Audited —·Last updated 31 Jul 2026·3 citations·Tier 3·0 uses

Molecular SO2 Calculator

Convert a wine's free SO2 and pH into molecular SO2, find the free SO2 a target level needs, and size the potassium metabisulfite addition.

Molecular SO2 Calculator

Measured with a calibrated meter, not estimated from titratable acidity. This is the field that dominates the answer — the free SO2 needed for a given molecular level roughly ten-folds for every whole pH unit.
Free SO2 in mg/L, which winemakers also write as ppm — the aeration-oxidation or ripper figure, not total SO2. Zero is a legal entry and returns zero molecular SO2.
mg/L
0.8 mg/L is the level at which Beech et al. (1979) measured a 10,000-fold reduction in viable Brettanomyces and certain lactic acid bacteria in white table wine over 24 hours. Zoecklein names 0.5 mg/L as the option for winemakers worried about sensory impact.
mg/L
The lot you are treating. A 225 L barrique is 225 L; a US gallon is 3.785 L; a hectolitre is 100 L. Used only to turn the mg/L addition into grams of powder.
L
Published values in this application are not identical: Zoecklein prints 1.8, most winemaking calculators and AWRI guidance use 1.81, and Butzke's dosage charts imply an effective value near 1.83. It is editable rather than baked in because the choice moves the required free SO2 by a few percent.
Molecular SO2
0.6003
The undissociated fraction of your measured free SO2 — the only form that crosses microbial cell membranes, and therefore the only form doing antimicrobial work. This is the scalar that drives the band below, and the band reads it before rounding.
Share of free SO2 that is molecular
2.00
Free SO2 needed for your target
39.9823 mg/L
Free SO2 to add
9.9823 mg/L
Potassium metabisulfite
3.4643 g
Potassium metabisulfite per hectolitre
1.7322 g/hL
Free SO2 needed if the pH were 0.1 higher
50.1276 mg/L
Where this sits
Between the two published levels — at or above the 0.5 mg/L molecular lower option, but below the 0.8 mg/L at which Beech et al. (1979) measured a 10,000-fold reduction in viable Brettanomyces and certain lactic acid bacteria over 24 hours.
Summary
At pH 3.50, 30.00 mg/L of free SO2 is 0.600 mg/L molecular — only 2.001% of the free SO2 is in the active molecular form. Reaching 0.80 mg/L molecular needs 39.98 mg/L of free SO2, an addition of 9.98 mg/L — about 3.464 g of potassium metabisulfite in 200.0 L, or 1.73 g per hectolitre. Some of any addition binds within 2 to 8 hours, so re-measure free SO2 afterwards rather than assuming the addition landed in full. One tenth of a pH unit higher, the same target would need 50.13 mg/L of free SO2 instead.

Background.

Free SO2 on its own does not tell a winemaker very much. Sulfur dioxide dissolved in wine sits in an acid-base equilibrium between the undissociated molecular form and the bisulfite ion, and only the molecular form crosses microbial cell membranes — so it is the molecular concentration, not the free concentration, that determines whether a wine is protected against Brettanomyces and spoilage bacteria. Because that equilibrium is governed by pH, the same 30 mg/L of free SO2 is comfortable protection in a pH 3.1 white and close to useless in a pH 3.9 red.

This calculator does the conversion in both directions. Give it a wine's pH and its measured free SO2 and it returns the molecular SO2 and what percentage of the free SO2 that represents. Give it a target molecular level and it returns the free SO2 that target requires at that pH, the addition needed to get there, and the weight of potassium metabisulfite that addition works out to for your volume. It also shows what the same target would cost in free SO2 if the pH were one tenth of a unit higher, because the relationship is exponential and that single figure makes the point faster than any explanation.

