How this instrument works
Potassium metabisulfite (KMS, K2S2O5) is the standard sulfite source home winemakers use to protect wine against oxidation and spoilage microbes, both at crushing and again before bottling. What actually does the protective work is sulfur dioxide (SO2), measured in parts per million (ppm) of "free" SO2 — the active, unbound portion available to do its job, as opposed to SO2 that's already bound up with other wine compounds.
This calculator uses the common home-winemaking dosing formula: grams of KMS = gallons × target ppm SO2 × 0.0066. It's a simplified shortcut, but it holds up well against the underlying chemistry — KMS is about 57.64% SO2 by weight, so 1 gram of KMS in 1 gallon of wine works out to roughly 152 ppm of SO2, and solving the same case through this formula (1 gallon, 1 gram KMS) gives about 151.5 ppm, a match within 1%.
A typical starting free-SO2 target for a finished, ready-to-age or ready-to-bottle wine is around 50 ppm, sometimes pushed to 75 ppm for wines meant for longer aging, and it usually needs to be adjusted based on the wine's pH and how much free SO2 it already has, which this calculator doesn't measure for you. If you're serious about precise sulfite management, pair this dose calculation with an actual free-SO2 test kit rather than assuming zero starting point every time.
- Enter your wine's volume in gallons.
- Enter your target free SO2 level in ppm (a typical starting point is 50-75 ppm).
- Read Potassium metabisulfite (KMS) needed for the grams to dissolve and stir into the batch.
- Dissolve the KMS in a small amount of wine or water before adding it to the full batch for even distribution.
- Retest free SO2 with a test kit if you're topping up an existing addition rather than dosing a fresh, untreated batch.
Worked example — a 5-gallon batch targeting 50 ppm free SO2
A 5-gallon batch of homemade wine is ready for its first sulfite addition, targeting a common starting point of 50 ppm free SO2: kmsGrams = 5 × 50 × 0.0066 = 1.65 grams of potassium metabisulfite, dissolved in a bit of water or wine and stirred into the full batch.
Push the target to the higher end of the commonly cited 50-75 ppm starting range for a wine intended for longer aging, still at 5 gallons: kmsGrams = 5 × 75 × 0.0066 = 2.475 grams — about half again as much as the 50 ppm dose, since the formula scales linearly with target ppm.
A small 1-gallon batch at the same 50 ppm target needs proportionally less: kmsGrams = 1 × 50 × 0.0066 = 0.33 grams, illustrating how directly the dose scales with volume for a fixed target concentration.
Questions
What's the difference between free SO2 and total SO2?
Total SO2 is every sulfur dioxide molecule in the wine, whether it's actively protecting the wine or already bound up with sugars, acetaldehyde, and other compounds. Free SO2 is the smaller, unbound portion still available to fight oxidation and microbial spoilage — it's the number that actually matters for protection, and it's what this calculator's target ppm refers to.
How much free SO2 should I target?
A commonly cited starting point for a finished wine is 50-75 ppm free SO2, with higher-pH wines generally needing more since SO2's protective molecular form (molecular SO2) is less available as pH rises. Wines intended for extended aging often run toward the higher end of that range. This calculator doesn't adjust for pH, so treat 50-75 ppm as a reasonable general target, not a precise number for every wine.
Does this calculator account for SO2 the wine already has?
No — it calculates the dose needed to reach the target ppm from zero, assuming an untreated batch. If you're adding a second or later sulfite addition to a wine that already has some free SO2 in it, test the current level with a free-SO2 test kit first and only dose for the remaining gap to your target, or you risk oversulfiting.
Is the 0.0066 constant exact?
It's a widely used, practically accurate shortcut rather than a lab-grade exact constant. Cross-checking it against the underlying stoichiometry — KMS is about 57.64% SO2 by weight — gives a very close match (within about 1%) for the same gallon/gram scenario, which is why this formula is trusted across home-winemaking references despite its simplicity.
Can I use Campden tablets instead of powdered KMS?
Yes, but you'll need to convert — a standard Campden tablet is commonly cited as roughly 0.44-0.5 grams of potassium metabisulfite (or sodium metabisulfite for some brands, which behaves similarly but isn't identical). Divide this calculator's gram result by your tablets' stated KMS-per-tablet weight to get the tablet count, and check your specific product's packaging since tablet weights vary by brand.