These are marked at 1-Gal and 1.5 Gal, with approximately 3in. of "head space" to the rim/lid. One could functionally use for up to 1.75-Gal if one had to. However, at the 1.5-Gal line, one can easily "swirly" the liquid without removing the lid as I like to do with a primary-fermentation. As far as air tight, I'd have to say, yes. Define that? - Well, I would not expect to drop it and not have it leak, or the lid to pop off (especially after the locking-band is pealed off of it). However, for what it is intended to be used for, --primary fermentation, it is perfect. Again, what does that mean? - Specifically, the Lid and drilled-hole-gasket does seal well enough that if fermentation exists, the "fermentation gases", quickly enough pressurize the container, displacing the "air" and causing "bubbling" in the "Air-lock". With standard, and good, sanitizing practice I have only had a good result with these. - For the price point I paid, I am satisfied with it. I would expect to use it for many numbers of primary-fermentation's. It appears the Lids inner, soft gasket can be replaced; and if those can be found to do so, --I would consider that. I would not expect the Lid, or Pail to fail or wear out for this use (aside from the gasket), for what I provide a guess at of, two-years of use. Use for me for this "use", would be 6 - 8 times a year, possibly up to 20 uses in two-years, --offered as an impression of use quality. The quality seems consistent across several lids/pails I have in hand. I would not want to pay more that I did and feel good about what it is, and is for. To offer further qualification of use by me to gauge for yourself: Presently 5-Gal batches are to big and/since I like have three batches of different things going, offset by a week or two. However, 1-Gal is a bit "small". Ideally a 2.5-Gal would be great for me and could just use the 5-Gal items for it; however, I am concerned by the amount of head space in the 5-Gal sized, and the general size for home-kitchen use (kitchen sink, etc.), and if one does not have a vigorous fermentation on occasion, --the 3in head space when these pails are at 1.5-Gal line seems ideal overall. And all equipment at this capacity can be handled well enough in a Kitchen space; not having to use a "slop-sink" or garden hose, etc. for maintenance.
