When I first started with all grain, I ended up knocking my head against the wall trying to determine the amount of water needed to mash…

…of course this wasn’t much of a problem for steeping grains, but then again I wasn’t doing it right either…

With extract brewing, there is a simple rule of thumb to follow… ‘boil big, steep small’… but with all grain brewing, boiling big is not always what you want and mashing has to be a little more precise than just ‘small’…

So we’ll get steeping out of the way and just go with proven ratio of using less than one gallon of water per pound of grains… and this is just a rule of thumb, because really what you are trying to accomplish by keeping the ratio this way is you pH levels below 6 among other things…

Now, the amount needed for all-grain is a whole different animal altogether…

**Grain Absorption**

First we have to account for the amount of water the grains will soak up during the mash, which can not be drained in the lauter tun… this usually is about .1 to .18 of the initial weight of the grain… so if you mash 10 lbs of grain, you will lose about 1 to 1.8 gallons of water to the grains…

In my system this is about 1.36 gallons, but you don’t need to be that precise…

**Lauter Tun Water Loss**

Depending on the configuration of your false bottom, there may be water left in the lauter tun that you were not able to drain, but was not absorbed by the grains…

My system drains the wort to within 1/8″ from the botton of the lauter tun so I only loose just under two cups of wort… to find out yours, you’ll have to remove the grains from your lauter tun and measure the amount of water left at the bottom…

This can be a PITA, but the good thing is that you only have to do it once and you’ll know from here on out…

**Loss of Water During Boiling**

Evaporation rates vary depending on how much you are boiling and your burner settings. 5-15% is about the range you’ll see, but you shouldn’t not be boiling too vigorous so the lower the evaporation rate you can get while maintaining a rolling boil the better… (there are a few exceptions though)

**Shrinkage**

When you cool down your wort to room temperature, you’ll see the volume decrease. This loss is usually about 4%…

You can leave this as a constant or you can exclude it and account for it by increasing your evaporation rate… so if you have 5% evaporation rate and 4% shrinkage, you could just say you lost 8-10% to evaporation…

**Equipment Losses**

Last, you want to account for any other equipment losses such as siphoning from your kettle to your fermentor and then to your bottling bucket or keg…

I usually include any loss of wort due to trub since I try not to siphon that into my bottling bucket…

The main thing to keep in mind here is that even with a big long formula that accounts for all of this, the amount of water needed is a mere approximation since you will have many variables…

Being precise is not the goal here… the goal is consistency and making sure you have enough water to brew with… if you are to err, you should do so by over-estimating…

So putting all of this together…

Say I want to mash 10 lbs of grain… Grain absorption will be between .1 to .18, so let’s choose .13…

That means I lose 1.3 gallons of water in the mash…

Next, I lose just under two cups of wort when lautering so we’ll say .45 gallons… so far, I’ve lost 1.75 gallons… this is __what you lose during the mash__ which means that if you want a 5 gallon batch you need to make sure you have 6.75 gallons so far…

But now we have to account for the boil losses… so you would calculate the loss due to evaporation rate… (evaporation rate * Boil time)… in this case I’ll boil for 1 hour and expect my evaporation rate to be 5% so I will lose about 5% of my wort due to evaporation…

That means you need to add the 5% you would lose to evaporation and that is done by dividing 6.75 by 95%… so you would get about 7.11 gallons of water you need…

Next you account for shrinkage of 4%, so you divide 7.11 by 96%… and that gives us 7.4 gallons…

Again, you can combine the evaporation and shrinkage and divide 6.75 by 91% (9% loss) and you would get about 7.41 gallons, which is close enough…

Last you account for any equipment losses (siphoning, trub, etc.)… if say you lose .5 gallons… then you would add that to 7.4 and you get 7.9 gallons of water needed…

This is a lot of math, but the good thing is that if you are consistent, then you just need to worry about the losses from the mash…

If you constantly brew 5 gallon batches then you could just make sure you collect 6 to 6.5 gallons of wort and make that a constant knowing that this will yield about 5 gallons everytime…

So the only math there is to do now is the mash math which we said it was 1.75 that we lost… and add that to your constant, which should be around 6 to 6.5 gallons needed pre-boil…

So depending on our constant, our easy math would yield 7.75 to 8.25 gallons of water needed…

You can also let some brewing software do this math for you…

How do you calculate the amount of water needed?

## 5 replies to "How To Determine Amount Of Water Needed To Brew"

Great info. I like using a constant and just worrying about the loss of water during the mash. How do you determine the boiloff rate?

This seems a little complicated, and that’s why I use brewing software.

@Jeff – That depends on your system and how fast your wort evaporates during the boil as well as how long you boil for… if you start with 6 gallons and you end up with 5.5 gallons after one hour, then you simply divide 5.5 by 6 and you get .92… that means your boil off rate is 8% per hour.

@William – I hear you… that’s how I determine it nowadays, but I used this 2 step technique for quite a while…

How did you determine that your system loses 1.36 gallons in the mash?

I think your math is a little off..”That means you need to add the 5% you would lose to evaporation and that is done by dividing 6.75 by 95%… so you would get about 7.11 gallons of water you need…” – Since we are assuming 6.75 gallons is what we put into the mash, with 1.75 gallons lost – we’d have 5.00 gallons in our kettle. So the math to account for evaporation should be based on 5.00 gallons and not 6.75 gallons as you’ve indicated. If we want to end up with 5.00 gallons after the boil, with 5% evaporation rate (disregarding shrinkage), then we need to start with 5.26 gallons (5.00/0.95); this means an addition 0.26 gallons of water is needed in addition to the 1.75 lost in the mash. This comes out to 7.01 gallons (1.75 + 0.26 + 5.00) total water needed