO2 needed: free SO2 = molecular SO2 × (1 + 10^(pH − 1.81)). Only the molecular form protects wine, and how much of the free SO2 is molecular depends on pH. 1.81 is the pKa of the sulfite equilibrium, and the usual targets are 0.8 ppm molecular for white, rosé and fruit wines and 0.5 ppm for reds, which get extra protection from tannins.
- Potassium metabisulfite: K2S2O5 is 57.6% SO2 by weight (two SO2 at 64.07 g/mol each, out of 222.3 g/mol). Grams = ppm × liters / 0.576 / 1000.
- Limits: some added SO2 binds straight away, so measure free SO2 with a test kit before topping up.
Sources
Frequently asked questions
How much potassium metabisulfite per gallon?
About 0.33 g (roughly 1/16 teaspoon) per US gallon adds 50 ppm of SO2. How much you actually need depends on your wine's pH and its current free SO2.
Why does pH matter for sulfite?
Only the molecular form of SO2 protects wine, and the share that is molecular falls as pH rises. A wine at pH 3.6 needs roughly four times the free SO2 of a wine at pH 3.0 for the same protection.
What molecular SO2 should I target?
Winemakers commonly target 0.8 ppm molecular SO2 for white, rosé and fruit wines, and 0.5 ppm for reds, which get extra protection from tannins.