The first time that I brewed with whole flower / leaf hops, my more-experienced homebrewing friend said that I should have more wort than usual at the start of the boil, because the dried leaves will reconstitute and absorb liquid. I followed her directions, and I was pleased with the ultimate result. Naturally, I forgot her exact directions.
These are the hops that I'm using.
Is having extra wort a proper adjustment for these types of hops, and if so, how much extra wort should I have?
An experiment to determine wort absorption by whole leaf hope is described here. The conclusion was that an ounce of hops absorbs 0.15 quarts of wort.
Whole leaves will absorb quite a bit of your wort during boil, but it really depends on what you are brewing. Lower hopped beers (e.g. German and Belgian beers) = lower absorption, higher = higher absorption. If you are brewing a double IPA (or Imperial IPA), you may want to account for absorption by increasing your final expected quantity.
I've often taken the mindset that for every 5 gallons of homebrew I wish to finish with, I'll add 1 gallon, making a 5 gallon batch, 6 gallons in total. 10 gallon batches become 12 gallon batches. The amount of hops and trub you end up with after primary and secondary may vary, but if you top off your fermentation bucket/carboy with 5 gallons, you will inevitably end up with less than 5 to go into kegs/bottles because of trub and yeast.
While it's my personal rule of thumb, if I plan on using whole leaf hops, or a lot of hops... Or screw it, anything in general that's excessive in hops or ABV, and want 5 gallons total output from my brew, I'll brew 6 gallons. Frankly, if I wind up with too much, and have to bottle off a few extra beers, or fill my keg to the point of overflowing, what a shame!
Dried leaf hops absorb 4-6 times their own weight in wort. So, if you add 4oz/100g of hops in the boil, that's in the region of 16-24fl.oz/400-600ml of wort that you could lose. Naturally, the losses become more significant the more hops you add to the boil.
Most brewers intentionally overshoot slightly on the batch volume, e.g. target 5.5 or 6 gallons for a 5 gallon batch. If you're brewing a beer with 8oz/200g or more of hops, then use the numbers above to compute the additional volume needed to counter the losses.
