Brew in a Bag (BIAB): A Simple Guide to One-Pot Brewing
The brew in a bag method, or BIAB brewing, is one of the simplest ways to get into all-grain brewing without needing a full multi-vessel setup. Using a single pot and a grain bag, the BIAB method means you can mash, boil and brew your beer in one place – allowing you to create authentic craft beer at home with minimal equipment and space requirements.
In this guide, we’ll walk through the brew in a bag method step by step, covering the equipment you need, how the BIAB process works and a few useful tips to help you get the best results. Whether you’re moving on from extract brewing or looking for a more streamlined way to brew all-grain beer, BIAB is a solid place to start.
Table of Contents
What Is Brew in a Bag (BIAB)?
Brew in a bag (BIAB) is a method of all-grain brewing where the entire hot-side process takes place in a single vessel. Instead of using separate vessels such as a mash tun, hot liquor tank and boil kettle (as you would in a more traditional three-vessel brewing system), the brewing bag is typically placed into the kettle first, with the grain then added into the hot liquor inside the bag for the mash. This helps keep fine grain particles within the mash, rather than losing them through the mesh.
Once the mash is complete, the grain bag is lifted out of the kettle and allowed to drain, removing the need for a separate sparging step. The wort can then be brought straight to the boil in the same pot, making the overall brew in a bag process much simpler than traditional multi-vessel setups.
Because everything happens in one pot, BIAB brewing is often referred to as one pot brewing. It’s a practical approach that reduces equipment requirements while still allowing you to brew full all-grain recipes at home.
In simple terms, BIAB brewing is all-grain brewing made more accessible by reducing the amount of equipment and steps involved.
Benefits of the Brew in a Bag Method
The brew in a bag method has become a popular choice in homebrewing because it simplifies the all-grain brewing process without taking away the control that brewers are looking for. By keeping everything in one pot, brew in a bag brewing reduces both the amount of equipment needed and the number of steps involved on brew day – offering several distinct advantages for both newcomers and experienced brewers.
1. Simplicity and Accessibility
One of the main advantages of brewing in a bag is how straightforward the process is. There’s no need for separate mash tuns, sparging setups or complex transfers between vessels. For many homebrewers, this makes the BIAB method an easier way to move into all-grain brewing without overcomplicating things.
2. Less Equipment and Lower Setup Costs
Because the brew in a bag process uses a single kettle and a grain bag, the overall setup is much simpler than a traditional all-grain system. This low barrier to entry makes it an excellent way to experiment with all-grain brewing before committing to more expensive setups.
3. Space-Saving Brewing
BIAB is well suited to smaller brewing spaces. Whether you’re brewing in a kitchen, garage or utility room, one pot brewing keeps the footprint of your setup manageable while still allowing you to produce full all-grain batches.
4. Full Control Over Your Beer
Even though the setup is simpler, the BIAB method still gives you full control over your recipe, ingredients and brewing process. You’re still working with grain, mash temperatures and hop additions in the same way as any all-grain brewer, just with a more streamlined setup.
What Equipment Do You Need for BIAB Brewing?
One of the biggest appeals of brew in a bag brewing is that the equipment list is shorter than a traditional all-grain setup. You still need the basics to mash, boil, chill and ferment, but the overall setup is much easier to manage because the hot-side process happens in one pot.
1. A Suitable Brew Kettle/Heat Source
The kettle is the centre of any brew in a bag setup, so choosing the right size matters. As a rule of thumb, your pot will usually need to hold a little under twice the volume of the batch you want to brew, as you need room for the full liquor volume as well as the grain.
Good starting points:
- The Malt Miller 35L Kettle – a good fit for standard 20L BIAB batches. You can also get this with a powerful 3kW electric element included.
- The Malt Miller 25L Kettle – for 10-12L batches, our 25L kettle has loads of extra room, allowing you to think about bigger beers that require much more grain (which means more liquor).
- The Malt Miller 12L Kettle – BIAB is also ideal for super small batches; this kettle will fit on a normal kitchen hob and is perfectly sized to make batches for a 5L fermenter or demijohn.
2. A BIAB Grain Bag
A good quality grain bag is what makes the BIAB method possible. The bag holds the crushed grain during the mash, then allows it to be lifted out once conversion is complete. Fine mesh bags are the usual choice, as they help hold back grain particles while still allowing wort to circulate.
Good starting points:
- TMM Tailored Grain Bag for 35L Kettle – a strong starting point for standard 20L brew in a bag batches.
- TMM Tailored Grain Bag for 25L Kettle – a good fit for smaller BIAB setups where you want to keep batch size more manageable.
- Grain Bag Fits all Kettles Up to 100L – a simple option for BIAB brewing in any sized kettle.
