Calculating and Adjusting Kettle / Brew Bucket Volumes

BYO recently published their article about calibrating your equipment and it got me thinking about how the volumetric calibrations can often leave a lot to be desired. I’ve gone and added some graduations to a couple of my smaller carboys, but have yet to get around to my larger 6.5 gallon ones. Another area that I never bothered to add graduations to are my brew kettle and the utility bucket I collect my first and second runnings into.

After mashing I’m usually under volume by 2 or 3 litres but the wort is typically at a higher gravity so diluting it to hit my target is a far easier operation. In the past I’d just wing it by adding a couple of litres of water and hoping for the best, but considering how I’m focusing on consistency in my brewing processes I figured that more time should be put into calculating what the actual adjustments should be.


It really is

At the teaching brewery we don’t use graduations in our boil kettle, instead we grab a ruler and hit a lookup table to figure out what volume we have. I took a similar approach for my own setup but instead of using a lookup table I decided use some basic algebra to get an approximation. I have the 30.28 (8 gal.) kettle from OBK, which has an inner diameter of 36.6cm. Whenever I’ve collected all my wort and am about to start the boil I figure out the how tall my headspace is.

With that volume in hand and knowing the density of water (1cm3 == 1ml) I’m able to figure out how much wort I actually have. This combined with a gravity reading makes figuring out how much water I need to add extremely simple.


Because I can never remember how to calculate the volume of a cylinder

For example, let’s say I had 10cm of headspace above my 1.055 SG wort and my target was 1.048. First I’d calculate how much volume I actually have:

Volume(total) = Volume(wort) + Volume(air)
Volume(total) = 30280
Volume(air)  = 0.25 * 3.14159 * square(36.6) * 10
             = round(10520.870751)
             = 10520
Volume(wort) = 30280 - 10520
             = 19760

Next up I need to do an equality calculation to figure out how many (total) litres of wort I’d need to have a gravity of 1.048. Remember, this is done by only taking the number after the decimal and treating it as a whole number.

19760 * 55 = 48 * X
round((19760 * 55) / 48) = X
22642 = X

Addition required: 22642 – 19760 = 2882 => 2.882L

So in order to drop my gravity by 7 points, I’d need to add approximately 2.9 litres of water.

While I’ve been brewing for quite some time, it’s been a very yolo approach to it and I never really cared if I was over gravity. I’d end up with a stronger beer, which wasn’t a huge problem. However, since one of my goals this year is to aim for consistency and I’m going to be brewing the same recipes way more frequently, nailing these things down is actually going to matter.

I’ve also been motivated by figuring out the laziest way possible to calculate how much volume I actually have in my kettle. I have a good idea of where my volume should be at, and can eyeball that, and markings on my kettle really won’t help that much. Math is a super simple tool that can give you far more accurate estimations, even if your model is assuming a perfect world (i.e. volume displacement from components, density of wort vs. distilled water, etc.) you are going to be able to make far more informed decisions and hopefully get closer to your targets.

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About Chris

If there is a place that serves or sells beer I've never heard of, I'll find a way to get there. A fan of the humble Pale Ale, though always willing to try a pint of something new. I also enjoy brewing my own beers and love sharing the science of the beer making process.