The wine sulfite calculator determines how much potassium metabisulfite (KMS) to add to achieve your target free SO2 level. Proper SO2 management protects wine from oxidation and microbial spoilage throughout aging and bottling.
SO2 Addition Calculator
How to Use the Wine Sulfite Calculator
Sulfite (SO2) management is one of the most important aspects of winemaking. The wine sulfite calculator takes the guesswork out of SO2 additions by accounting for your wine's pH, current SO2 level, and batch size.
Why pH Matters for SO2 Additions
At wine pH, sulfur dioxide exists in three chemical forms: sulfurous acid (H2SO3), bisulfite (HSO3−), and sulfite (SO3²−). Only the molecular SO2 form — a tiny fraction — provides antimicrobial protection against Brettanomyces, acetic acid bacteria, and other spoilage organisms. The proportion of molecular SO2 decreases as pH rises, so high-pH wines require much more total free SO2 to achieve the same protective effect.
Target Molecular SO2 Levels
Most winemakers target 0.5–0.8 ppm molecular SO2. At pH 3.5, achieving 0.5 ppm molecular SO2 requires about 22 ppm free SO2. At pH 3.8, the same protective level requires 45 ppm free SO2. High-pH wines (above 3.6) are more difficult to protect and benefit from techniques that lower pH before fermentation.
How to Add Potassium Metabisulfite
Dissolve KMS powder in a small amount of cool wine or water before adding to the batch. Never add it dry directly to the wine — it dissolves unevenly and can create localized hot spots. Stir the dissolved solution gently into the wine, then test free SO2 again after 24 hours to confirm uptake. Some wines bind SO2 more aggressively than others, particularly those high in aldehydes from oxidation.
When to Test and Add SO2
Test free SO2 every 2–3 months during barrel or tank aging, before and after racking, and before bottling. SO2 dissipates over time as it binds with wine components. Wines stored in wood lose SO2 faster than those in stainless steel or glass. Keep a log of additions and test results to understand your wine's SO2 consumption rate.
FAQ
Is this wine sulfite calculator free?
Yes, completely free with no signup required. All calculations run in your browser.
Why does SO2 target depend on wine pH?
Sulfur dioxide exists in wine in three forms: sulfurous acid, bisulfite, and sulfite. Only the molecular SO2 form (a tiny fraction at wine pH) is antimicrobial. Higher pH means less molecular SO2 per unit of free SO2, so you need more total free SO2 to achieve the same protective effect.
What is the target molecular SO2 for wine?
Most winemakers target 0.5-0.8 ppm molecular SO2 for antimicrobial protection. At pH 3.5 that requires about 22-35 ppm free SO2. At pH 3.8 you need 53-85 ppm free SO2 to hit the same molecular target.
What is the difference between potassium and sodium metabisulfite?
Potassium metabisulfite (KMS) is preferred in winemaking because it adds only potassium and SO2 — no sodium. Sodium metabisulfite works the same way but adds sodium to the wine, which most winemakers prefer to avoid.
How often should I add sulfites to wine?
Test free SO2 every 2-3 months during aging and before racking, bottling, or bulk storage. Free SO2 gradually binds with wine components and dissipates. Keeping levels in the target range protects against oxidation and microbial spoilage throughout the aging process.
Can I sulfite wine too much?
Yes. Excessive free SO2 causes off-aromas (burnt match, garlic) and can inhibit or kill malolactic bacteria if MLF is desired. Always test current free SO2 before adding more. In the US, wines for sale cannot exceed 350 ppm total SO2.