- Stainless Steel Mesh Basket for 50L Kettle – worth considering if you prefer a basket-style setup rather than a fabric bag. This can make it easier to apply heat during the mash, as there is less risk of the material coming into contact with an exposed element.
3. Temperature and Brewing Tools
Mash temperature matters in any all-grain method, and BIAB is no different. A reliable thermometer helps you stay on target, while a long spoon or paddle makes it easier to stir the mash and manage the boil. Heatproof gloves are also worth having, particularly once the grain bag is full of hot, wet grain.
Good starting points:
- Brewing Thermometers – essential for checking mash temperature and keeping the BIAB process on track.
- Mash Paddles and Brewing Spoons – useful for stirring in the grain, breaking up dough balls and managing the mash more evenly.
- Heatproof Brewing Gauntlets – strongly recommended when lifting and handling a wet grain bag at the end of the mash.
4. Fermentation and Cooling Equipment
Once the boil is finished, the process becomes much more familiar. You still need to cool the wort, transfer it cleanly and ferment it properly. BIAB simplifies the hot side, but you still need the usual back-end equipment to finish the batch well.
Good starting points:
- Wort Chillers – the easiest way to cool wort quickly and get on with the rest of the brew day.
- Fermenters – for standard BIAB batches, a good fermenter is still the main bit of kit you’ll need once the wort is cooled.
- Demijohns and Small Fermentation Vessels – especially useful if you’re brewing micro BIAB batches in a 12L or smaller kettle setup.
5. Cleaning and Sanitising Essentials
Good starting points:
- Cleaning Products – for getting kettles, fermenters and other brewing kit properly clean before and after use.
- No-Rinse Sanitisers – ideal for anything touching the cooled wort, including fermenters, taps and transfer equipment.
Optional but Useful Kit:
- A hop spider or hop sock for containing hops
- A bazooka filter or similar for clearer transfers
- Basic water treatment supplies
- A method for cooling your wort
How to Brew in a Bag (Step-by-Step)
Once you’ve got your BIAB setup ready, the actual process is fairly straightforward. The main difference compared with a more traditional all-grain setup is that the mash and boil both happen in the same pot, with the grain held in a mesh bag throughout the mash.
Step 1: Water Preparation (The Liquor)
Start by working out your full liquor volume, as BIAB brewing uses all of the brewing water in the same pot from the beginning. That means you need to allow not just for the volume you want in the fermenter, but also for boil-off and the liquor absorbed by the grain.
A useful rule of thumb is to allow around 3 litres per hour of boil-off and roughly 1 litre per kilo of grain for grain absorption.
For example, with a target batch size of 20L, 5kg of grain, and a 1 hour boil:
- Boil-off losses = 3L per hour x 1 hour = 3L
- Grain absorption = 1L per kg x 5kg = 5L
- Batch size (20L) + boil-off (3L) + grain absorption (5L) = begin with 28L in your pot
Once you know your volume, heat the water to approximately 70–74°C. This will usually drop to your target mash temperature, typically around 65-68°C, once the room-temperature grain is added.
If you want to be more precise, you can use a strike water calculator to fine-tune your mash liquor temperature based on your setup and grain bill.
One of the key practical checks in BIAB homebrew is making sure your kettle has enough space for both the full liquor volume and the grain before you begin.
Step 2: Add the Grain Bag and Mash In
Once the water has reached the right temperature, fit your grain bag into the kettle and slowly add the crushed grain, stirring well as you go to avoid dry clumps or dough balls.
At this stage, the grain bag should hold the full grain bill while still allowing the liquor to circulate through it. Once all the grain is in, check the mash temperature and adjust if needed.
Step 3: Hold the Mash Temperature
Maintain the mash temperature for around 60 minutes. During this time, the starches in the grain are converted into fermentable sugars.
It helps to stir the mash occasionally to keep the temperature even throughout the pot. If you want to improve consistency, you can also gently recirculate the wort by scooping from the bottom and pouring it back over the top. On more advanced setups, a temperature controller and pump can be used to maintain a more stable mash temperature and keep wort moving more consistently throughout the mash.
Temperature Management Tips:
- Insulate the pot with something like a duvet or sleeping bag to help hold mash temperature steady
- Apply gentle heat only if needed, taking care that the bag does not touch the heat source
- Stir from time to time to avoid hot or cool spots in the mash
Step 4: Lift and Drain the Bag
Once the mash is complete, carefully lift the grain bag out of the pot and allow it to drain. This is one of the defining parts of the brew in a bag process, as removing the bag effectively replaces the more traditional lautering stage.
A wet grain bag can be surprisingly heavy, so take care here. Heatproof gloves or gauntlets are strongly recommended, particularly for larger batches. Many brewers let the bag drain naturally into a clean bucket or vessel, then return the drained wort to the kettle.
