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Wine SO2 Calculator.

Calculate molecular SO2 from free SO2 and pH, the target free SO2 needed for any wine style, and the potassium metabisulfite dose for your batch volume. Same chemistry applies to cider. Only molecular SO2 has antimicrobial activity — pH determines how much of your free SO2 is actually working.

Inputs

Measure pH with a calibrated meter at wine temperature; measure free SO2 with a Ripper or aeration-oxidation test.

Typical wine range 3.0–3.9.
Enter 0 for a fresh wine that hasn't been dosed yet.
Targets are molecular SO2 in ppm.
Volume to dose.
Current molecular
—
From current free SO2 + pH
Free SO2 needed
—
To hit target molecular
Add to current
—
Additional free SO2 ppm
K-meta dose
—
For your batch volume

How this is calculated.

Show the formulas and citations

SO2 in solution exists in three forms: molecular (active), bisulfite (HSO3⁻), and sulfite (SO3²⁻). Only the molecular form is antimicrobial. Their equilibrium is pH-dependent, governed by the pKa of SO2/HSO3⁻ (1.81 at 20 °C):

molecular_SO2 = free_SO2 / (1 + 10^(pH − 1.81))

To hit a target molecular level, rearrange:

target_free = target_molecular × (1 + 10^(pH − 1.81))

K-metabisulfite (K2S2O5) is 57.6% SO2 by mass. 1 g of K-meta dissolved in 1 L delivers approximately 332 ppm free SO2 (at typical commercial purity):

K_meta_grams = (additional_free × volume_L) / 332
Sources: Boulton, R. B., Singleton, V. L., Bisson, L. F. & Kunkee, R. E. (1999). Principles and Practices of Winemaking. Chapman & Hall. Iland, P. et al. (2004). Chemical Analysis of Grapes and Wine. pKa value of 1.81 confirmed against AWRI guidance.

Worked example.

A dry white wine at typical pH

pH 3.50 · Free SO2 0 ppm (fresh wine) · Target 0.8 molecular · 20 L batch

Factor: 1 + 10^(3.50 − 1.81) = 1 + 49 = 50
Free SO2 needed: 0.8 × 50 = 40 ppm
Additional: 40 − 0 = 40 ppm
K-meta: 40 × 20 / 332 = 2.4 g

Common mistakes.

  • Treating all free SO2 as equally effective. A wine at pH 3.3 with 30 ppm free SO2 is well protected; the same 30 ppm at pH 3.8 is barely doing anything. Always compute molecular, not just free.
  • Not measuring pH at wine temperature. Calibrate the pH meter at the same temperature as the wine sample. A 5 °C swing changes pH by about 0.02 — small, but it compounds.
  • Confusing free, bound, and total SO2. The calculator works on free SO2 only. Test kits sold to home winemakers usually measure free; if your kit returns total, ask the manufacturer for the conversion.
  • Adding K-meta dry, then stirring vigorously. K-meta off-gasses SO2 if hit hard. Dissolve in a small amount of wine first, then gently mix into the bulk to avoid losses and oxidation.

Trademark notice.

Vinmetrica® is a trademark of Vinmetrica, Inc. Accuvin is a trademark of Accuvin, LLC. References to these instruments are for context only; this calculator is not affiliated with or endorsed by any test-instrument manufacturer.

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Frequently asked.

What molecular SO2 should I target?

Dry red wines run at 0.5–0.6 ppm molecular SO2 — anthocyanins bind some sulfite, lowering the effective need. Dry whites and rosés want 0.6–0.8 ppm. Off-dry to semi-sweet wines aim for 0.8–1.0 ppm. Sweet wines and ports go to 1.0+ ppm to prevent re-fermentation.

Why does pH matter so much?

Only molecular SO2 has antimicrobial activity. The proportion of free SO2 that exists in molecular form is pH-dependent — at pH 3.0, about 6% is molecular; at pH 3.8, only about 1%. Higher-pH wines need much more free SO2 to reach the same molecular protection.

Ripper or aeration-oxidation — which test method?

Both measure free SO2 but with different chemistry. Ripper (iodometric titration) is older and cheaper but interferes with red wine pigments. Aeration-oxidation (AO) is more accurate, especially for reds. Vinmetrica and similar electrochemical analysers use AO chemistry.

How much K-meta is in 1 gram?

Potassium metabisulfite is about 57.6% SO2 by mass. One gram dissolved in 1 litre delivers approximately 330 ppm free SO2 — though commercial K-meta varies in purity. The calculator uses 332 ppm/g/L as a conservative figure.