Wine SO₂ Calculator (free SO₂ for a molecular target & potassium metabisulfite to add)

Understanding the wine SO₂ calculator — concept and principle

This calculator works out how much free sulfur dioxide (SO₂) a wine needs for a chosen molecular SO₂ target at its pH, and how many grams of potassium metabisulfite to add to get there. Home winemakers use it at racking, before bulk ageing and before bottling.

At higher pH less free SO₂ is in the active molecular form, so more is needed.

How it is calculated

Molecular share = 1 ÷ (1 + 10^(pH − 1.81))

Free SO₂ needed (mg/L) = molecular target ÷ molecular share

Free SO₂ to add = needed − measured now (not below 0)

Metabisulfite (mg) = mg/L to add × litres ÷ 0.57; grams = mg ÷ 1,000

Metabisulfite per litre = mg/L to add ÷ 0.57.
The page also checks whether the free SO₂ needed falls within a commonly recommended 15–40 mg/L.

The principle

Free SO₂ in wine exists mostly as bisulfite, with a small share in the molecular form, which is the part that protects against microbes. The balance depends on acidity: as pH rises, the molecular share falls steeply. The formula above is the acid-base balance with a pKa of 1.81, and it reproduces the extension chart values of 2.51 % at pH 3.4 and 1.60 % at pH 3.6. So a high-pH wine needs much more free SO₂ for the same protection. Potassium metabisulfite (K₂S₂O₅) releases about 57 % of its weight as SO₂, hence the division by 0.57.

Worked example

Default inputs: 23 L of wine, pH 3.4, free SO₂ now 10 mg/L, molecular target 0.8 mg/L.

Molecular share = 1 ÷ (1 + 10^(3.4 − 1.81)) = 1 ÷ (1 + 38.9) = 2.51 %.
Free SO₂ needed = 0.8 ÷ 0.02506 = 31.9 mg/L, which is inside the 15–40 mg/L range.
To add = 31.9 − 10 = 21.9 mg/L.
Metabisulfite per litre = 21.9 ÷ 0.57 = 38.5 mg/L.
Total = 21.9 × 23 ÷ 0.57 = 885 mg = 0.88 g.

Each point is the calculator re-run with only “Molecular SO₂ target (mg/L)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Molecular SO₂ target (mg/L)	Potassium metabisulfite to add (g)
0.2	0
0.4	0.24
0.6	0.56
0.8	0.88
1	1.21
1.2	1.53
1.6	2.17

How to read the result

Targets of about 0.8 mg/L molecular SO₂ are often used for white wines and around 0.5–0.6 for reds, but follow your own reference. If the free SO₂ needed is far above 40 mg/L because the pH is high, adjusting acidity may be better than adding a lot of sulfite. Part of each addition binds to the wine and is no longer free, so measure again a day or two later and top up if needed.

Common mistakes

Using total SO₂ instead of free SO₂ as the current value.
Measuring pH with old or uncalibrated equipment.
Adding the powder without dissolving it in a little water or wine first.
Forgetting that the free level drops over time.

Keep in mind: Sulfites can cause reactions in sensitive people, and many countries set legal limits on total SO₂ in wine. Weigh small amounts with a scale that reads to 0.01 g.

Frequently asked questions

How much potassium metabisulfite per gallon?

It depends on pH and the current free SO₂; enter your numbers. As a reference, 1 g in 23 L adds about 25 mg/L of free SO₂ before binding.

Why does pH matter so much?

The molecular share roughly halves for every 0.3 pH units, so higher-pH wines need much more free SO₂.

What is a Campden tablet?

A pressed tablet of metabisulfite. Check the pack for its SO₂ content per tablet.

Sources: Oklahoma Cooperative Extension Service, A. Essary, "Understanding Free Sulfur Dioxide (FSO₂) in Wine", fact sheet HLA-6263 (2024)

Checked 9 Nov 2026. Oklahoma Cooperative Extension Service, Aaron Essary, “Understanding Free Sulfur Dioxide (FSO₂) in Wine”, fact sheet HLA-6263 (May 2024): Chart 3 molecular share by pH for 0.8 mg/L, recommended free SO₂ 15–40 ppm, KMBS grams = ppm to add × litres ÷ 0.57 (÷ 1,000 for grams). The pKa 1.81 reproduces Chart 3.

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