For a simple BIAB process, there is no real need to squeeze the bag aggressively. Letting it drain well on its own is often enough.
Step 5: Bring the Wort to the Boil
With the grain removed, bring the wort to a boil and begin your hop additions according to the recipe. In most cases, you’ll be aiming for a 60 minute boil, although this can vary by beer style.
Boil Management Tips:
- Boil with the lid off once the wort is properly underway
- Watch closely at the start, as boil-overs are most likely early on
- Add hops gradually if needed to avoid sudden foaming
- Keep a spray bottle of clean water nearby to knock back foam if the boil starts to climb
- Stir occasionally to reduce the risk of scorching, especially on smaller or more direct-heat setups
- Pro tip: adding your first hop addition just before the wort reaches a rolling boil can help reduce the risk of boil-overs, as it helps break the surface tension
Step 6: Cool the Wort
Once the boil is complete, cool the wort as quickly as possible before transferring to the fermenter. There are a few ways to do this, depending on the setup you are using.
Common Cooling Options:
- Overnight cooling in a sanitised fermenter or a no chill cube (a good option if you want to reduce water usage)
- Ice bath cooling
- Immersion chiller
- Plate chiller
The best method will depend on your setup, batch size and how quickly you want to move into fermentation.
Step 7: Transfer and Ferment
Once the wort is cooled, transfer it into a clean, sanitised fermenter and pitch your yeast. From here, the process is much the same as any other all-grain or extract brew.
As always, anything touching the cooled wort needs to be properly cleaned and sanitised. Good cold-side hygiene is just as important in BIAB brewing as it is in any other brewing method.
Check out our video on BIAB, and remember to subscribe to our YouTube channel for all the latest brews and reviews from The Malt Miller.
FAQs: Brew In a Bag
What is the difference between brew in a bag and traditional all-grain brewing?
Both methods are forms of all-grain brewing, so the main difference is the setup rather than the principle. With brew in a bag, the mash and boil both happen in one pot, with the grain held in a mesh bag during the mash. Traditional all-grain brewing usually uses separate vessels for mashing, lautering and boiling.
That means BIAB is generally simpler, more compact and easier to manage at home, especially if space or equipment is limited. Traditional systems can offer more process separation, but BIAB brewing still gives you full control over recipe, mash temperature and hop additions.
Is BIAB beer good for beginners?
Yes, BIAB brewing is one of the best ways for beginners to get started with all-grain beer. The process is simpler than a traditional multi-vessel setup, the equipment list is shorter, and the overall brew day is easier to manage.
That makes the BIAB method a good stepping stone for brewers who want more control than extract brewing offers, but do not want to jump straight into a more complex all-grain system. You still learn the core parts of all-grain brewing, just with fewer moving parts.
How long does the brew in a bag process take?
For most homebrewers, a full brew in a bag process will usually take somewhere around 4 to 6 hours, depending on batch size, heating power and cooling method.
That usually includes heating the water, mashing for around 60 minutes, boiling for around 60 minutes, then cooling and transferring to the fermenter. Some setups will move faster than others, but BIAB is generally one of the more streamlined ways to make all-grain beer at home.
Can you do a full-volume mash with a brew in a bag system?
Yes – that is one of the defining features of the brew in a bag method. In BIAB brewing, the full volume of brewing liquor is usually added to the pot from the start, rather than splitting the mash and sparge across multiple vessels.
This is what makes kettle size so important in a BIAB setup. Your pot needs enough room for the full liquor volume as well as the grain, with enough headspace to manage the mash and later the boil safely.
How do you improve efficiency in BIAB brewing?
The main ways to improve efficiency in BIAB brewing are to use the right crush, keep mash temperature steady, stir the mash well, and give the grain bag enough time to drain properly. In most cases, starting with a standard crush size is more than adequate and gives a good balance between efficiency and ease of use.
Some brewers experiment with a finer crush to improve extraction, but this depends on your setup and needs to be dialled in carefully. At The Malt Miller, we offer fine, standard and uncrushed grain options, so you can either choose a crush that suits your system or mill your own at home if you want more control.
Consistency matters too. If your liquor volumes, mash temperatures and process are steady from batch to batch, it becomes much easier to understand where efficiency is being gained or lost. In practice, good BIAB efficiency usually comes from getting the basics right rather than adding complexity.
Is BIAB better than extract brewing?
Not necessarily better in every situation, but it does give you more control. With BIAB, you are working directly with malted grain rather than relying on malt extract, so you have more influence over mash temperature, fermentability and the overall makeup of the wort – which also means you have more control over the final flavour profile of your beer!
That said, extract brewing is still a very useful method and often the quickest way to get started. BIAB sits in a nice middle ground for brewers who want to move into simple all grain brewing without taking on a full traditional all-grain setup.
