Free tools for winemakers
SO₂ addition calculator
Cellar addition — SO₂ preservative
Add
Type the volume you are treating.
Type the level it is at now, or leave it empty to dose from zero.
Type the wine’s pH to work out the free SO₂ it needs.
0.8 mg/L is the usual target for whites and rosés, 0.5 for reds. Change it if you aim for more or less.
— The free SO₂ that keeps this much in molecular form, the part that actually protects the wine. Some of what you add binds to the wine, so measure free SO₂ again the next day.
Total SO₂ will likely pass the limit.
High pH. Each 0.1 rise in pH needs about a quarter more free SO₂. Above pH 3.6 the target gets hard to reach without passing the total-SO₂ limits. Lowering the pH may protect the wine better than more SO₂.
Nothing entered, so this doses as if there were none there to start with.
57.6% is the standard for potassium metabisulfite — change it if your product is different.
How do we calculate this?
The exact formulas used in this tab:
Units
What this calculator does
Works out how many grams of potassium metabisulfite (KMBS) to add to bring free SO₂ (sulfur dioxide, often typed SO2) up to a target, for the volume you are treating.
- Type the free SO₂ target yourself, or set it from the wine’s pH and a molecular SO₂ target.
- The lower the pH, the less free SO₂ the wine needs.
- In “Add an amount” mode it doses a fixed number of ppm instead.
The formula
Sulfite (g) = (ppm added × volume L) ÷ (1000 × active share)active share = 57.6% (editable above)To a target: ppm added = target − currentFrom pH: target = molecular SO₂ × (1 + 10^(pH − 1.81))
Worked example
A target you type
- 500 L of wine
- Free SO₂ now: 20 ppm
- Target: 50 ppm
- Powder 57.6% active
- How much to raise free SO₂ 50 − 20 = 30 ppm
- How much powder (30 × 500) ÷ (1,000 × 0.576) = 26 g
Add 26 g of potassium metabisulfite
That is the theoretical addition: some of the SO₂ binds to the wine, so measure free SO₂ again the next day.
A target set from pH
- A white at pH 3.4
- Molecular target: 0.8 mg/L
- Free SO₂ now: 20 ppm
- 500 L
- Free SO₂ target 0.8 × (1 + 10^(3.4 − 1.81)) = 31.9 ppm
- How much to raise it 31.9 − 20 = 11.9 ppm
- How much powder (11.9 × 500) ÷ (1,000 × 0.576) = 10.3 g
Add 10.3 g of potassium metabisulfite
Questions
Why is the dose based on the increase, not the target?
Because the wine already holds some free SO₂. If it reads 20 ppm and you want 50 ppm, you only add the 30 ppm difference; dosing the full 50 ppm would add far more than you need. Part of any SO₂ you add binds to the wine, so measure again after adding.
What is molecular SO₂, and why does pH matter?
Only a small part of free SO₂ is in the molecular form that actually protects the wine against microbes, and that part shrinks as pH rises. So the free SO₂ a wine needs depends on its pH; the last line of the formula above shows the calculation. The usual targets are 0.8 mg/L molecular for whites and rosés and 0.5 mg/L for reds. For 0.8 mg/L that is about 13.2 ppm free at pH 3.0, 31.9 ppm at pH 3.4 and 79 ppm at pH 3.8. Choose “From pH” to have the calculator set the target this way.
Zoecklein et al., Wine Analysis and Production; AWRI, “Understanding molecular SO₂”.
Why does it warn me when the pH is high?
Each 0.1 rise in pH asks for about a quarter more free SO₂, and above about pH 3.6 the free SO₂ needed gets hard to reach without passing the legal limits on total SO₂. Free SO₂ is usually a third to a half of the total, so the calculator also warns when the free target alone reaches half the EU limit for a dry wine of that style: 150 mg/L total for a red, 200 mg/L for a white or rosé. The US allows 350 mg/L for any wine. Lowering the pH, for example with an acid addition, may protect the wine better than adding ever more SO₂. A pH outside 2.5–4.5 is refused: no wine reads there, so it is almost certainly a typo.
EU: Regulation (EU) 2019/934, Annex I Part B. US: 27 CFR 4.22.
What does “57.6% active” mean?
Potassium metabisulfite powder isn’t pure SO₂: about 57.6% of its weight becomes SO₂ in the wine. If your product’s label gives a different figure, type it in “Active SO₂ in the product” and the dose follows.
Metabisulfite, “meta”, KMBS — is it all the same powder?
Usually, yes. Potassium metabisulfite is sold as KMBS, “meta” or just metabisulfite, and it is the powder this calculator assumes at 57.6% active. Sodium metabisulfite is a different salt with about 67% SO₂; if that is what you have, type its share in “Active SO₂ in the product”.
Free or total SO₂ — which do I enter?
Free SO₂. It is the part that protects the wine, and it is what the target refers to. Total SO₂ also counts SO₂ already bound to other compounds in the wine, which no longer protects it. The legal limits are on the total, so measure total SO₂ too.
Does the calculator choose a target for me?
Only if you ask it to. With “From pH” it works the free SO₂ target out from the pH and the molecular target, which you can change. Otherwise it doses the target you type; the quick buttons are shortcuts for typing, not a recommendation.
Guides that use this calculator
-
Malolactic fermentation: how to do it and when it's done
Malolactic fermentation turns malic acid into lactic, cutting TA by 0.56 g/L per g of malic. Run it at 18–22 °C with no SO₂; it's done below 0.1 g/L.
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How much sulfite to add to wine, and when
Aim for 0.5 mg/L molecular SO₂ in reds and 0.8 in whites. At pH 3.6 a red needs about 31 ppm free SO₂. A pH table, a worked example and a free calculator.