BIAB water calculations

nickwalters@icloud.com

New Member
Hi, I'm new to homebrewing and am mostly trying to figure it out on my own. There's so much differing information online, and most of it is imperial units, which makes it harder for me to follow.

I've already made two BIAB batches, an IPA and an APA, both of which turned out surprisingly well. However, I'm having trouble figuring out how to make calculations about how much water to use at each stage, so some advice would be great!

I have a 14 L fermentor and a 16 L kettle (a regular pot with no dead volume). Using a bag to mash 3.5 kg grains, I want to end up with 14 L in my fermentor. In my first two batches I made the mistake of covering the kettle during boiling, and my more experienced friend could taste a little DMO, though not loads, so next time I will leave uncovered during boiling and cooling. I don't yet know the evaporation rate from the kettle if leaving uncovered.

Because the kettle is only slightly bigger than the fermentor, and because the grains absorb some water, I would like to know how much water to use at each stage. I was thinking that one way to solve this would be to batch sparge the mashed grains. My brewhouse efficiency last time was around 58%, so sparging might also help improve that slightly. So would something like the following work?

• Prepare 13 L strike water at 68°C
• Mash 3.5 kg grains in 13 L water at 66°C
• Batch sparge grains in 4 L water at 76°C
• Boil the two batches together
• If the cooled volume is under 14 L and the OG is high enough to do so, top up with water to 14 L

I'm largely just guessing here, because I'm having trouble figuring out the calculations because the information online varies so much, so your help would really be appreciated!

Sounds plausible. What grain are you looking to use? Probably not that helpful, but a bigger pot is going to make things much simpler to predict.

Sounds plausible. What grain are you looking to use? Probably not that helpful, but a bigger pot is going to make things much simpler to predict.
Thanks for the reply, Mark. I'm mainly use pale ale malts so far, with up to 8% crystal T50. I agree that a bigger pot would make this much easier (the one I ordered was advertised as 17 L but turned out to be 16 L. I may invest in one in future, but right now would like to see if I can make it work with what I've got.

So using my BIAB system as an example, I'm:
• putting 12.8 litres of water in the kettle
• expecting to lose 2.8 litres of water to grain absorption when mashing
• starting the boil with 10 litres
• losing 3 litres for the 60 minute boil
• ending with 7 litres at 1.109
Problems with that are that I'm assuming 70% efficiency. I'd probably go lower to 60-65% efficiency because of the thicker mash and the amount of grains. But to keep the thought experiment going...

If I just dilute that with 7 litres of water to get to 14 litres in the fermenter I get 1.055, so with an average attenuation I'm probably getting something around 5.8-5.9 % ABV.

The dunk sparge and boil in the other pot will give you a few more points so if everything went perfectly you may be in the low 6% ABV range for a finished beer. There's a lot of unknowns and you should get your boil off rate worked out before you try a batch, but your idea still sounds plausible.

No idea about what that amount of dilution does to the taste, but you're not doing anything that hasn't been done before (though it may be more dilution than is generally the case).

Good info from Mark there!

To figure out your boil off rate, put a measured amount of water in your kettle, boil it for 30 minutes. When it cools measure how much water is left. Deduct that from the starting volume, multiply by 2 and there is your approximate voil off rate to put into your calculations.

Craigerr has a good suggestion. The metric imperial conversions are a pain in the ass so I tend to have a bunch of google calculator windows up while doing a brew day.

Thank you all for your replies, and sorry for my slow response.

On the day I posted this, I made a small batch (5.5 L) of grapefruit IPA. I did a 15 min batch sparge (instead of just pouring water over the grains in the bag) and had enough space in my pot to boil both batches together. My brewhouse efficiency was >79%, compared to 58% before. As this was higher than expected, my OG was very high, and I was able to add water in the fermentor. The beer's not ready yet (only started bottle conditioning a few days ago), but it tastes ok, despite finishing a little sweet. Using Irish moss for the first time did some interesting things to the trub in the fermentor.

Now I'm going back to the APA I'd made early (with 58% efficiency) and trying to scale up to completely fill my fermentor. Yesterday I tried to achieve 14 L, but due to the limitation of my pot, even with two batch sparges and fairly high gravities, I only managed to get 11.75 L into my fermentor, and this involved topping up. I think it's going to be a decent batch, as I also did several things that should improve it, including removing the hot and most of the cold break, using Irish moss, and aerating the cool wort before pitching.

I think the only decent solution for me can be to buy a bigger pot, so I'm waiting for a 30 L pot to come back into stock in my city. Below are some water management estimates, which show that a 30 L pot should have plenty of room for a 15 L batch (but let me know if you disagree with my numbers!). Next batch I will try to make this same APA again, but with the 30 L pot, using larger quantities of water and attempting to get 15 L into the fermentor without needing to top up, so that I can bottle 14 L. From the next batch onwards, I will make one change at a time so I can track what each variable does.

I'll let you know how it goes with the new pot.

Mashing 3.5 kg grains in bag in 18 L water in 30 L kettle; loss to grain absorption ~2 L; yield 16 L
Batch sparging grains in bag in 6 L water in 15 L kettle; yield 6 L
Boiling mixed wort batches (22 L) in 30 L kettle; loss to evaporation, hop absorption & hot break ~5 L; yield 17 L
Cooling 17 L wort in kettle in sink; loss to shrinkage & evaporation ~0.5 L; yield 16.5 L
Transferring 16.5 L wort from kettle to fermentor; loss to cold break in kettle ~1.5 L; yield 15 L
Transferring 15 L beer from fermentor to bottles; loss to trub & dry hop absorption in fermentor ~1 L; yield 14 L

IIRC, the general rule of thumb for kettle size with BIAB is the kettle should be at least double your batch size. You might want to get a 10.5 gallon (40 liter) kettle, it will give you a bit of breathing room, and if you ever want to mike a higher gravity beer, you will have the space for the additional grains needed.

Problem is I'm mashing and boiling on an induction stove (which is really good at keeping the temperature steady), so I can't go any bigger than 30 L, because they are so wide they would cover and possibly switch off the touch controls of the stove. Plus I live in an apartment, so wouldn't want to have anything much bigger to store. Do the estimates above not sound reasonable?

30L kettle should be fine, if you do higher gravity beer you could just top up with water in the fermenter.

Sounds good. And who knows, in a year I might band together with some mates and buy some bigger gear and rent a space to brew in...

“Start where you are. Use what you have. Do what you can.”
– Arthur Ashe

Good info from Mark there!

To figure out your boil off rate, put a measured amount of water in your kettle, boil it for 30 minutes. When it cools measure how much water is left. Deduct that from the starting volume, multiply by 2 and there is your approximate voil off rate to put into your calculations.

Hi so I just ran this test as I’m planning to do my first BIAB next weekend.

started with 8L H20, heated it up and as soon as it hit a boil, I started a timer kept it at a low roll for 30 minutes exactly, removed from heat let it cool down and I was left with 6.4L. So that would be a 33% boil off rate?

So next weekend If I’m shooting for 10L to hit the fermenter I should start my boil with approx 16L?

I think you'll be more accurate using a boil off rate instead of a percentage. Since you boiled off 1.6 liters in 30 minutes, your boil off rate is 3.2 liters per hour. So if you want 10 liters and plan to boil for an hour, start with 13.2 liters.

Keep in mind that the boil off may vary a bit due to weather, burner setting, boil gravity, etc. Just make an adjustment at the end of the boil.

Haha. I was overthinking it obviously. That is way simpler. Thanks Bubba.

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