Three limits are worth reading before the number. First, the page works entirely in free SO2 and does not model binding: a real addition partially combines with acetaldehyde, anthocyanins and sugars within a few hours, so the free SO2 you measure afterwards will be lower than the addition implies and you should re-measure rather than assume. Second, total SO2 in finished wine is capped at 350 mg/L by 27 CFR 4.22(b)(1) and by the OIV, and any wine over 10 mg/L must carry a sulfite declaration; free SO2 is only one part of total, so the cap is yours to check against a total measurement, not something this page can do for you. Third, the dissociation constant is an editable field rather than a hidden literal, because published values genuinely differ — 1.8, 1.81 and an implied 1.83 all appear in reputable winemaking sources, and the spread moves the answer by a few percent.

What is molecular so2 calculator?

Molecular SO2 is the fraction of a wine's free sulfur dioxide present as undissociated H2SO3 rather than as the bisulfite ion. It matters because it is the species that actually inhibits microbes: it is uncharged, so it diffuses through cell membranes, and once inside a cell at near-neutral pH it ionises, which both traps it and pulls more molecular SO2 in behind it.

The split between the two forms is set by wine pH through the first dissociation of sulfurous acid. At low pH the equilibrium sits toward the molecular form; at high pH it sits almost entirely on bisulfite. Across the range of ordinary wine pH values the molecular share runs from a few percent down to well under one percent, which is why a fixed free-SO2 target applied to every lot is a poor practice and why the pH must be measured.

This page converts between free and molecular SO2 in either direction, and turns the resulting addition into grams of potassium metabisulfite for a stated volume.

How to use this calculator.

  1. Measure the wine's pH with a calibrated meter and enter it. A tenth of a unit is worth about 25 percent of the answer, so this is not a field to estimate.
  2. Enter the measured free SO2 in mg/L — the aeration-oxidation or ripper figure, not total SO2.
  3. Set the target molecular level. 0.8 mg/L is Beech et al.'s tested level; 0.5 mg/L is the lower option for winemakers concerned about sensory impact.
  4. Enter the volume of the lot so the addition can be converted into grams of powder.
  5. Leave the pKa at 1.81 unless you have a house value; the field exists because published values differ and the choice is worth being explicit about.
  6. Read the molecular SO2 and the band, then the addition. Add the sulfite dissolved in a little wine or water, mix thoroughly, and re-measure free SO2 after a day — some of the addition will have bound.

The formula.

MSO₂ = FSO₂ ⁄ (1 + 10^(pH − pKa)) · FSO₂ required = MSO₂ target × (1 + 10^(pH − pKa))
MSO₂ · molecular sulfur dioxide, mg/L — the antimicrobially active formFSO₂ · measured free sulfur dioxide, mg/L (= ppm)pKa · first dissociation constant of sulfurous acid; 1.81 by default, editableKMBS (g) = addition (mg/L) × litres ⁄ 1000 ⁄ 0.5763 — K₂S₂O₅ is 57.63% available SO₂

The relation is the Henderson-Hasselbalch equation rearranged for the protonated species. Sulfur dioxide in solution equilibrates as H2SO3 against H+ and HSO3-, so the ratio of bisulfite to molecular SO2 is 10 raised to the power (pH − pKa), and the molecular share of the free SO2 is one divided by one plus that ratio. Zoecklein's Virginia Tech enology module prints it directly as MSO2 = FSO2 / (1 + 10^(pH−1.8)).

The worked example takes a 200 litre lot at pH 3.5 measuring 30 mg/L free SO2, with the pKa left at 1.81. The exponent is 3.5 − 1.81 = 1.69, so 10^1.69 = 48.978 and the denominator is 49.978. Molecular SO2 is therefore 30 / 49.978 = 0.600 mg/L, and the molecular share of the free SO2 is 100 / 49.978 = 2.001 percent — one part in fifty.

Running it the other way, a 0.8 mg/L molecular target needs 0.8 × 49.978 = 39.98 mg/L of free SO2, which is 9.98 mg/L more than the wine has. Over 200 litres that is 1.996 g of SO2, and since potassium metabisulfite is 57.63 percent available SO2 by mass — K2S2O5 releases two SO2, so 2 × 64.058 / 222.3116 — the addition is 3.464 g of powder, or 1.73 g per hectolitre.

