Last updated: August 23, 2026
Day three of your first sauerkraut, you lift the towel off the jar and the top inch of cabbage is sitting proud of the brine. It has gone gray at the edges, there is a soft bloom on the shreds that were exposed, and the rest of the jar underneath looks and smells exactly as it should. That is the single most common way a home ferment goes wrong, and it is not a recipe problem.
Cabbage floats. Cucumbers float. Sliced peppers, carrot sticks and green beans all float. The brine is denser than the vegetable, and lactic fermentation produces gas that lifts pieces further. A weight is the small, dull piece of gear that stops all of it, and choosing the right one is mostly a question of jar mouth, jar shape and how much fiddling you are willing to do.
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Quick Answer
Do you really need fermentation weights?
- You need something holding the vegetables under the brine. The National Center for Home Food Preservation says cabbage and cucumbers must be kept one to two inches under brine while fermenting.
- A purpose made weight is the easy version of that. NCHFP’s own low tech answer is a plate weighted down with sealed water filled quart jars.
- Match the weight to the jar mouth, not to the brand of your lid. Wide mouth and regular mouth weights are different diameters and are not interchangeable.
- Glass is the default. Ceramic suits crocks, and a spring follower suits tall jars where a disc alone rides up.
Bottom line: buy the weight that fits your jar mouth and covers most of the surface, because everything above the brine is where the trouble starts.
Why Anything Above the Brine Goes Wrong
Lactic fermentation works because the vegetables sit in a salty, oxygen poor liquid where lactic acid bacteria outcompete everything else. A cabbage shred that pokes above the surface is in a completely different environment. It has air on one side, it dries out, and it becomes the landing pad for airborne yeasts and mold spores that cannot get a foothold down in the brine.
That is why the growth you see on a ferment almost always starts at the surface and almost always starts on the exposed pieces. Submerging the vegetables does not make the jar sterile, and it does not stop a surface film from forming on the brine itself. What it does is remove the dry, oxygen rich real estate that gives mold somewhere to grow.
Surface Film Is a Separate Question
Even a perfectly weighted jar can develop a thin white film on the brine. NCHFP describes scum on a fermenting brine as wild yeasts and bacteria that feed on the acid, and its instruction is simply to remove the scum as often as needed. Fuzzy, raised, colored growth is a different matter, and our kahm yeast versus mold guide walks through how to tell them apart before you decide what to do with the jar.
What NCHFP Actually Recommends for Covers and Weights
The official guidance is more low tech than the gear aisle suggests. NCHFP tells home fermenters to set a dinner plate or glass pie plate inside the container, slightly smaller than the opening but large enough to cover most of the vegetables, then weight that plate down with two or three sealed quart jars filled with water.
Two things are worth pulling out of that. First, the goal is coverage, not just mass: the plate is doing as much work as the jars on top of it. Second, NCHFP also notes that if you use jars as the weight you have to check the ferment two or three times a week and remove scum, whereas a properly sealed cover lets the batch sit until bubbling stops. Modern weights and airlock lids are a convenience version of exactly that trade.
Fermentation Weights by Situation
The Default for Wide Mouth Mason Jars
A flat glass disc sized for a wide mouth jar is what most people should buy first. It covers nearly the whole surface, it is inert so the acid does not touch it, it goes in the dishwasher, and it stacks in a drawer between seasons. Buy them in a set of four rather than singly, because you will almost always have more than one jar going at once.
If Your Jars Are Regular Mouth
Regular mouth weights are a smaller diameter and often come with a notch or a small handle, because getting a plain disc back out of a narrow neck with wet fingers is genuinely annoying. Check the listed diameter against your jar before ordering, since a wide mouth disc simply will not pass through a regular mouth shoulder.
For a Crock Rather Than a Jar
Stoneware crocks are usually sold with, or matched to, two half moon ceramic weights that together cover the whole surface and can be lowered in one at a time. That two piece design exists because a single disc big enough for a crock is heavy and awkward to lift out of a full vessel. If you are still choosing the vessel itself, our fermentation crock guide covers the water seal question first.
