Brew-in-a-bag mash bag

A filter that fits the kettle

A brew-in-a-bag mash bag is a large fabric filter that sits inside the kettle used for an all-grain beer mash. Crushed malt is held in the bag while hot strike water circulates through it. The fabric lets wort drain while retaining most of the grain. Once the mash is finished, the brewer lifts the grain-filled bag away, leaving the wort in the kettle for boiling. The bag is equipment for the mash and lautering stages; it is not the kettle, a fermentation vessel, or the complete beer-brewing system.

In the simplest single-vessel, no-sparge version, the same kettle holds the full mash volume and later the wort boil. The American Homebrewers Association also describes BIAB variations that add a sparge, in which the bag is rinsed or dunked in more hot water. Those options change water volumes and sometimes require a second vessel; they do not change what the bag is. The full-volume method in Fermentea's Monk’s Ration recipe is an example of the bag's role, without naming a brand or model.

Mesh, fabric and construction vary

There is no single standard construction. The AHA guide describes voile as widely used by homebrewers and says that bags can be made from curtain or bulk fabric. Premade examples have sewn rims, reinforced seams, lifting loops, straps, drawstrings or other ways of holding the opening at the kettle rim. One manufacturer identifies its bag as polyester; another lists a polyester-mesh product with reinforced nylon seams. These are documented choices, not proof that all BIAB bags share one fibre or seam design.

Mesh opening matters because wort must pass through while the bag retains grain. The AHA notes that a coarse bag can let more grain sediment into the boil and that a very fine bag may impede the passage of wort sugars. Retail products illustrate the range rather than define a standard: Craft a Brew lists 63-micrometre mesh for its product, while Craft Hardware sells a 200-micrometre version and advises against pumping through that particular bag. Neither figure is a universal target. A bag should be chosen for its kettle, grain bill and intended drainage or recirculation setup.

The bag works by physical separation. It is not a mash basket: a rigid basket is built for a particular kettle or appliance, while the bag is flexible fabric that must be supported at the kettle rim or by its sewn tabs. Nor is it a mash tun, where a false bottom or other filter supports the grain bed and wort is run off separately. The main advantage of the bag is that it can replace that separate mash vessel and later be lifted out of the same kettle.

Fit and kettle size set the limits

The bag must match the kettle. The AHA guide says it should fit around the kettle's circumference without resting on the bottom, both to retain grain and to avoid scorching. The Brew Bag's instructions for its own product call for a loose fit with the top overhanging the kettle rim by two to four inches. The exact fit and fastening method depend on the bag design; a bungee, drawstring, sewn loops or rim clips are not interchangeable requirements.

For a full-volume mash, the kettle must hold the strike water and grain together, with enough additional room for the planned pre-boil volume. The AHA recommends a kettle of at least ten gallons for its example five-gallon batch, assuming a 7–7.5 gallon pre-boil volume, and notes that higher-gravity beers may need more space. This is a kettle recommendation tied to its stated example, not a capacity rating for the bag or a rule for every batch. No common bag capacity or dimensions are established across products.

The bag should also clear the heat source during use. The AHA says some brewers insert it after heating the water to reduce the chance of scorching. The Brew Bag maker says its polyester bag must not touch the kettle bottom while its flame is on and warns that the material is not fireproof. That advice is specific to the maker's product; a different material may carry different instructions. A bag being sold for brewing does not make its fabric flameproof or suitable for every kettle configuration.

Lifting separates the grain from wort

After mashing, the brewer lifts the bag so wort drains back to the kettle. A bag with sewn loops can be lifted by hand or attached to a support; larger wet grain bills make the load heavier. One maker suggests using suitable support in the brewing space and cautions against putting uneven pressure on the kettle. A community photograph of a very large BIAB rig shows a hoist lifting a filled bag, but that image documents one unusual scale and is not a typical household setup.

Drainage advice differs. The AHA's guide says to let its example bag drip and does not recommend squeezing. The Brew Bag's manufacturer demonstrates squeezing its own product as an option. The evidence does not establish one handling rule for every bag and batch. A brewer should follow the exact bag instructions and recipe process, with a stable kettle and support that can carry the wet load.

A BIAB bag can also be used in a kettle with a sparge or with additional equipment such as a pump. These are modified setups. The Craft Hardware seller warns that its 200-micrometre bag is not suitable for pumped recirculation and recommends a coarser product from its range for that purpose. This is a useful example of how a particular mesh can limit a setup, not proof that all bags or pumps behave alike.

Buying, making and cleaning a bag

Premade bags are sold by homebrew shops, and the AHA guide also describes homebrewers sewing their own from curtain or bulk fabric. A purchased bag should match the kettle's internal size and have material and heat instructions for its intended use. A DIY bag needs fabric that is suitable for the job; the AHA's description of voile as widely used does not certify every textile sold under that name. The intended use is hot wort and grain, so the source of the material and the bag's construction matter.

Care instructions also differ by product. The Brew Bag says its mash-use bag can be shaken out and rinsed thoroughly on both sides, then hung to dry. It advises handwashing after the bag is used for hop additions, because hop oils remain. These instructions concern that manufacturer's sewn bag. The AHA guide's mash procedure says not to squeeze its example bag; another manufacturer's directions demonstrate optional squeezing. Rinse, cleaning agents and drying should therefore follow the product's own guidance rather than an assumed rule for all fabrics.

Contemporary product pages show bags sold across North American, New Zealand and German/EU home-brewing markets, alongside DIY approaches described by the AHA. That supports treating the modern bag as internationally available without claiming a single country invented it. Brewshop's historical account suggests an Australian origin for BIAB, but also says the exact history is difficult to trace; the sources gathered here do not establish an origin date for this equipment form.