The pH sensitivity is the part worth internalising. Move the same wine to pH 3.6 and the denominator becomes 62.66, so the same 0.8 mg/L target now needs 50.13 mg/L of free SO2 instead of 39.98 — a quarter more, for a tenth of a pH unit. That is why the calculator prints that figure as an output, and why a pH meter that has drifted is more expensive than it looks.

Rounding happens once. Every quantity is carried at forty significant digits and rounded at the end to ten decimal places; the band is decided from the unrounded molecular value and does not restate the number, so it cannot contradict the two-decimal figure beside it. The addition is floored at zero rather than going negative, because adding sulfite cannot lower free SO2.

A worked example.

Example

A 200 litre lot of red at pH 3.5, measuring 30 mg/L free SO2 at racking, with the winemaker aiming for Beech et al.'s 0.8 mg/L molecular level before the wine goes into barrel. Thirty milligrams per litre sounds like a healthy figure until it is converted. At pH 3.5 only 2.001 percent of the free SO2 is in the molecular form, so the wine is carrying 0.600 mg/L molecular — above the 0.5 mg/L lower option but short of the 0.8 mg/L at which Beech and colleagues measured a ten-thousand-fold reduction in viable Brettanomyces and certain lactic acid bacteria over twenty-four hours. Reaching 0.8 mg/L molecular at this pH takes 39.98 mg/L of free SO2, an addition of 9.98 mg/L. Over 200 litres that is 3.464 g of potassium metabisulfite, or 1.73 g per hectolitre — a small enough quantity that it needs a decent scale rather than a spoon. Some of it will bind within two to eight hours, so the free SO2 measured the next day will be below 39.98 and a second, smaller top-up is normal. The last output is the one that changes behaviour. If the same wine had been read at pH 3.6 rather than 3.5, the same 0.8 mg/L target would have needed 50.13 mg/L of free SO2 instead of 39.98. A tenth of a pH unit — well within the error of a meter that has not been calibrated recently — is worth a quarter of the dose.

free So2 Mgl30
p Ka1.81
wine Ph3.5
wine Volume Liters200
target Molecular So2 Mgl0.8

Frequently asked questions.