For Tall Jars Where a Disc Rides Up
In a half gallon jar packed with whole cucumbers or long carrot spears, a flat disc can float up on the gas the ferment produces. A stainless spring or follower braces against the underside of the lid and pushes down continuously, which keeps pressure on the pack instead of relying on dead weight alone.
Comparing the Weight Types
| Type | Fits | Strength | Watch out for |
|---|---|---|---|
| Glass disc | Wide or regular mouth jars | Inert, dishwasher safe, covers most of the surface | Hard to lift out without a notch or handle |
| Ceramic half moons | Stone and stoneware crocks | Heavy enough for a deep pack, lifts out in two pieces | Chips if knocked; check the glaze is food safe |
| Stainless spring | Mason jars under a standard lid | Constant downward pressure, works in tall jars | Needs a lid to brace against, so not for open crocks |
| Water filled bag or jar | Any container | Free, and the NCHFP fallback method | More scum checking; a leaking bag dilutes the brine |
The water filled jar and brine bag methods in the last row are the ones NCHFP publishes directly in its fermenting instructions; the rest are convenience gear built around the same principle.
Getting the Size and Fit Right
The number that matters is the outside diameter of the weight against the inside diameter of the jar at its narrowest point. A weight that clears the mouth but leaves a wide gap around the edge lets shreds escape up the sides, which is exactly the failure you were trying to prevent. Aim for a disc that covers most of the surface with only a small margin.
Depth matters too. NCHFP asks for the vegetables to sit one to two inches under brine, so you need headroom above the weight for that brine plus room for the foam a lively ferment throws up in the first few days. Packing a jar to the shoulder and then adding a weight is how people end up mopping brine off a shelf.
Weights and Lids Are Two Different Jobs
A weight holds food down. A lid manages the gas and keeps insects out. Neither substitutes for the other, and a lot of first ferments go sideways because someone bought an airlock kit and assumed it included the submersion problem. Our fermentation lids guide covers the gas side, and the fermentation kit guide covers what a sensible starter set contains.
Mistakes That Make a Weight Useless
Overpacking is the big one. A jar crammed to the rim leaves no space for brine above the weight, so the top layer is weighted and still not submerged. Pack to leave real headroom, then check the level again after a day.
The second is using a metal object that is not stainless. A fermenting brine is acidic and salty, and reactive metal will corrode and taint the batch. NCHFP’s list of acceptable fermenting equipment is food grade unchipped enamelware, glass, stainless steel and stoneware, and it warns outright against brass, copper and zinc utensils.
Frequently Asked Questions
My sauerkraut keeps floating above the brine, what do I do? Push it back down, top up with brine if the level has dropped, and put a weight on it. NCHFP asks for cabbage and cucumbers to stay one to two inches under brine, and a flat glass disc sized to your jar mouth is the simplest way to hold that.
Can I just use a small jar or a zip bag of brine instead? Yes, and that is close to the published method. NCHFP describes weighting a plate with sealed water filled quart jars, and it notes you then need to check for scum two or three times a week. Fill any bag with brine rather than plain water so a leak does not dilute the batch.
What weight do I need for a wide mouth versus a regular mouth jar? Different diameters, and they do not swap. A wide mouth disc will not fit through a regular mouth neck, and a regular mouth disc in a wide mouth jar leaves a gap around the edge where shreds float free.
Do glass weights stop mold completely? No. They remove the exposed food that mold grows on best, which is most of the problem, but a film can still form on the brine surface. NCHFP treats scum as wild yeasts and bacteria to be removed as needed, while fuzzy colored growth is a different call.
Are ceramic weights safe for food? Look for a food safe glaze from a known fermenting or pottery brand, and retire any weight that is chipped or crazed. NCHFP lists unchipped stoneware among acceptable fermenting equipment, with the emphasis on unchipped.
The Bottom Line
Fermentation weights solve one specific problem, and it happens to be the problem that ruins most first batches. Get the diameter right for your jar mouth, leave enough headroom that brine can actually cover the weight, and treat the weight and the lid as two separate jobs. Glass discs cover almost every mason jar setup, ceramic half moons belong in crocks, and a spring earns its place in tall jars that gas up. From here, the sauerkraut brine ratio guide covers the salt side of the same batch, and the fermentation jars guide covers what to put it all in.