Three numbers, one additive
Sulphur dioxide in wine exists in two broad states. Bound SO₂ is tied up with acetaldehyde, sugars, anthocyanins and other compounds; it does very little. Free SO₂ is unbound and does the protecting. Total SO₂ is the two added together, and is what legal limits are written against.
Free SO₂ itself is a mix of forms, and only one — molecular SO₂ — is strongly antimicrobial. The share of free SO₂ in molecular form depends almost entirely on pH.
| On your report | What it tells you |
|---|---|
| Free SO₂ (mg/L) | How much SO₂ is available to protect the wine |
| Total SO₂ (mg/L) | Everything added so far, free and bound — check against legal limits |
| pH | Combine with free SO₂ to work out molecular SO₂ |
Why pH changes everything
Molecular SO₂ = free SO₂ ÷ (1 + 10pH − 1.81). In practice, each 0.1 rise in pH means you need roughly 25% more free SO₂ for the same protection. The table shows the free SO₂ needed for common targets.
| pH | 0.5 mg/L molecular | 0.6 mg/L | 0.8 mg/L |
|---|---|---|---|
| 3.0 | 8 | 10 | 13 |
| 3.2 | 13 | 15 | 20 |
| 3.3 | 16 | 19 | 26 |
| 3.4 | 20 | 24 | 32 |
| 3.5 | 25 | 30 | 40 |
| 3.6 | 31 | 38 | 50 |
| 3.7 | 39 | 47 | 63 |
| 3.8 | 49 | 59 | 79 |
| 3.9 | 62 | 74 | 99 |
What to aim for
- Whites and rosé: about 0.8 mg/L molecular is the common working target.
- Reds: 0.5–0.6 mg/L is common, because tannins and pigments give some protection and bind SO₂ readily.
- Brettanomyces control: keep molecular SO₂ at or above about 0.6 mg/L, especially in barrel.
- Around 2 mg/L molecular and above, many tasters can smell the sulphur.
Making additions
Work out the free SO₂ you need for your target, subtract what you have, and add that much. Then expect to fall a little short: part of every addition becomes bound, more so in wines high in acetaldehyde or sugar. Add in stages, mix thoroughly, and re-test free SO₂ 24–48 hours later.
Free SO₂ also drifts down during maturation as it binds and oxidises, faster with ullage, racking and warmth. Monthly free SO₂ and pH checks are the simplest insurance there is.
Do the maths for your own wine.
Open the SO₂ calculatorLegal limits
Limits on total SO₂ depend on where the wine is sold, and on its sugar content. As an example, the EU caps dry red wine at 150 mg/L and dry white and rosé at 200 mg/L total SO₂. Check the current rules for every market you export to before finalising additions.
FermLabs guide · general information only — your own wine's analysis and trials come first.