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"top off to desired boil volume"

Hey there,

I was wondering if someone would be able to answer my question regarding the stage in partial mash brewing that the brewer “tops off to desired boil volume” (to quote NB PM instructions).

For as well explained as NB’s instructions are, I think this instruction which is simply stated is perhaps more complex, and for me, something I might be getting wrong.

Doesn’t the amount of water in your boil greatly affect whether or not your OG will be on target? For me, I think a common problem I have is topping off either too much or not enough, seldomly I hit my target OG AND have a perfect 5 gal. Lately, I’ve prioritized hitting my target OG, but in doing so have sacrificed a full 5 gal carboy.

I’d love a advice on this stage. How does one figure out their ‘desired boil volume.’ Is it better to have a lower boil volume and top off with more h20 at the end? Or, better to have a higher boil volume and top off with less?

Thanks in advance,

If I understand the directions on the partial mash kit, the step you are talking about happens after the partial mash but before you start the boil. I would assume this means they want you to bring the volume in the boil kettle up to what you normally would use to do an extract boil. I haven’t done extract in a while but I think it’s around 2-2.5 gallons. You may need to also add water at the end of the boil to get to 5 gallons into the fermentor, I have done this with ice water to cool and get the right volume at the same time.
Adding water will certainly affect your OG, but the kits are designed for 5 gallons. If you are doing the partial mash right, you should be getting pretty close to the OG (+/- a point or two) if you have 5 gallons into the fermentor. So the answer to your last two questions is: it doesn’t matter, the same amount of sugar will be in the wort regardless of volume. I hope this helps, best of luck.


My impression is that “top off” water is added to the fermenter. If you don’t have a pot big enough to boil 6-7 gallons and end with 5-5.5 in the fermenter you “top off” to get to your volume.

If you are coming up short on your OG, it could be a number of issues. The wort may not be mixed well with your “top off” water, so you have a low gravity. Your volume is greater than you think, giving you a low volume. You didn’t get all the LME/DME in the the mix.

Depending on how low you are, you could add up to a pound of table sugar. That would give you about 9 additional points. Or you can order an extra pound of DME to add to the boil. That is also bout 9 points for 5 gallons.

It is better to boil as much as you can. You reduce the chance of burning the wort or caramelizing the sugars. If you don’t have a 8 gallon pot, a big burner and a way to cool the wort, then boil as much as you can. Many people then will only add 1/2 of the LME/DME to the boil to avoid the caramelizing.

I have brewed several partial mash kits and yes, if you follow the directions, you end up with less than 3 gallons or so of “boil volume”. What I have done is mash in a bag, rinse with hot sparge water while the bag sits on a strainer. Then I will continue rinsing with cool water until I reach the volume I am looking for, for the boil.

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