Why do two wines with the same free SO2 need different treatment?
Because free SO2 is a total of two species and only one of them is antimicrobially active. The undissociated molecular form is uncharged and diffuses into microbial cells; the bisulfite ion is charged and largely does not. Which of the two dominates is set by pH, and the relationship is exponential rather than proportional: at pH 3.2 about 3.9 percent of free SO2 is molecular, at pH 3.5 about 2.0 percent, and at pH 4.0 only 0.64 percent. So 30 mg/L of free SO2 gives 1.17 mg/L molecular in the first wine and 0.19 mg/L in the third — a six-fold difference in protection from an identical laboratory number. This is also why high-pH wines are so difficult: the free SO2 needed climbs fast enough that the sensory threshold and the 350 mg/L legal ceiling on total SO2 both start to become real constraints.
Should I target 0.5 or 0.8 mg/L molecular?
Both figures come from the same place and mean different things. Beech and colleagues (1979) found that 0.8 mg/L molecular free SO2 in white table wine achieved a ten-thousand-fold reduction in twenty-four hours in viable Brettanomyces, certain lactic acid bacteria and other spoilage organisms — that is a tested antimicrobial level, and it is the default here. Zoecklein notes that winemakers concerned about the negative sensory impact of excessive sulfur dioxide may elect to use a lower concentration such as 0.5 mg/L, which is a trade rather than an equivalent. The right target also depends on what you are trying to prevent: a wine that has finished malolactic fermentation and is going into a clean barrel is in a different position from one with residual sugar and a Brettanomyces history. This page gives you the arithmetic, not the decision.
Why is the pKa an editable field instead of a fixed number?
Because the sources do not agree, and pretending otherwise would hide a real few-percent uncertainty. Zoecklein's module prints the equation with 1.8. Most winemaking calculators, and AWRI's published guidance, use 1.81. Butzke's Purdue Extension dosage charts — which give 79 to 112 mg/L of free SO2 at pH 3.95 depending on alcohol — imply an effective constant nearer 1.83. Using 1.8 instead of 1.81 asks for about 2.3 percent more free SO2 for the same molecular target, which is smaller than the error in most cellar SO2 measurements but is not nothing. Exposing it as a labelled field lets a winery use its own house value and keeps the assumption visible instead of buried in the code.
Will adding this much potassium metabisulfite actually raise my free SO2 by that amount?
No, and the shortfall is not a rounding error. The calculation gives the theoretical addition assuming every milligram stays free. In practice sulfite binds to acetaldehyde, anthocyanins and residual sugars; Zoecklein describes monitoring after an addition as showing a rapid rise in the free form followed by a decrease over two to eight hours as the bound percentage rises. How much you lose depends on the wine — reds with high anthocyanin, wines from botrytised or sour-rot fruit, and sweet wines all bind more. Treat the number as the first dose, mix the addition in thoroughly, and re-measure free SO2 the following day. Also bear in mind that dry sulfite loses potency in storage, particularly in a warm or humid cellar, so an old tub delivers less than the theoretical 57.6 percent available SO2.
Does this calculator keep me inside the legal limit?
It does not, and it cannot, because it works in free SO2 and the law is written about total. Under 27 CFR 4.22(b)(1) the presence in finished wine of not more than 350 parts per million of total sulfur dioxide shall not be precluded, and the OIV limit is the same figure; TTB has required a sulfite declaration on the label since 1987 for wine containing 10 mg/L or more. Total SO2 is free plus bound, and the bound portion is frequently the larger of the two, so a wine can be well inside its free-SO2 plan and still be approaching the ceiling. The only way to know is to measure total SO2. If you are making wine commercially, your compliance obligations sit with your own analysis and your regulator, not with a calculator.
Can I use this for cider, mead or fruit wine?
The chemistry is the same — the equilibrium between molecular SO2 and bisulfite depends on pH and nothing else — so the conversion between free and molecular SO2 holds for any aqueous, alcoholic beverage in the guarded pH range. What does not carry across is the target. The 0.8 mg/L figure was measured in white table wine, and the microbial population, the alcohol content, the sugar and the binding compounds are all different in cider or mead. Alcohol in particular matters: Butzke notes that ethanol acts synergistically with molecular SO2, which is why his dosage charts ask for less free SO2 at 14 percent alcohol than at 12 percent for the same protection. Use the arithmetic, but take the target from a source that studied your product.

References& sources.

  1. [1]Zoecklein, Bruce W. "Sulfur Dioxide (SO2)." Virginia Tech Enology-Grape Chemistry Group, winemaking module — prints the equation [M SO2] = [FSO2] / (1 + 10^(pH−1.8)); reports Beech et al. (1979) that "for white table wines, 0.8 mg/L molecular free sulfur dioxide achieved a 10,000-fold reduction in 24 hours in the number of viable Brettanomyces spp., certain lactic acid bacteria, and other wine spoilage organisms"; names 0.5 mg/L as the lower option; states "theoretically, available SO2 makes up 57.6% of the total weight of potassium metabisulfite"; states the TTB and OIV maximum total SO2 of 350 mg/L and the 10 mg/L labelling threshold; and describes the post-addition decline in free SO2 "over 2 to 8 hours" as binding proceeds.
  2. [2]27 CFR 4.22(b)(1) (Alcohol and Tobacco Tax and Trade Bureau, Labeling and Advertising of Wine) — "the presence in finished wine of not more than 350 parts per million of total sulfur dioxide, or sulphites expressed as sulfur dioxide, shall not be precluded under this paragraph." The statutory ceiling this page tells the reader to check a total-SO2 measurement against; the page itself computes free SO2 only.
  3. [3]IUPAC Commission on Isotopic Abundances and Atomic Weights, Standard Atomic Weights 2021 — K 39.0983, S 32.06, O 15.999, giving potassium metabisulfite K2S2O5 = 222.3116 g/mol and SO2 = 64.058 g/mol. Two SO2 per K2S2O5 is 128.116/222.3116 = 57.629 percent available SO2 by mass, which is the figure behind the potassium metabisulfite outputs and which reproduces Zoecklein's stated 57.6 percent.
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