Brew in a Bag: A Complete Guide

You might not think homebrewing has much in common with enjoying a nice cup of tea. But as homebrewers who use the brew in a bag (also known as BIAB) method can attest, you can make a delicious, high-quality beer with the same convenience and as rustling up a mug of Earl Grey.

Stay with us, we promise this makes sense. The technical term for the boil in a bag method is all-grain, single-vessel, no-sparge brewing.

Brew in a Bag: A Complete Guide

Photo Credit: James Spencer of Basic Brewing.

All-grain brewing means you’ll be using crushed malts. The “vessel” in question here is the boiling pot—that’s our mug equivalent. The “no-sparge” bit means you can skip rinsing your grains, a process also known as lautering.

And as you’ve no doubt figured out, the “tea bag” is the bag you’ll use to steep your grains and create beer.

You’ll still need to ferment the resulting wort, of course, and there’s some math involved when you’re setting up. But trust us—if you can make tea, you can brew in a bag.

An Easier Way to Homebrew

Whether you’re adding a new set of tools to your existing setup, want to try an alternative method of all-grain brewing, or are brand-new to homebrewing and want to keep things simple, the brew in a bag method makes it easy to get started.

Regardless of your preferred method, the goal when homebrewing is to transform grain, water, hops, and yeast into that nectar of the gods we call beer. This process has, at its simplest, four steps:

  1. Transforming the starches in your grain malt into sugars by mashing
  2. Separating the wort, or liquid portion of your brew, from the grain
  3. Adding hops and boiling the liquid to achieve the desired bitterness, and then cooling
  4. Adding yeast to transform the sugars in your brew into alcohol to create beer

Brewing in a bag simplifies this process by allowing you to do it with fewer steps and with less equipment than you’d need for a traditional homebrew, saving you time, money, and effort.

Brew in a Bag Supplies

You don’t need much to go from zero to brew with this method. For the brewing, you’ll need:

Armed with these few items, you’ll have everything you need to get your brew ready for mashing.

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A Note on Kettle Size

Because the brew in a bag method uses just one vessel for mashing and boiling, you’ll need to get yourself a larger brew kettle than you would if you were planning to do a traditional multi-vessel brew.

If this seems like overkill, remember you need to have enough room for your pre-boil recipe as well as the volume of wort you’ll be boiling to transfer to your fermenter after chilling.

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The Right Bag for the Right Brew

You can find any number of high-quality brewing bags online or at your local homebrewing supply store. But if you’re really into the DIY spirit that comes with being a homebrewer, you can also find designs online to make your own brewing bag from voile fabric or nylon.

You might also find that some folks recommend paint-straining bags, but voile is a finer fabric, and making your own means you can customize it for your kettle.

Whatever bag you choose, be sure it’s sized to fit into the kettle without touching the bottom during the boil as direct contact with the hot metal can scorch your grains or even melt your bag! We’re betting that won’t make for a tasty beer.

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Preparing Your Brew in a Bag Setup

As with any other style of brewing, the first thing you want to do is clean and sterilize all your equipment. Because this is an all-grain brewing method, you’ll also need to make sure you have your grains pre-crushed and ready to mix.

Crushing the grains will ensure they’re open and ready to have their starches released and transformed into sugars. Your supplier can usually handle this for you, unless of course you’re a little more hardcore and have also learned how to crush your own malts.

Let’s Brew in a Bag!

Once you’ve gathered your supplies, you’re ready to get started. But first, you’ll need to run a few calculations.

Running the Numbers

You’ll want to take note of two particular numbers in your recipe, and both involve specific gravity, or the ratio of a liquid’s density to water. For example, a liquid with a specific gravity of 1.040 is 1.04 times the density of water.

In homebrewing, specific gravity is used to help measure sugar concentration and, by extension, the qualities of the beer produced by a given amount of sugar.

The two numbers you’ll be focused on are the target original gravity (OG), which refers to the specific gravity of your mash before it’s been fermented, and the target final gravity (FG), which is the specific gravity of your finished beer.

You’ll note that FG is always lower than OG. This is because by the time you measure the FG, the yeast added during the fermentation process will have eaten up most of the sugar in the wort. Sugar is denser than water, but once it’s converted to alcohol, it’s less dense than water.

For example, the beer we’ll be using in our explanatory calculations has an OG of 1.054, and an FG of 1.014.

Make note of both OG and FG on your recipe or brewing sheet, as you’ll be referencing them throughout the brewing process.

You’ll also need to calculate a few numbers on your own. Unlike traditional brewing, the brew in a bag method does not use the homebrewer’s standard of 1 to 2 quarts of water per pound of grain.

Instead, you’ll calculate the volume of strike water, which is simply a name for the water that gets added to your grains to create your mash, based on the desired volume of your wort.

Note: The calculations that follow are in Imperial units. To convert for metric, simply use these conversions:

1 pound = 2.2 kilograms

1 gallon = 3.74 liters

1 gallon per pound = 8.4 liters per kilogram

Note: If just the idea of doing math makes your brain cramp, you can use a BIAB calculator like this one, and then skip to the next step in the process.

How Much Strike Water Do You Need?

To determine this value, you need to know three variables:

So let’s say your recipe calls for a finished batch size of 5 gallons of beer. Using those three variables, you can find your pre-boil volume, or the total volume of wort you need to boil to produce your five-gallon batch of beer.

