Ibu's and effects of adding water post boil

**Aschecte** · 01-30-2012, 04:02 PM

How would adding top up water effect the Ibu's of the beer. A specific example is that I had done a 8 gallon boil which I calculated at 64 Ibu's and 1.069 post boil based on beersmith calculation for a 5.5 gallon batch. I had a very bad brew day and only ended up with 3.75 gallons of useable wort so I topped of with 1.75 gallons of fresh water. I understand how to calculate my dillution for the SG but what about my Ibu's as I'm sure that is effected as well.

Is this formula possibly correct ?

(Ibu's x wot collected) + (fresh water addded) / total volume of fermenter
(64 x 3.75 ) + (1.75 ) / 5.5
240 + 1.75 = 241.75 / 5.5 = 44 Ibu's

I'm really winging that formula but am I on track or can someone tell me a way to calculate this problem.

**ajdelange** · 01-30-2012, 05:53 PM

If you indeed have 64 IBU wort have 64 units of bittering in 3.75 gal of wort you would have (approximately) 64 mg/L isomerized bittering principle for a total of 64 mg/L * 3.75 gal * 3.78 liter/gal. Diluting to 5.5 gal would result in 64 mg/L * 3.75 gal * 3.78 liter/gal / (5.5*3.75) or 64*3.75/5.5. But you calculated the hops charge such that you got the 64 IBU based on 5.5 gal implying total bittering of 64 mg/L * 5.5 gal * 3.78 liter/gal. Dividing the total bittering by 5.5 * 3.75 shows that the bittering is unchanged.

The only problem here is that if you boil the hops in more concentrated wort the hops utilization goes down and you probably have, therefore, extracted less than 64 IBU's. Calculated bittering levels are little more than WAGs anyway so I wouldn't worry about it. Call it 64.