Sulfur dioxide (SO2) is the backbone preservative of home and commercial winemaking, guarding against oxidation, bacterial spoilage, and unwanted wild yeast activity. This calculator converts a target free SO2 level into the exact grams of potassium metabisulfite (KMeta) needed for your batch, so additions at crush, racking, and bottling stay consistent and measured rather than guessed.
How the Wine SO2 Addition Calculator works
The calculator starts from the gap between your current free SO2 reading and your target level, then multiplies that ppm gap by the batch volume in liters (since 1 ppm equals 1 milligram of SO2 per liter). Because potassium metabisulfite is only about 57% available SO2 by weight — the remainder is potassium and other inert compounds — the raw milligram figure is divided by 0.57 to arrive at the actual grams of KMeta powder to weigh out. This 57% figure is the standard home-winemaking approximation used across sources like Jack Keller's winemaking reference and WineMaker Magazine's technical guides.
Inputs and what they mean
Batch volume sets the scale of the addition and can be entered in gallons or liters. Current free SO2 should come from a titration test kit or a prior known addition — enter 0 for a freshly crushed, untreated must. Target free SO2 is the level you want to reach; picking a wine-type preset (red, white/rosé, or dessert) fills in a typical target, or you can enter a custom ppm value if your winemaking notes call for something different. The optional pH field does not change the KMeta math directly — it only informs the molecular SO2 effectiveness note, since pH affects how much of your free SO2 is actively protective.
Limits and edge cases
This calculator assumes your current free SO2 reading is accurate — an outdated or estimated reading will throw off the dosage. It also does not account for SO2 lost to off-gassing during vigorous racking or headspace exposure, so retesting after a significant addition is good practice. If the target you enter is at or below your current free SO2, no addition is needed and the calculator will not return a dosage. For anything beyond home-batch scale, or if you are unsure about your titration results, consult a winemaking supply shop or an enology reference before dosing a large or valuable batch.