Only the molecular form of SO₂ protects wine from microbes, and how much of the free SO₂ is molecular depends strongly on pH: at pH 3.2 about 4% is molecular, at pH 3.6 under 2%. Higher-pH wines therefore need much more free SO₂.
How to use the SO₂ Wine Calculator
- Free SO₂
- Wine pH
- Target molecular SO₂
- Custom target
- Volume of wine
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the current free SO₂, the wine’s pH, the target molecular SO₂ and the volume of wine.
Tips
- Measure gravity and temperature carefully – hydrometers are calibrated for a specific temperature.
- Keep alcohol calculations conservative: never drink and drive, and know your limits.
- Write down every measurement in a brewing log so you can repeat good batches.
- Chill drinks in an ice-water bath for the fastest, safest cooling.
Common mistakes to avoid
- Reading a hydrometer at the wrong temperature without correcting it.
- Mixing up ABV (% by volume) and ABW (% by weight).
- Over-priming bottles – too much sugar can make bottles burst.
Inputs explained
| Field | What to enter |
|---|---|
| Free SO₂ | In mg/L |
| Wine pH | A number |
| Target molecular SO₂ | Choose one: Red wine (0.5 mg/L), White / rosé (0.8 mg/L), Sweet / dessert (1.5 mg/L), Custom |
| Custom target | In mg/L |
| Volume of wine | Units: L, ml, US gal, fl oz, cups |
What each result means
- Current molecular SO₂ = [Free SO₂]÷(1 + 10^([Wine pH] – 1.81))
- Free SO₂ to add = max([Free SO₂ needed for the target] – [Free SO₂], 0)
- Potassium metabisulfite to add = [Free SO₂ to add]×[Volume of wine]÷1000÷1000÷0.576
- Campden tablets (0.44 g KMS each) = [Potassium metabisulfite to add]÷0.44
Example values
How the main result changes when Free SO₂ changes and everything else stays at the example values above:
| Free SO₂ (mg/L) | Current molecular SO₂ (mg/L) |
|---|---|
| 12 | 0.24 |
| 18 | 0.36 |
| 24 | 0.48 |
| 30 | 0.6 |
| 36 | 0.72 |
| 48 | 0.96 |
Formula
Molecular SO₂ = free SO₂ ÷ (1 + 10^(pH − 1.81))
Required free SO₂ = target molecular × (1 + 10^(pH − 1.81))
Potassium metabisulfite (g) = (required − current free) × volume (L) ÷ 1000 ÷ 0.576
Worked example
Inputs — Free SO₂: 24 mg/L; Wine pH: 3.5; Target molecular SO₂: White / rosé (0.8 mg/L); Volume of wine: 23 L.
Results — Current molecular SO₂ = 0.48 mg/L; Free SO₂ needed for the target = 40 mg/L; Free SO₂ to add = 16 mg/L; Potassium metabisulfite to add = 0.64 g; Campden tablets (0.44 g KMS each) = 1.5.
Frequently asked questions
What is molecular SO₂?
The active, antimicrobial form of sulfur dioxide in wine.
How much molecular SO₂ do I need?
About 0.5 mg/L for reds and 0.8 mg/L for whites.
How much SO₂ does potassium metabisulfite give?
About 57.6% of its weight.