# How do folks calculate ABV after a step feed with volume change?

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#### Gabriel Sayegh

##### New Member
Hi all, I'm running into a reoccurring situation where I end up with too much wort for primary by ~1 gallon (more than could go into the carboy and account for blow off). So, I've been reserving it in a separate container during primary and after racking the partially fermented wort off the trub into a new carboy for secondary, I step feed with that additional unfermented wort and get some volume back, example below. I'm wondering how to calculate the total ABV at the end of all this:

Pre-boil volume = 7 gallons
Post-boil volume = 5.5 gallons
Since this would reach the top of my 5.5-gallon carboy, I reserved ~0.75 gallons into a second glass jug and placed it in the fridge for later.

Primary fermenter volume = 4.75 gallons @ 1.065 OG
After 2 weeks, SG = 1.012
Racked to secondary fermenter, resulted in 4.25 gallons (lost ~1/2 a gallon of trub)
Step fed with ~0.75 gallons of wort reserved from the boil (still at 1.065 since no yeast was added to this wort)
New SG at the beginning of secondary for ~5 gallons total = 1.022
After 2 additional weeks, FG = 1.014

At this point, I added dextrose and bottled, but what's the ABV?

Best I can guess is that the primary fermenter took 1.065 -> 1.012, so that's ~7% ABV, and then the secondary took 1.022 -> 1.014, so that's an additional ~1%, so do I call it 8% in this case (or 8.5% after accounting for increased ABV due to bottle conditioning & carbonation)?

Or does the fact that the first wort was a smaller size than the second wort play a factor in calculating the resulting ABV of the combined beer?

I recognize this is potentially complex. I appreciate any thoughts or insight folks might have!

Seems risky to me to add the second lot of wort after 2 weeks. Freezing it for 2 weeks, defrost, reboil and add it in one go would be safer.
If I understand 5.5 gallons at 1.065 ferments down to 1.014.
Doesn't matter that it arrived at the FG figure after the addition.
Your dextrose for bottling will affect the abv.

I can't see any way that adding MORE of the SAME WORT to an already fermenting wort would change the abv in any way.

If you were combining different worts, then just get a weighted average... Here's a contrived example:
Wort #1: 4 gallons, at 1.055
Wort #2: 1 gallon, at 1.065

so combined "original gravity" WOULD HAVE BEEN (had they been combined prior to yeast addition):
(4 * 1.055) + (1 * 1.065) / 5
or 1.057

Anyways, in your specific case: you had 5.5 gallons of 1.065 wort, and it all finished at 1.014 (even if it didn't all go at the same time). So your ABV would be 6.69%. Bottle priming to 2.7 volumes adds 0.5% to your actual ABV.

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I recognize this is potentially complex.
I can't see any way that adding MORE of the SAME WORT to an already fermenting wort would change the abv in any way.

But skip the secondary and just ferment the extra wort in any one gallon jug that you can stick an airlock onto.

Short of getting a new fermenter (much advised over using a dangerous glass carboy) it would be wiser to adjust your profile to achieve the volume that fits your fermenter. You may end up with less than a full 5 gallons at the end but you will also end up with the beer you intended.

Thanks for all the replies, I must be overcomplicating this, and I appreciate the reassurance that this is simple. If I am reading the calculations correctly, would it be fair to say that the increase in volume from the step feed offsets the additional alcohol creation in secondary since the worts started at the same gravity?

I ask because I got hung up on how the original wort in my example went from 1.065 -> 1.012, so it would have been ~7% (~6.9-7.2% depending on calculation) before the addition. So, the fact that more wort was added and more alcohol was produced leads me to believe the ABV has to now be higher (or at least different...), but both the 'A' and the 'V' have increased, so I guess it makes sense that the additional fermentation observed from 1.022 -> 1.014 (~1-1.1%) would be offset by the fact the wort is now ~0.75 gallons larger as well.

If that makes sense to everyone else, I'll try to put it to rest in my mind and just calculate the total as many of you have suggested. Thanks again!

Do you make yeast starters? If you have extra wort, pour it (hot) into Mason jars and freeze it.

I ask because I got hung up on how the original wort in my example went from 1.065 -> 1.012, so it would have been ~7% (~6.9-7.2% depending on calculation) before the addition. So, the fact that more wort was added and more alcohol was produced leads me to believe the ABV has to now be higher (or at least different...), but both the 'A' and the 'V' have increased, so I guess it makes sense that the additional fermentation observed from 1.022 -> 1.014 (~1-1.1%) would be offset by the fact the wort is now ~0.75 gallons larger as well.

If that makes sense to everyone else, I'll try to put it to rest in my mind and just calculate the total as many of you have suggested. Thanks again!

If you've got a typical homebrew hydrometer (0.990 -> 1.160), finding a 2 point difference (0.002) is easily enough explained by margin-of-error.

As far as adding the same wort to the FV days later, the FG you reach with it will be the ABV you calculate regardless. You increased the volume when you added that wort and it's OG was the same as the wort you put in the FV originally.

If you were to add say just plain sugar to the FV which didn't appreciably increase the volume, then you can just plug that number in your beer software to figure out what that predicted increase of the sugar would have been if added prior to taking the OG reading.

If you added a certain volume wort or other sugary liquid of a known but different SG to your FV, you can also just figure out the dilution of the added solution on the contents of the FV when it was at it's initial reading (OG) and use that as your OG to figure your ABV from whatever your FG winds up to be.

There are calculators to figure out the dilution of SG between two liquids of known volume and density. There is a fairly simple formula too. I just don't know it!

It's not complicated. But describing it is, and maybe wrapping your head around it is. And while it's true there might be some variables to the above that affect it's accuracy, then we'd have to also ask how accurate just calculating the ABV for a normal brew without your added complication is for giving us a precise value for the alcohol content.

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Do you make yeast starters? If you have extra wort, pour it (hot) into Mason jars and freeze it.
Do you reboil for 15 minutes after thawing?

Do you reboil for 15 minutes after thawing?

In theory, it should come out sterile, as long as it's not exposed to outside air while thawing. But I wouldn't leave it to chance. So, yes, boiling would probably be a good idea.

In theory, it should come out sterile, as long as it's not exposed to outside air while thawing. But I wouldn't leave it to chance. So, yes, boiling would probably be a good idea.
Thanks. I made a 6 liter batch of wort some time ago, used half of it directly in a yeast starter, and froze the rest. I just thawed it out a couple of days ago and boiled it for 15 minutes. It likely wasn't necessary since I put the wort in the bottles at about 180 F and closed them immediately before freezing, but I didn't want to take a chance. Do you add the wort when it's still at boiling temperature, or do you wait for it to cool down a bit first? Have you ever had one of your Mason jars crack?

Thanks. I made a 6 liter batch of wort some time ago, used half of it directly in a yeast starter, and froze the rest. I just thawed it out a couple of days ago and boiled it for 15 minutes. It likely wasn't necessary since I put the wort in the bottles at about 180 F and closed them immediately before freezing, but I didn't want to take a chance. Do you add the wort when it's still at boiling temperature, or do you wait for it to cool down a bit first? Have you ever had one of your Mason jars crack?

It's been a while since I canned wort, but IIRC, I boiled the jars prior to pouring in the wort, so thermal shock wasn't an issue.

I've made invert sugar a few times, and did the same with canning it.

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