Sulfur dioxide protects wine from oxidation and microbes, but the dose that matters is not the dose you add — it is the tiny fraction present as molecular SO2, and pH decides how tiny. The same 30 mg/L of free SO2 guards a pH 3.2 wine firmly and a pH 3.8 wine barely at all. This SO2 wine calculator converts your wine’s pH and your molecular target into the free SO2 you actually need, and optionally into the grams of potassium metabisulfite that deliver it.
The Formula Behind the Number
In wine, free SO2 exists mostly as bisulfite, with only a small share in the antimicrobial molecular form — and pH sets the share. The governing relation is free SO2 = molecular SO2 × (1 + 10^(pH − 1.81)), where 1.81 is the effective pKa in wine conditions. At pH 3.4 the multiplier is about 40, so the standard white-wine target of 0.8 mg/L molecular requires 0.8 × 40 ≈ 32 mg/L free SO2 — this page’s default result. Drop the pH to 3.2 and the multiplier falls to about 25, so the same protection needs only ≈ 20 mg/L. That steep pH leverage is the entire lesson: protection is bought with acidity far more cheaply than with sulfur.
The Molecular Targets
The widely used enology targets: 0.5 mg/L molecular SO2 for reds (whose phenolics do part of the antimicrobial work and whose colour binds SO2 eagerly), 0.8 mg/L for whites and rosés, and 1.5 mg/L for sweet and botrytised wines, where residual sugar invites refermentation and spoilage yeasts. These are pre-bottling protection targets, not legal limits — total SO2 caps are a separate, regulatory number, and much of what you add becomes bound and inactive. Note that the calculator solves for free SO2: if your lab reports total SO2, the difference (bound SO2) cannot be recovered by this formula, and chasing molecular targets off a total-SO2 reading is a category error.
The Addition: Metabisulfite Math
Potassium metabisulfite (KMBS, “meta”) is 57.6 percent available SO2 by weight, which gives the addition formula the widget applies: grams of KMBS = (SO2 to add in mg/L × volume in litres) ÷ 1000 ÷ 0.576. The worked example: a 100 L white at pH 3.4 measuring 12 mg/L free needs 32 mg/L, an addition of 20 mg/L — that is 20 × 100 ÷ 1000 ÷ 0.576 ≈ 3.47 g of KMBS. Always dissolve the measured powder in a small amount of wine or water first and stir it into the full volume; dumping crystals into a tank doses the neighbourhood of the hatch and leaves the rest unprotected. Campden tablets are pre-dosed KMBS (roughly 0.44 g SO2 each) for small carboys, where a scale’s resolution argues for them.
Measure, Wait, Measure Again
Free SO2 is a moving quantity. A fresh addition binds rapidly in the first hours as it reacts with oxygen, acetaldehyde and sugars, so the free level you measure the next day will sit below the arithmetic — the wine “consumes” a portion of every dose, more in reds and in wines rich in binding compounds. The professional rhythm is to add about two-thirds of the calculated amount, wait 24 hours, measure, and trim to target. Over-sulfiting has its own costs: harshness, a struck-match note at the extreme, bleached colour in rosés, and legal ceilings in many jurisdictions. The calculator gives the theoretical requirement; the lab bench has the final word.
pH First: The Cheaper Lever
Because the multiplier roughly doubles for every 0.3 of pH, a high-pH wine (3.8 and above) can demand free SO2 levels that are impractical or unlawful to reach — at pH 3.8 the 0.8 target needs about 100 mg/L free. The enologically sound response is usually to adjust acidity (tartaric additions pre-fermentation, blending) rather than to sulfur a wine into submission. This is why cellar records track pH and free SO2 as a pair, and why two wines at an identical 30 mg/L free can be in completely different states of protection — one snug, one exposed.
Common Mistakes
Dosing to a fixed free-SO2 number regardless of pH — the error this page exists to retire. Confusing total SO2 with free SO2 when planning an addition. Measuring pH with uncalibrated strips and then calculating to one decimal place. Adding KMBS dry and unstirred. And forgetting that this is a cellar-management aid for home and small-lot winemaking: commercial producers work under regulatory limits and lab verification that a calculator complements but never replaces.
Related Tools
The drinks bench continues in the Food Calculators hub — the brewing family there starts with the ABV and dilution tools, and the Cold Brew Ratio Calculator covers the morning side of the cellar door.
Frequently Asked Questions
How much free SO2 does my wine need?
It depends on pH and target: free SO2 = molecular target × (1 + 10^(pH − 1.81)). A white at pH 3.4 aiming for 0.8 mg/L molecular needs about 32 mg/L free; at pH 3.2, about 20 mg/L.
What molecular SO2 level should I target?
Standard enology targets are 0.5 mg/L for reds, 0.8 mg/L for whites and rosés, and 1.5 mg/L for sweet or botrytised wines. These are antimicrobial protection targets for the free fraction.
How much potassium metabisulfite do I add?
Grams = (mg/L of SO2 to add × litres of wine) ÷ 1000 ÷ 0.576, since KMBS is 57.6 percent SO2. Raising 100 L by 20 mg/L takes about 3.47 g, dissolved first and mixed thoroughly.
Why does pH matter so much for SO2?
Only the molecular fraction of free SO2 is strongly antimicrobial, and pH controls that fraction: each 0.3 rise in pH roughly doubles the free SO2 needed for the same protection. High-pH wines are far more expensive to protect.
What is the difference between free and total SO2?
Free SO2 is the unbound, active pool (bisulfite plus molecular). Total SO2 includes the bound fraction already reacted with sugars, acetaldehyde and pigments, which no longer protects. This calculator works with free SO2 only.
Why is my measured free SO2 lower than what I added?
Part of every addition binds within hours — to oxygen, acetaldehyde and other wine compounds — especially in reds. Add about two-thirds of the calculated amount, wait a day, remeasure, and trim to the target.
Can I just keep free SO2 at 30 mg/L for every wine?
That habit over-protects low-pH wines and under-protects high-pH ones. At pH 3.8, 30 mg/L free delivers only about 0.24 mg/L molecular — well under the white-wine target — while at pH 3.1 it overshoots. Dose to pH.