Mastering Mash Efficiency: The Role of Grain Absorption Rate in Brewing

Grain Absorption Rate

When brewing your own beer, there’s a lot that you need to keep in mind – and one of the most important is grain absorption rate. 

Grain actually accounts for a very small percentage of the content of your beer overall. The vast majority of your beer is nothing more than simple water – more than 90%, in fact. That said, it’s important that you have an overall idea of how much water your grain will be absorbing (the grain absorption rate). 

Nowadays, there are a good many homebrew calculators that take all the hassle out of figuring out numbers and figures and make things nice and uncomplicated for even novice homebrewers. This is all fine and dandy, of course, and some people can go from A to C without needing to know B– but some of us maths nerds need to know the B of it. 

With that said, let’s take a look at grain and grain absorption rate. 

What Is Grain Absorption? 

What Is Grain Absorption

In terms of brewing, grain absorption is the term used for when the grain you’re using absorbs some of the water during the mashing process. Grain is relatively porous, and can hoover up a considerable amount of water during this process

It’s important that you have an idea of how much water is going to be absorbed; if you’re not sure about this and use too much or too little water, the beer you end up with is going to be very different to the beer you wanted. 

How Much Space Does A Pound Of Grain Take Up?

How Much Space Does A Pound Of Grain Take Up

A single pound of grain has a volume of about 0.3 gallons, or about 3 cups.

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How Can Grain Absorption Affect Your Beer? 

How Can Grain Absorption Affect Your Beer

Typically, the issue with getting grain absorption wrong (or not correctly accounting for it) is that your yield is less than you might have hoped, purely because you didn’t use enough water. It can also affect the final gravity of your beer, which can have implications for the final taste of your brew. 

Grain absorption isn’t the only way that you can lose water during the brewing process, however. Some water is inevitably lost during the process of home-brewing, and it’s important to account for this (usually with a brew calculator, as we mentioned earlier). 

How Much Water Does Beer Grain Absorb? 

How Much Water Does Beer Grain Absorb

Figuring out grain absorption can be a little tricky, because there are so many factors at play. The grain itself and how it’s crushed and mashed can play a part in grain absorption, but also something as unpredictable as the humidity on the day can alter this. 

There are two ways of figuring out grain absorption; one is a quick (but less accurate) measurement, and the other is more precise. 

Calculating By The Rule Of Thumb 

A number of respected brewers and brewing companies have, after much trial and error, arrived at a few independent figures for grain absorption rates. These are: 

  • 0.2 gallons/lb of grain
  • 0.12 gallons/lb of grain 
  • 0.1 gallons/lb of grain 

Roughly speaking, then, if you figure that your grain absorption rate will be between 0.1 and 0.2 gallons per pound of grain, you should be good. 

For those who find this a tad imprecise, however, there is another way of figuring this out… 

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True Grain Absorption Calculation 

This method is a little more finicky and in-depth, but hey – isn’t that why you’re here? 

This method assumes that you have already calculated the amount of water lost during your brewing process. If this is something you’re not aware of, you’ll have to figure it out during your next brew. 

Figuring out your grain absorption loss is, if the above is taken into account, quite simple. Simply measure how much water you have at the start of your mash, and the water remaining in the extracted wort during sparging. The difference between these two numbers is your grain absorption rate. 

Aside From Grain Absorption, How Else Is Water Lost? 

Aside From Grain Absorption, How Else Is Water Lost

As mentioned, grain absorption is not the only way you can lose water in a brew. It’s also important to account for the following: 


Obviously, when you’re boiling your wort, there’s going to be water lost through evaporation. Depending on the strength of your heat source, you can expect to lose 1-3 gallons per hour. 

Because of this, you should start with more water than you want to finish with. Either top up the wort before pitching, or add water before boiling. Make sure that the kettle you’re using is big enough to accommodate the extra water. 

Mash Tun Loss 

Depending on how your mash tun is designed, there is always going to be a small amount of wort lost during the mash process. It’s usually not a lot, but it’s worth knowing how much in order to correctly calculate your water loss. 

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The good news is that if you always use the same mash tun, you only need to make this calculation once. The calculation is also very easy: simply put a certain amount of water into the mash tun, extract it, and work out the difference in volume. That’s your mash tun loss. 

Tube Loss 

When racking your wort, a minimal amount is going to be lost in the process. This amount is insignificant and so trace that it’s not worth bothering trying to measure it. 

Trub Loss 

Trub is all the left-over stuff that’s sitting at the bottom of your fermentation container after you’ve racked your beer into bottles or kegs. Because you don’t want any solids in your final brew, you don’t want to try to extract liquid from this layer of gunk. It’s therefore inevitable that you’ll lose some liquid when racking your beer. 

How Much Water Is Absorbed When Using BIAB?

How Much Water Is Absorbed When Using BIAB

Beer In A Bag (BIAB) is an easy and convenient way of brewing your own batch, and takes a lot of the complexity out of home-brewing. However, it’s still important to make sure that you’re accounting for grain absorption. 

Typically, one pound of BIAB can be expected to absorb 0.1 gallon, though this can sometimes go as high as 0.125 gallons. This difference is negligible; stick to assuming 0.1 gallon per pound and you’ll be fine. 



Though figuring out your grain absorption rate can be a little intimidating, the truth is that you can’t go too far wrong simply using rule-of-thumb measurements. If, however, you want a bit more precision, that’s completely doable too – it’s just a matter of sitting down and making a few calculations in your next batch, before applying those calculations to the following batch. It does involve a bit of effort and busywork, but that’s the price of perfectionism. 

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Whatever method you opt for, you’ll have a lovely batch of beer brewed up in no time (fingers crossed)! 

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Lisa Hayden-Matthews

Lisa Hayden-Matthews

A bike rider, triathlon enthusiast, amateurish beach volleyball player and nature lover who has never lost a dare! I manage the overall Editorial section for the magazine here and occasionally chip in with my own nature photographs, when required.

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