How it calculates
Molecular SO₂ is the part of free SO₂ that actually protects the wine, and it depends on pH: molecular SO₂ = free SO₂ ÷ (1 + 10^(pH − 1.81)). It's the same formula the LagarLab lab uses.
The general reference is to keep it between 0.5 and 0.8 mg/L. Between 0.3 and 0.5 the wine is under-protected, below 0.3 it is unprotected, and above 0.8 it can smell pungent and stall the yeast.
For the dose, the calculator accounts for the 15% of added SO₂ that binds right away and never becomes free. To convert to product it uses the fact that potassium metabisulfite delivers 57.6% of its weight as SO₂: 1 g per hectoliter raises about 5.8 mg/L.
This is a theoretical calculation: every wine binds a different share of SO₂ over the following days. Measure free SO₂ again after adding and correct if needed.
Why not try it on a tank?
In LagarLab you add SO₂, change the pH and watch day by day how molecular SO₂ moves, without risking a real batch.
Vinify for free: Agrelo (red)No sign-up. Prefer a white? Try Chablis.
Frequently asked questions
How much free SO₂ does my wine need?
It depends on pH. To reach 0.5 mg/L of molecular SO₂ you need about 12.8 mg/L of free SO₂ at pH 3.2, about 25 mg/L at pH 3.5 and about 31.3 mg/L at pH 3.6. For 0.8 mg/L, about 20.4, 40 and 50.1 mg/L respectively.
How much potassium metabisulfite per liter?
Each gram of potassium metabisulfite delivers about 0.58 g of SO₂. In practice, 1 g per hectoliter (100 liters) raises SO₂ by about 5.8 mg/L; raising it by 20 mg/L in 20 liters takes about 0.7 g. Since part of it binds when you add it, measure again afterwards.
Why does the result depend so much on pH?
Because pH decides what share of free SO₂ is in molecular form. At low pH that share is larger, so the same free SO₂ protects a pH 3.2 wine far better than a pH 3.8 one.
What if I go above 0.8 mg/L of molecular SO₂?
The wine can smell pungent and, if it is still fermenting, the excess is toxic to yeast and can stall it. That is why 0.8 mg/L is the upper limit of the reference window.