Sulphite (SO2) Addition Calculator

Work out sulphite (so2) addition instantly with clear inputs, formula shown and shareable results.

About Sulphite (SO2) Addition Calculator

Only molecular sulphur dioxide is antimicrobial, and its fraction of free SO2 collapses as pH rises: about 6% at pH 3.0 but under 2% at pH 3.6. Targeting 0.8 mg/L molecular SO2 therefore requires far more free SO2 in a high-pH wine. Potassium metabisulphite is 57% available SO2.

Formulas

Molecular fraction

Fraction = 1 / (1 + 10^(pH - 1.81))

Addition

SO2 (g) = volume x deficit / 1000; KMS (g) = SO2 / 0.57

Worked example

With the default values below, Sulphite (SO2) Addition Calculator returns Potassium Metabisulphite Required of 31.5789. Change any input above and every figure here is recalculated in your browser.

Inputs used

Wine Volumeminimum 0 L: 1,000 L
Current Free SO2minimum 0 mg/L: 12 mg/L
Target Free SO2minimum 0 mg/L: 30 mg/L
Wine pHaccepts 2.5–4.5: 3.5

Results

Potassium Metabisulphite Required: 31.5789
Sulphur Dioxide Required: 18.0000
Molecular SO2 At This pH: 0.6003
Free SO2 Needed For 0.8 mg/L Molecular: 39.98

Step by step

1Values used
2Molecular fraction
3Addition
4Potassium Metabisulphite Required= 31.5789
5Sulphur Dioxide Required= 18.0000
6Molecular SO2 At This pH= 0.6003
7Free SO2 Needed For 0.8 mg/L Molecular= 39.98

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Sulphite levels are legally capped and must be declared above threshold levels. Sulphites cause serious reactions in sensitive individuals; measure rather than estimate free and total SO2.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Sulphite levels are legally capped and must be declared above threshold levels. Sulphites cause serious reactions in sensitive individuals; measure rather than estimate free and total SO2.

Frequently Asked Questions

Why does pH matter more than the SO2 number?

Because the same free SO2 gives radically different protection at different pH. A wine at pH 3.7 needs roughly three times the free SO2 of one at pH 3.2 for equal antimicrobial effect.

What limits total SO2?

Legal maxima, typically 150-200 mg/L total for dry wines and higher for sweet, plus sensory thresholds. Labelling of sulphites above 10 mg/L is mandatory in most markets.