Brew in a Bag (BIAB) UK Guide: All-Grain Brewing in One Pot

Brew in a bag is the method that made all-grain brewing accessible. Instead of a three-vessel system with a mash tun, hot liquor tank and boil kettle, you mash the grain in a large mesh bag inside your boil kettle, lift the bag out, and boil in the same vessel. One pot, one heat source, and a considerable saving in money and washing up. This guide is based on desk research: supplier specifications, established brewing references, and UK home-brewing practice rather than first-hand testing. Below you will find the equipment you need, the method step by step, honest notes on efficiency, and how BIAB compares with the alternatives.

What You Need

Prices are indicative and were correct at the time of writing. Always check current prices before buying.

Why BIAB Instead of Three Vessels?

Traditional all-grain brewing uses three vessels: a hot liquor tank for heating water, a mash tun with a false bottom or manifold for the mash, and a boil kettle. It works well and remains the standard for larger batches, but it means three vessels to buy, store, clean and heat.

BIAB collapses this to one. The mesh bag replaces the mash tun’s false bottom — the bag holds the grain and lets the wort through, and you lift the grain out rather than draining the liquid away from it.

The advantages:

• Substantially cheaper to get started, and the kettle is the only significant purchase.
• Far less cleaning, and much less to store — a real consideration in a British house or flat without a garage.
• Faster, typically by 45 to 60 minutes, since there is no separate sparging step in the full-volume method.
• Fewer transfers between vessels means fewer opportunities to introduce infection or lose heat.

The trade-offs:

• Efficiency is usually a few percentage points lower than a well-run three-vessel system, though a fine crush recovers most of that.
• You must lift a bag of wet grain — for a 23-litre batch, roughly 5kg of grain holding several litres of water, so 10kg or more. This is the single most common complaint and the main practical limit.
• Batch size is capped by the size of pot you can safely heat and handle.
• The kettle must hold the full volume of water plus grain, so it needs to be considerably larger than the batch size.

For most UK home brewers making 19 to 23 litre batches in a domestic kitchen or garage, BIAB is the sensible choice.

Sizing Your Kettle

This trips people up, so work it out before buying. For a full-volume BIAB you need to hold, at the start of the mash:

• your final batch volume (say 23 litres), plus
• boil-off, typically 3 to 4 litres per hour, plus
• water absorbed by the grain, roughly 1 litre per kilogram, plus
• losses to trub and hop material, perhaps 2 litres, plus
• the physical volume of the grain itself, and
• enough headroom to avoid a boil-over.

For a 23-litre batch with 5kg of grain, that lands somewhere around 32 to 35 litres of capacity. A 30-litre pot is marginal; 35 to 40 litres is comfortable. Buying too small is the most common early mistake, and boil-overs on a kitchen hob are genuinely unpleasant to clean up.

If your kettle is too small, you can use BIAB with a sparge: mash in a smaller volume, lift the bag, and rinse the grain with additional hot water in a separate container. This recovers most of the efficiency and reduces the peak volume, at the cost of an extra step and vessel.

Choosing a Bag

The bag material matters more than it appears. You want a fine mesh — commonly polyester voile — that holds grain particles back while letting wort flow freely. Too coarse and you get grain in your boil; too fine and it drains slowly.

Look for:

• Food-grade material rated for boiling temperatures. Cheap curtain voile is used by many brewers, but confirm it is untreated and suitable for food contact.
• Sizing to your kettle, with enough excess to fold over the rim and secure with clips or a bungee.
• Strong seams, since the bag carries the entire weight of wet grain. Reinforced or double-stitched seams are worth having.
• A drawstring or handles, which makes lifting far easier.

Several UK suppliers sell purpose-made BIAB bags sized to common pot diameters, which is easier than making one. Expect them to last many brews if you rinse and dry them properly.

The BIAB Method, Step by Step

1. Calculate and heat your water. Add all the water for the batch to the kettle. Heat to your strike temperature, which needs to be a few degrees above target mash temperature because the room-temperature grain will cool it — typically 2°C to 4°C higher for a full-volume BIAB, less than the 8°C to 10°C a three-vessel system needs, because the water-to-grain ratio is much higher.

2. Fit the bag. Line the kettle with the bag and fold the excess over the rim, securing it with clips so it cannot slip in.

3. Mash in. Turn off the heat. Add the crushed grain slowly, stirring as you go to break up clumps. Dough balls — dry pockets inside lumps of wet grain — are the enemy, as the starch inside never converts. Stir thoroughly.

4. Check and hold the temperature. Aim for a mash temperature in the region of 64°C to 68°C for most beers. Lower in that range (64°C to 65°C) favours beta-amylase and produces a drier, more fermentable wort; higher (67°C to 68°C) favours alpha-amylase and leaves more body and sweetness.

