# SO₂ Wine Calculator (Molecular and Free SO₂)

URL: https://speedycalculator.com/calculators/food/molecular-so2/
Last updated: 2026-10-30

> The SO₂ wine calculator works out the molecular SO₂ in a wine from its free SO₂ and pH, the free SO₂ needed to reach a target molecular level (0.5 mg/L for reds, 0.8 mg/L for whites), and how much potassium metabisulfite to add for your batch.

## Inputs

- Free SO₂ (mg/L)
- Wine pH
- Target molecular SO₂
- Custom target (mg/L)
- Volume of wine (L, ml, US gal, fl oz, cups)

## Results

- Current molecular SO₂
- Free SO₂ needed for the target
- Free SO₂ to add
- Potassium metabisulfite to add
- Campden tablets (0.44 g KMS each)

## 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
```

## Explanation

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

FieldWhat to enterFree SO₂In mg/LWine pHA numberTarget molecular SO₂Choose one: Red wine (0.5 mg/L), White / rosé (0.8 mg/L), Sweet / dessert (1.5 mg/L), CustomCustom targetIn mg/LVolume of wineUnits: 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)120.24180.36240.48300.6360.72480.96

## 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.

## FAQ

**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.


## Sources

- [UC Davis – Winemaking: sulfur dioxide](https://wineserver.ucdavis.edu/)