Our variables are set as follows:

Total Grain Weight: 12.5 lb
Grain Absorption: 0.1 gallons per pound of grain
Boil-off Rate: 1 gal per hour
Batch Volume: 5 gallons
Boil Time: 1 hour

To find our pre-boil volume, we just need to plug them into the following formula:

T x B + V = P


T is the Boil Time (hours)

B is the Boil-Off Rate (gallons per hour)

V is the Batch Volume (gallons)

P is the Pre-Boil Volume (gallons)

Which gives us:

1 x 1 + 5 = 6

We’ll need a pre-boil volume of six gallons to brew five gallons of beer.

Now that we know our pre-boil volume, we can calculate our strike water volume, which uses the following formula:

A x G + P = SV

T is the Boil Time (hours)

B is the Boil-Off Rate (gallons/hour)

A is the Grain Absorption Rate (gallons/pound of grain)

V is the Batch Volume (gallons)

G is the Total Grain Weight (pounds)

P is the Pre-Boil Volume (gallons)

SV is the Strike Water Volume

Giving us:

.1 x 12.5 + 6 = 7.25

We’ll need to add 7.25 gallons of strike water to our grains to brew our 5-gallon batch of beer.

Feeling Hot, Hot, Hot: Calculating Strike Water Temperature

Another important factor you’ll need to manage during the brew in a bag process is temperature. Your target mash rest temperature is the value specified for your mash while it rests before you remove the bag and proceed to the boil.

A good rule of thumb to follow for determining how hot your strike water should be is to add between 8°F and 12°F (between 4.4°C and 6.6°C) to the desired mash rest temperature.

For our example, let’s say the recipe calls for a 60-minute mash rest at 153°F (67.2°C). Following our rule, the strike water should be heated to between 161°F and 165°F (between 71.6C and 73.8°C).

Staying in the target range for your brew is important because it helps your grains release the sugars that will feed the yeast that turns your wort into delicious beer!

Let the Brewing Begin: Making Mash

Now that you have all the information and materials you need, it’s time to brew. The first thing to do is get the mash ready and allow it to rest.

  1. Add the strike water you calculated from your recipe to the kettle, and heat it to the target temperature.
  2. While wearing brewer’s gloves, add the brew bag to the kettle and secure it with clips, bungee cords, etc., to prevent it from touching the bottom of the kettle.
  3. Add your ground malted grains to the bag; stir and cover.
  4. Use a thermometer to make sure you hit the target mash temperature. If you need to adjust it, you can add ice to cool things down, or add more hot water to increase the temperature. You can also turn on the burner for a bit to raise the temperature.
  5. Remove the kettle from heat and wrap in the insulation of your choice.
    • Important: We recommend moving the kettle away from the heat source. But if you decide to insulate in place, keep anything flammable away from the base of the kettle and triple-check you’ve turned off the heat.
  6. Allow the kettle to rest as specified in the recipe you’re using.

If possible, resist the urge to check the temperature until the end of the rest, as this will help your mash conserve its heat.

Hoisting the Bag

Once the timer goes off and you’ve unwrapped your rested mash, check the temperature and make sure you’re within the target range. Slight variations of up to 4° are normal, so don’t panic if the temperature has dropped.

Now you can lift the bag and allow it to rest on the hook or hoist or whatever device you’ve selected to hold up the bag while it drains, which should take about 15 minutes. You can speed up the drain by putting on your brewer’s gloves, grabbing a friend, and gently squeezing the bag to encourage the extra wort to flow into the kettle. It’s kind of like squeezing a tea bag, only with two people and light industrial equipment.

Once the drain is complete, you can dispose of the grains simply by carrying the bag to the garbage can and giving the bag a shake. Your spent grains also make an excellent addition to your compost pile.

Rinse the bag out thoroughly and set it out to dry, or if you’re adding hops during the boiling stage rather than dry-hopping your beer, give it a rinse and use it to hold your hops.

It’s also time to test the OG of your wort at this stage. For this, you’ll use a hydrometer. Try to stay within plus or minus .05 of the target value.

If the specific gravity is too low (i.e., too much water to sugar in the ratio), you can boil off some liquid to raise it, then test it again. If it’s too high (i.e., too much sugar to water in the ratio), you can add some water to lower it. Or, if you’d like, leave it alone and give your brew a little extra kiss of alcohol. We promise not to tell!

Note: When you’re adding hops, remember they will increase the bitterness of your beer the longer they’re boiled. So if, for example, you test the OG of your homebrew before boiling and it’s too low, let evaporation reduce the volume of your wort to the correct level and then add your hops and boil as directed by the recipe for your particular beer style.

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Beyond the Bag and into the Boil

With your wort ready to roll, it’s time to heat things up once more. Boil your wort for 60 to 90 minutes, as directed by the recipe you’re using. Add hops when desired or as directed.

If you’re adding the hops to the boil and reusing the brewing bag to do so, be sure not to let it touch the bottom or sides of the kettle to avoid scorching. If you used a tripod or pulley hoist to suspend your bag, you can use it again here to keep the bag away from the kettle’s interior.

Alternatively, use a hop spider to keep things tidy and scorch-free.

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Bag it Up!

When you’ve finished the boil, your brew in a bag is complete. Kill the heat, remove your hops, and that, as they say, is that. All that’s left is to cool your wort and transfer it to the fermenter of your choice for what may be the only hard part of this and any brew: waiting for your beer!

The thing we love the most about the brew in the bag method is its accessibility. It doesn’t matter if you’ve brewed a thousand batches or are just starting out—if you want to brew beer, this approach makes it easy.

So roll out the hoist, grab your favorite grains, and polish up that kettle. Chances are, your next great beer is in the bag.

Happy Brewing!