5. Hold for 60 minutes. Insulate the kettle — a sleeping bag, camping mat, or old duvet wrapped around it works well — to limit heat loss. Check the temperature occasionally and apply brief gentle heat if it drops more than about 2°C, stirring while you do so to avoid scorching the bag against the base.

6. Optional: mash out. Raising to around 75°C for 10 minutes halts enzyme activity and thins the wort, making it drain more freely. Not essential, but it helps.

7. Lift and drain the bag. This is the physical part. Lift slowly and let it drain over the kettle. A pulley on a beam, a hoist, or simply resting the bag on a grid over the pot all make this easier and safer than holding several kilos of hot wet grain at arm’s length.

8. Squeeze — yes, really. Long-standing brewing advice warned that squeezing the grain extracts tannins. In practice BIAB brewers squeeze routinely and report no astringency, provided the mash pH and temperature were sensible. Squeezing recovers a meaningful amount of wort and improves efficiency. Use gloves; it is hot.

9. Boil as normal. Bring to a rolling boil, add hops on your schedule, and boil for 60 minutes. Watch carefully during the first few minutes, as the hot break is when boil-overs happen.

10. Chill, transfer, pitch. Cool the wort as quickly as you can — an immersion chiller is the usual approach — then transfer to the fermenter, take a gravity reading, and pitch your yeast.

Efficiency, and How to Improve It

Brewhouse efficiency is the proportion of available sugar you actually extract. Three-vessel systems commonly achieve 75% to 80%. Full-volume BIAB typically lands around 65% to 75%.

This does not matter as much as it sounds, provided you know your figure — you simply add slightly more grain to hit the same gravity. What matters is consistency: knowing your system does 70% every time lets you build recipes that land where you intended.

To improve it:

• Crush finer. This is the single biggest lever. Because the bag does the filtering rather than the grain husks, BIAB does not need an intact husk bed the way a three-vessel mash does. Many BIAB brewers set their mill to a noticeably tighter gap than standard, and gain several percentage points. The limit is a bag that will not drain.
• Stir during the mash. A couple of gentle stirs improves contact between enzymes and starch.
• Squeeze the bag properly. Worth several percent on its own.
• Consider a short sparge. Rinsing the lifted bag with 2 to 3 litres of water at around 75°C recovers additional sugar for very little effort.
• Mash a little longer. 75 or 90 minutes instead of 60 gives enzymes more time, which particularly helps with a coarser crush.

BIAB vs All-in-One Electric Systems

Units such as the Grainfather, BrewZilla and similar all-in-one electric brewing systems are, in essence, BIAB automated. They use a perforated malt pipe instead of a fabric bag, add a pump to recirculate the wort, and include integrated temperature control and often a built-in chiller.

They cost considerably more — typically several hundred pounds against under a hundred for a pot and bag — and in exchange they remove the manual lifting, hold mash temperature automatically, and recirculate for better efficiency and clarity. Grain is lifted out in a rigid basket rather than a floppy bag, which is far easier to handle.

Whether that is worth it depends on how often you brew and how much you mind the manual steps. Someone brewing monthly and enjoying the process may be perfectly happy with a pot and a bag indefinitely. Someone brewing weekly, or with back or mobility issues that make lifting wet grain a genuine problem, will find the money well spent.

FAQ

What size pot do I need for BIAB? For a standard 23-litre batch, aim for a 35 to 40 litre kettle. You need to hold the full batch volume plus boil-off (3 to 4 litres per hour), water absorbed by the grain (roughly 1 litre per kilogram), losses to trub, the physical volume of the grain itself, and headroom to prevent boil-overs. A 30-litre pot is workable but tight and leaves little margin. If your pot is smaller, mash in less water and sparge the lifted bag separately to make up the volume.

Is BIAB efficiency worse than three-vessel brewing? Usually by a few percentage points — around 65% to 75% against 75% to 80% for a well-run three-vessel system. This matters far less than consistency, because once you know your system’s typical efficiency you simply adjust the grain bill to suit. A finer crush is the most effective way to close the gap, since the bag rather than the husk bed does the filtering, so BIAB tolerates a much tighter mill setting than traditional mashing.

Can you really squeeze the bag without extracting tannins? In practice, yes. The traditional warning against squeezing comes from concerns about tannin extraction, but the conditions that actually cause astringency are high mash pH and excessive sparge temperatures rather than mechanical pressure. Large numbers of BIAB brewers squeeze routinely and report no off-flavours, while gaining useful extra wort and efficiency. Keep your mash temperature sensible, avoid sparging above about 78°C, and squeeze away — with heatproof gloves, because it is hot.

Do I need to sparge with BIAB? Not with the full-volume method, which is the main attraction — all the water goes in at the start, so there is no separate sparging step and the brew day is shorter. That said, a short sparge is a cheap way to recover efficiency: after lifting the bag, rinse it with 2 to 3 litres of water at around 75°C and add that to the kettle. It is also the standard workaround if your pot is too small to hold the full volume at once.