Keeping An Eye On Water And Wort Volume By Tracking Losses

My obsession with knowing more about my brew process continues. This time I’m looking closely at wort volumes.

This article explains why and how to measure the changing volumes of liquid on brew day, and how that makes recipe planning more effective.

wort volume in home brewing brew pot

Rather than let things stagnate, I’m currently making a concerted effort to improve my brew process.

It’s not that I’m not happy with my brews, just that I could be more in control and make better beer.

My approach, apart from naturally building up experience by brewing different beers, is to explore each component alone and in detail.

In parallel with explorations of single hop and malt flavours, I’m also looking at different parts of the brew process.

Which leads me back to wort and water volumes.

Why Measure Volumes On Brew Day?

When planning recipes from scratch the process usually follows a similar path.

You think about what you want to make and how you can do it based on what you already know.

If you’re like me, you also cross-check other recipes to look for common themes or interesting ideas.

The process typically ends with balancing numbers in brewing software.

I’ve come to realise that for this to be useful, the beer that arrives in the glass needs to bear some resemblance to the one you planned.

Leaving aside IBUs and bitterness for now, the gravities of the beer (original and final) usually get a lot of attention. To a large extent they drive the amount of malt and fermentables used.

Gravity in simple terms is the amount of sugar in a beer. Specific gravity of a liquid, which is what brewers usually talk about (1.047, 1.010, etc.), is a measure of density which means that it changes in relation to the volume.

The same amount of sugar in a larger volume of water makes a lower gravity wort, and vice versa.

So you need to know how much liquid you’ve got if you want to make plans about gravity.

Even if you’ve only brewed one batch, you’ll know that the starting volume of water is not the quantity of beer you end up with. There are losses to evaporation, grain, trub, spillages and more.

To predict beer characteristics such as strength and gravity you need to know how much sugar you’re going to extract (absolute gravity points, independent of volume) and how much liquid you retain after brewing.

I’m saving gravity points for another day, so the rest of this article is about measuring volume.

Hopefully by now you’re convinced it’s something worth bothering with.

When To Measure Wort Volume

There are certain moments on brew day when it’s useful to check the volume, usually at a change from one part in the process to the next.

wort volume: when to measure wort and water volumes

When To Measure Wort Volume: 1. Starting volume (of water); 2. After removing the bag; 3. Adding back drained wort (pre-boil volume); 4. Post-boil volume; 5. Volume in fermentor;
6. Beer volume.

This diagram describes brew in a bag (BIAB). Whichever brewing method you use, you need to take measurements at similar points in the process.

1. Starting Volume

Fairly self-explanatory, this is the amount of water you add to the brew pot at the start.

Usually this is determined with brew software, or is an educated guess based on experience.

2. After Removing the Bag

This is the liquid that’s left in the pot when you remove the brew bag.

It’s not strictly necessary to measure this, but it’s interesting to compare the wort in the pot with what drains from the bag.

3. Pre-Boil Volume

When you mix in the drained wort you can measure your pre-boil volume.

After several brews you can compare pre-boil and post-boil volumes with boil length to work out your evaporation rate.

This is usually measured as volume per hour or as a percentage. I use this simple formula to get litres per hour:

{{Pre-Boil Volume}-{Post-Boil Volume}}/{Time}={Evaporation Rate}

Pre-Boil Volume = Litres
Post-Boil Volume = Litres
Time = Hours
Evaporation rate = Litres per hour

I can’t think of a reason why the formula would be different working in gallons.

I prefer litres per hour because when I make small batches I lose much more in percentage terms than with large batches. I’m still checking how constant my evaporation rate is across batch sizes though.

Most brewing calculators let you enter the evaporation rate to feed into water volume calculations, so straight away you get more dependable predictions out of the software.

Knowing the evaporation rate you can also estimate your post-boil volume based on the pre-boil one, in real time on brew day.

That opens up the possibility of changing the boil length or adding additional sugars to ensure you hit your predicted gravities.

4. Post-boil volume

As 3. above.

5. Volume in Fermentor

This is the post-boil volume with hop and other debris removed.

6. Beer volume

This is the beer you bottle or keg. Trub in the fermentor, losses during siphoning and breakages all mean that this is less than the volume in the fermentor.

In my experience the losses here vary too widely from batch to batch to make averages useful.

It largely depends on how well I run the siphon. If I have to stop and start it for a blockage, for instance, I lose more beer. Cracking a bottle loses more.

I’m happy to just add an extra litre of water at the start to account for this. Sometimes I lose more, sometimes less. In any case, the gravity’s pretty fixed by this point so it’s not as important.

You may want to measure the liquid you add with the priming sugar, if it’s a significant amount.

How To Measure Wort Volumes

To track the losses you need to measure the liquid in the brew pot and in the fermentor.

I’m fortunate that my brew pot is a perfect cylinder so it’s fairly easy to calculate volumes. Using my thermometer, I measure the distance from the top of the brew pot to the surface of the water.

I know how much water I started with and the difference between the measurements represents the loss.

If your brew pot is unevenly shaped (an old keg for example) you need to calibrate it manually.

That’s how I calibrated my ever so slightly conical fermentor to show how much beer is in it, to the nearest litre.

Problems With The Method

Because water expands when heated you’re not really measuring volumes relative to each other. It’s similar to the way that gravity changes with temperature.

But as usual I’m trying to be consistently inaccurate and establish methods that I carry from batch to batch, rather than from brewer to brewer.

For now, I’m happy with this rough approach. It’s a lot better than not measuring anything.

If you want more accurate results for yourself you could try measuring volume by weight instead.

I haven’t included specific numbers for my set-up in the article because I want to build a substantial set of measurements before extracting average values.

But more importantly, it would be almost meaningless anyway because your brew kit is different.

If you don’t calibrate your own system, you may as well use the average values from your brew software.

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  1. Keith

    John: I really appreciate your website and articles. Thanks! I understand this article…but what should be done? Add a little extra at fermentation?

    Ex: I just made my first ever batch and was surprised that the original 5 gallons (sorry, but we used gallons), which I thought would turn out to be almost 2.5 cases turned out to be 47 bottles. We did 2 weeks in the fermenter and then two more in a secondary. I assume the large amounts of trub in both account for the loss of volume…

    • John


      Yes, and there will also be volume lost during the transfer between primary and secondary.

      You need to work out how much you are losing each brew and add that right at the start. You could also top up just before fermentation, but I would use that more as a way of adjusting the strength if you extracted more or less sugar from the malt than expected.

      The only way to really find out how much extra to start with is by brewing several batches, accepting that in the short term the final amount of beer could be a bit of a lottery.

      Cheers and good luck.

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