Sulfite (SO₂) Calculator

Find the grams of potassium metabisulfite (KMBS) needed to raise free SO₂ to your target ppm in a batch of wine or must.

litres

Quick Facts

Method
grams KMBS = ppm increase × litres ÷ 1000 ÷ SO₂ yield
Potassium metabisulfite yields about 57% of its weight as SO₂.

Your Results

Calculated
Potassium metabisulfite to add
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Weigh this on a 0.01 g scale
SO₂ increase needed
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Target minus current, in ppm
Total SO₂ delivered
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Milligrams of SO₂ into the batch

Ready

Enter your batch volume, current and target free SO₂, then calculate.

What this sulfite calculator does

Sulfur dioxide (SO₂), added as a sulfite salt, is the workhorse preservative of winemaking and many other fermented and dried foods. It protects against oxidation and inhibits spoilage yeasts and bacteria. This calculator answers one practical question: how much potassium metabisulfite do you weigh out to raise the free SO₂ of a specific batch from its current level to a chosen target?

The formula

Concentration in wine is measured in parts per million, and 1 ppm equals 1 milligram of SO₂ per litre. So the total SO₂ you need to add is simply the increase you want multiplied by the batch volume:

SO₂ needed (mg) = (target ppm − current ppm) × volume in litres

Potassium metabisulfite (K₂S₂O₅, abbreviated KMBS or "meta") is not pure SO₂ — it releases about 57% of its weight as SO₂ when it dissolves. To find the weight of the salt, divide by that yield fraction:

grams of KMBS = (target ppm − current ppm) × litres ÷ 1000 ÷ 0.57

The 0.57 figure comes from the molar mass ratio: metabisulfite has a molar mass of about 222.3 g/mol and releases two SO₂ units at 64.07 g/mol each, giving 128.1 ÷ 222.3 ≈ 0.576. Many suppliers and winemaking references round this to 57% (some to 58%); the field on the calculator lets you enter the exact yield printed on your product, or switch to sodium metabisulfite, which yields about 67%.

Why free SO₂ matters

Only a small portion of the SO₂ in wine stays "free" and molecular — the rest binds to sugars, acetaldehyde and pigments and becomes largely inert. It is the free molecular SO₂ that actually protects the wine, and how much of the free pool is molecular depends on pH: at pH 3.2 you need roughly 21 ppm free SO₂ for 0.8 ppm molecular, while at pH 3.6 you need about 40 ppm. Always dose against a measured free SO₂ number, not total SO₂.

Common reference points

  • Typical free SO₂ targets: around 25–35 ppm for reds and 30–40 ppm for whites, adjusted up at higher pH.
  • Legal ceilings on total SO₂: 350 mg/L for most table wines in the US, and 150 mg/L for red / 200 mg/L for white wine under EU rules for conventional wine.
  • KMBS yield: ≈ 57% SO₂. Sodium metabisulfite yield: ≈ 67% SO₂.
  • Campden tablets: one standard tablet supplies roughly 0.44 g of active metabisulfite — about 0.25 g of SO₂, enough to raise 1 US gallon (3.785 L) by roughly 65 ppm.

Frequently Asked Questions

How do I convert the grams into Campden tablets?
One standard Campden tablet contains about 0.44 g of potassium (or sodium) metabisulfite. Divide the grams this calculator returns by 0.44 to estimate the number of tablets, then round up. Crush and dissolve them fully before adding, because undissolved fragments dose unevenly.
What if I use sodium metabisulfite instead of potassium?
Sodium metabisulfite (Na₂S₂O₅) releases about 67% of its weight as SO₂ rather than 57%, so change the "SO₂ yield" field to 67. It adds sodium to the wine, so many winemakers prefer potassium metabisulfite, but both deliver the same SO₂ chemistry.
Why did my free SO₂ come out lower than the target after dosing?
Some of the SO₂ immediately binds to acetaldehyde, sugars and pigments, especially in young or oxidized wine, so the free reading rises less than the full addition. Wait until mixing is complete, re-measure free SO₂, and top up with a smaller second addition if needed rather than overshooting on the first dose.

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