The purpose of this guide is to clarify what kind of wort gravity and batch sizes you can expect to accomplish with certain vessel sizes.
First we'll look at traditional 3 vessel brewing and then get to Brew In a Bag.
In a multi vessel system, the batch size is first limited by the boil kettle size. Aside from using an inefficient partial boil technique, all grain wort must be boiled down to final volume. 5 gallon batches generally start out as 6.5 to 7 gallon preboil worts and require at least a gallon of headspace (7.5 to 8 gallons of kettle space minimum). For a 10 gallon batch, a 15 gallon kettle is practical.
More importantly, the standalone mash tun size is what will determine the maximum gravity you can achieve for a given batch size. To best explain this, we've created a chart.
Just for example, a 10 gallon mash tun can do 5 gallons of 1.118
barleywine (~11% ABV) and 10 gallon batches of 1.060 IPA (~6% ABV). New all grain brewers will probably wonder why the chart has different efficiency examples like 70% and 80%. To be clear, the sugar production and extraction is technique and process dependent so you never really know what your efficiency will be until you brew a while and track it.
Combining the kettle boil requirements and mash tun data, you can make some decisions. 5 gallon batches can make use of a minimum of 8 gallon boil kettle and 5 or 10 gallon mash tun. 10 gallon batches (up to 1.088 OG) will need a 15 gallon kettle and 10 gallon mash tun.
Brew In A Bag is taking a brewing community by storm and even the old timers are starting to relax their grip on traditional three vessel all grain brewing techniques. BIAB has several advantages such as less cleaning, less liquid transfer, less equipment, more stable pH throughout the brew and generally a faster brew day. The downsides are minor and probably the most important one is the upper limit of wort gravity given certain kettle sizes. Of course it's only a downside if you already own a certain size kettle and don't want to replace it.
BIAB is typically done as a full volume mash with no sparging which means when the grain bag is removed, you are left with enough wort to begin boiling down to the finished batch size immediately. This makes the total kettle size important. The chart below shows the max gravity you can achieve per batch and kettle size assuming a no sparge process.
| Batch Size | Kettle Size (gallons) | Max Grain Weight (LBS) | Max Gravity at 70% efficiency | | 5
| 10 | 22 | 1.106 | | | 5 | 15 | 50 | 1.220 | | | 10 | 15 | 22 | 1.077 | | | 15 | 20 | 25 | 1.058 | | | 15 | 25 | 50 | 1.116 | |
As you can see, by contrast BIAB will require a larger kettle than multi-vessel brewing all else being equal.
The good news is that there is an easy trick to getting a little more gravity out of a given system by performing a small sparge. Since grain displacement is such a small portion of the overall kettle volume requirement, it's the strike water that takes up most of the space. Let's take the 10 gallon batch in a 15 gallon kettle scenario for example. At full volume, it can only take 22 pounds of grain because you'll already have 13.25 gallons of water. If you remove 2 gallons of that water, the kettle can hold a whopping 38 pounds of grain! Since you'll still need that water for the volume, the easiest way to add it back in without losing efficiency is to dunk the grain bag into the 2 gallons of water in a separate container like a bucket. Of course in this example, you could not possibly submerge 38 pounds of grain under only 2 gallons of water so you can suspend the bag over the kettle with a colander or grate and do your best to pour the 2 gallons through the grain as evenly as you can. It will pick up at least some of the residual sugars on the grain which is better than simply topping off with water in the kettle.
Note that the max gravity in the chart assumes 100% starch conversion and your mileage may vary.
A decent conclusion to make is that a 10 gallon pot is still completely viable for just about any 5 gallon batch and frankly, that's about the smallest kettle you'd want to boil 7 gallons in anyway. For people that may want to do the occasional batch or low to moderate strength beer for parties and such, a 15-16 gallon pot will let you do that without losing the ability to brew 5 gallon batches as well. When you get up to 15 gallon batches, the 20 gallon pot is quite limited so you would want to look at the 25 gallon. In recent years, we're finding seemingly odd capacity kettles that happen to work great. For example, we now stock a Brewmaster kettle at 18.5 gallons which is perfect for 10 gallon BIAB with no sparge needed.
Summary:
If you already have an 8 gallon pot, the fastest way to all grain is to add a 10 gallon cooler for a mash tun and sky's the limit for 5 gallon batches. If you have a pot smaller than 8 gallons, you may as well upgrade the kettle to a 10 to 15 gallon kettle and do BIAB since you'll need the kettle upgrade anyway.
There are other considerations for mashing in separate vessels like beverage coolers for hands off temperature stability as well as tricks for holding temps in BIAB kettles, but it's out of the scope of this guide.
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