Sulfite Addition Calculator

How to Use This Sulfite Calculator

Enter your batch size in gallons or liters, your measured free SO2, and your target. The calculator finds the ppm gap, converts it to grams of potassium metabisulfite using the 57% SO2 content, and shows teaspoons as a backup measure. It gives the straight dose; the guidance below explains why your retest will read 10 to 20% lower after binding.

The Dosage Formula

The 570 divisor folds together the 57% SO2 content of K-meta and the mg/L-to-ppm equivalence: ppm x liters gives milligrams of SO2 needed, divided by 570 gives grams of powder. A level teaspoon of K-meta weighs about 5.6 grams, but powder density varies, so grams are the reliable unit. Campden tablets at 0.55 g each are pre-measured but cost more per dose.

Worked Example

Five gallons (18.93 liters) of wine at 10 ppm free, targeting 35 ppm: gap = 25 ppm. Grams = 25 x 18.93 / 570 = 0.83 g. That is about 0.15 teaspoons, or 1.5 Campden tablets. After adding, expect the retest to show roughly 30 ppm free instead of 35, because 10 to 20% binds on contact. Top up with a small second addition if needed.

Practical Tips for Sulfite Additions

Always dissolve powder before adding; dry powder dumped into a carboy distributes unevenly and can bleach color locally. Add at racking when the wine is already moving. Keep a log of every addition with dates; total SO2 is cumulative and the legal limit counts everything. Store K-meta powder sealed and dry; it loses strength in humid air. If you overshoot, splash-racking and time reduce free SO2, but prevention beats correction.

Addition Calculator vs Molecular SO2

This calculator answers "how much powder do I weigh out" once you know the ppm gap. The molecular SO2 calculator answers "what should my target ppm be" from your wine's pH. Run the molecular calculator first to get the target, then this one to get the dose. Target first, dose second; never dose without a target.

Second worked example: 5 gallons of wine at crush

Five gallons is 18.9 liters. The standard crush addition is 50 ppm, which is 50 mg per liter: 18.9 times 50 is 945 mg of SO2. Potassium metabisulfite (KMS) is 57.6 percent SO2, so divide 945 by 0.576: about 1.64 grams of powder, roughly one-third teaspoon. Dissolve it in a cup of must or water first; dry powder dumped on top dissolves unevenly and can bleach patches of skins. In Campden tablets, one tablet per gallon adds roughly 50 ppm, so 5 tablets does the same job. Stir the must well after adding and let it sit 12 to 24 hours before pitching yeast.

Why sulfite at crush matters

Freshly crushed grapes carry wild yeast and bacteria on every skin. SO2 at crush knocks those populations down so your chosen yeast strain wins the fermentation instead of fighting for it. It also blocks the oxidation enzymes (polyphenol oxidase) that turn white juice brown within hours of crushing. Without that addition, wild fermentations start unpredictably and whites darken before fermentation even begins. The 50 ppm standard is a compromise: enough to sanitize the must, low enough that a healthy yeast pitch powers through it after the 12-hour wait.

Total versus free versus bound: what actually protects

Only free SO2 protects the wine; bound SO2 is tied to acetaldehyde and sugars and does nothing. Total SO2 is the sum, and the US legal limit is 350 ppm total. Over a wine's life, free SO2 keeps binding and fading, which is why you test and top up every few months. A wine can show 100 ppm total and be unprotected if only 5 ppm is free. Additions should always be calculated from the free SO2 gap, not the total. Keep a log of every addition with the date; when the total creeps toward 200 ppm in a white, start thinking about whether the wine needs another dose at all.

Addition mistakes

Adding sulfite to a fermenting wine and stalling it is the classic: yeast under 5 percent alcohol can stick at high SO2, so wait until fermentation finishes. Dosing by Campden tablets without checking tablet size is second; tablets range from 0.44 to 0.55 grams, a 25 percent swing. Adding powder to a full carboy without stirring leaves pockets of high concentration that smell like burnt matches for weeks. And the health worry is mostly misplaced: the sulfites in a glass of wine are a fraction of what dried apricots carry, and true sulfite allergy is rare. Asthma sensitivity is the real, though uncommon, concern.

Worked Example: The 54-Gallon Barrel

A 54-gallon (204 L) barrel of red wine needing 50 ppm SO2. Milligrams needed: 50 × 204 = 10,200 mg, or 10.2 g of SO2. Potassium metabisulfite is 57 percent SO2 by weight: 10.2 / 0.57 = 17.9 g of powder. Dissolve in a cup of wine, stir into the barrel gently but thoroughly. That single addition protects the wine for 2 to 3 months, after which free SO2 is remeasured and topped up.

The common home-scale version: a 5-gallon carboy at the same 50 ppm needs 0.95 g, about 1/4 teaspoon of powder (a level 1/4 tsp of K-meta weighs roughly 1.4 g, so a scant quarter teaspoon). This is why the "1/4 tsp per 5 gallons" rule of thumb exists: it lands near 50 ppm without a scale. Rules of thumb are fine; the scale is better.

How SO2 Actually Protects Wine

Sulfite protects two ways: antimicrobial (it inhibits wild yeast, bacteria, and the spoilage organisms that make vinegar and mousiness) and antioxidant (it scavenges oxygen and binds the aldehydes that make wine taste flat and brown). The active form is molecular SO2, a tiny fraction of the total: at wine pH 3.5, only about 2 percent of free SO2 is molecular, which is why pH matters so much.

Lower pH wines need less sulfite: at pH 3.2 you need about 25 ppm free for 0.8 ppm molecular; at pH 3.8 you need about 60 ppm free for the same protection. This is the real reason the calculator asks about pH: the same "50 ppm addition" protects very differently at different acidities. High-pH wines are sulfite-hungry; low-pH wines are naturally defended.

Potassium Metabisulfite vs. Campden Tablets

Potassium metabisulfite powder (K-meta) is the flexible choice: cheap, measurable to the tenth of a gram, no binders. One gram in one gallon gives about 150 ppm, so small additions are easy with a gram scale. Campden tablets are pre-measured convenience: one tablet per gallon gives roughly 50 to 75 ppm depending on the tablet (sodium vs. potassium metabisulfite, 0.44 g vs. 0.55 g).

Tablets win for 1-gallon batches and for sanitizing (one tablet per gallon of water makes a no-rinse sanitizer). Powder wins for 5-plus gallons, where tablet counting gets silly and precision matters. Avoid sodium metabisulfite for wine additions if you watch sodium intake; potassium is the wine standard. Either way, store airtight: metabisulfite degrades in humid air within months.

Pro Tips for Small-Batch Winemakers

Buy a $20 gram scale. Volumetric measures (teaspoons) of powder vary 20 percent with packing and humidity; at sulfite levels, that variance is the difference between protected and over-sulfured. The scale is the cheapest quality upgrade in home winemaking.

Add sulfite to the receiving vessel before racking, not after: the turbulence of racking mixes it perfectly with zero extra stirring (and zero extra oxygen pickup). And keep a log: date, amount, free SO2 reading, pH. Six months later, the log tells you your wine's consumption rate, and the next vintage gets easier.