Work out exactly how much sulphite to add to reach your target free SO₂, accounting for your wine's pH (lower pH means more active molecular SO₂). Supports potassium metabisulphite (KMS), sodium metabisulphite and Campden tablets in litres or gallons.
Calculate how much sulphite to add to reach your target free SO₂ level. Accounts for wine pH — lower pH means more active molecular SO₂.
0 if starting fresh
Typical: whites 25–35, reds 20–30 mg/L
Lower pH = more effective SO₂
Sets the legal total-SO₂ ceiling below.
Addition required
1.2 g
SO₂ to add
30 mg/L
Molecular SO₂ at target
0.6 mg/L
Track this in your batch log
MakeWine is a free winemaking app — log additions, lab readings and fermentations, and get a deterministic assistant. No card required.
KMS is about 57.6% SO₂ by weight. To raise free SO₂ by X mg/L in V litres, add (X × V ÷ 1000) ÷ 0.576 grams of KMS. Enter your target and current free SO₂ above to get the exact amount.
Molecular SO₂ is the antimicrobially active fraction of free SO₂. It is calculated as free SO₂ ÷ (1 + 10^(pH − 1.81)). At lower pH a larger fraction is molecular, so you need less free SO₂ for the same protection. Aim for ~0.5–0.8 mg/L molecular SO₂.
A standard Campden tablet provides ~0.253 g SO₂. Choose Campden tablets and your volume (litres, imperial or US gallons) above and the calculator converts for you.
Calculations are estimates based on standard winemaking formulas (Henderson–Hasselbalch for SO₂, Duncan & Acton for wine ABV, standard addition rates). Always verify critical additions with a reference or lab analysis. The decision and responsibility for any addition rest with you.