How it works
How the addition is calculated
The calculator takes the gap between your target and current free SO₂ (mg/L, the same as ppm) and multiplies it by the volume. That gives grams of pure SO₂. Dividing by the SO₂ content of the product gives the weight to add: potassium metabisulfite (KMBS) is about 57% SO₂, sodium metabisulfite about 67%. For an SO₂ solution the result comes out in mL.
This assumes everything you add stays free. In practice part of it binds straight away to acetaldehyde, sugars and other compounds, so a wine often reads a little under target after an addition. Measure free SO₂ again a day or two later and top up if needed.
Why pH decides how much free SO₂ you need
Only the molecular form of SO₂ protects the wine against yeast and bacteria, and the share of free SO₂ in that form drops steeply as pH rises. The calculator uses the standard relation: molecular SO₂ = free SO₂ ÷ (1 + 10^(pH − 1.81)). Common targets are about 0.8 mg/L molecular for whites and rosés and about 0.5 mg/L for reds.
In red wines, part of what a lab reads as free SO₂ is loosely bound to pigments, so the molecular figure is less reliable there. Treat it as a guide.
Free SO₂ needed by pH
Free SO₂ (mg/L) that gives each molecular SO₂ target, from the same formula the calculator uses.
| pH | 0.5 mg/L molecular | 0.8 mg/L molecular |
|---|---|---|
| 3.0 | 8 mg/L | 13 mg/L |
| 3.1 | 10 mg/L | 16 mg/L |
| 3.2 | 13 mg/L | 20 mg/L |
| 3.3 | 16 mg/L | 26 mg/L |
| 3.4 | 20 mg/L | 32 mg/L |
| 3.5 | 25 mg/L | 40 mg/L |
| 3.6 | 31 mg/L | 50 mg/L |
| 3.7 | 39 mg/L | 63 mg/L |
| 3.8 | 49 mg/L | 79 mg/L |
| 3.9 | 62 mg/L | 99 mg/L |
Worked example (US units)
100 gal (378.5 L) of Chardonnay at 18 ppm free SO₂, target 30 ppm. The gap is 12 ppm, so 12 × 378.5 ÷ 1,000 = 4.5 g of pure SO₂, or 4.5 ÷ 0.57 ≈ 8.0 g of potassium metabisulfite.
Questions winemakers ask
Is mg/L the same as ppm?
For wine, yes. Wine’s density is close to water’s, so 1 mg/L is treated as 1 ppm.
How much potassium metabisulfite per gallon?
1 g of KMBS in 1 US gallon (3.785 L) adds about 150 ppm of SO₂. Usual additions are 20–50 ppm, so enter your real volume in the calculator rather than scaling a per-gallon rule.
What are the legal limits for SO₂?
In the US, total SO₂ may not exceed 350 mg/L, and labels must say “Contains sulfites” from 10 ppm. In the EU the limit is 150 mg/L for dry reds and 200 mg/L for dry whites and rosés, higher for sweet wines.